NAME¶
Image::MetaData::JPEG - Perl extension for showing/modifying JPEG (meta)data.
SYNOPSIS¶
use Image::MetaData::JPEG;
# Create a new JPEG file structure object
my $image = new Image::MetaData::JPEG('somepicture.jpg');
die 'Error: ' . Image::MetaData::JPEG::Error() unless $image;
# Get a list of references to comment segments
my @segments = $image->get_segments('COM', 'INDEXES');
# Get the JPEG picture dimensions
my ($dim_x, $dim_y) = $image->get_dimensions();
# Show all JPEG segments and their content
print $image->get_description();
# Retrieve a specific value from Exif meta-data
my $image_data = $image->get_Exif_data('IMAGE_DATA', 'TEXTUAL');
print $image_data->{DateTimeOriginal}->[0], "\n";
# Modify the DateTime tag for the main image
$image->set_Exif_data({'DateTime' => '1994:07:23 12:14:51'},
'IMAGE_DATA', 'ADD');
# Delete all meta-data segments (please, don't)
$image->drop_segments('METADATA');
# Rewrite file to disk after your modifications
$image->save('new_file_name.jpg');
# ... and a lot more methods for viewing/modifying meta-data, which
# are accessed through the $file or $segments[$index] references.
DESCRIPTION¶
The purpose of this module is to read/modify/rewrite meta-data segments in JPEG
(Joint Photographic Experts Group format) files, which can contain comments,
thumbnails, Exif information (photographic parameters), IPTC information
(editorial parameters) and similar data.
Each JPEG file is made of consecutive
segments (tagged data blocks), and
the actual row picture data. Most of these segments specify parameters for
decoding the picture data into a bitmap; some of them, namely the
COMment and
APPlication segments, contain instead meta-data,
i.e., information about how the photo was shot (usually added by a digital
camera) and additional notes from the photograph. These additional pieces of
information are especially valuable for picture databases, since the meta-data
can be saved together with the picture without resorting to additional
database structures. See the appendix about the structure of JPEG files for
technical details.
This module works by breaking a JPEG file into individual segments. Each file is
associated to an
Image::MetaData::JPEG structure object, which contains
one
Image::MetaData::JPEG::Segment object for each segment. Segments
with a known format are then parsed, and their content can be accessed in a
structured way for display. Some of them can even be modified and then
rewritten to disk.
- $JPEG::show_warnings
- This package variable must be used to inhibit the printing of warnings: if
it is false, warnings are silently ignored. Otherwise, warning messages
come with a detailed back-trace and description of the warning location.
$Image::MetaData::JPEG::show_warnings = undef;
Managing a JPEG structure object¶
- JPEG::new
- [arguments: "($input, $regex, $options)"] The first thing you
need in order to interact with a JPEG picture is to create an
Image::MetaData::JPEG structure object. This is done with a call to
the new method, whose first argument is an input source,
either a scalar, interpreted as a file name to be opened and read,
or a scalar reference, interpreted as a pointer to an in-memory
buffer containing a JPEG stream. This interface is similar to that of
Image::Info, but no open file handle is (currently) accepted. The
constructor then parses the picture content and stores its segments
internally. The memory footprint is close to the size of the disk file
plus a few tens of kilobytes.
my $file = new Image::MetaData::JPEG('a_file_name.jpg');
my $file = new Image::MetaData::JPEG(\ $a_JPEG_stream);
The constructor method accepts two optional arguments, a regular
expression and an option string. If the regular expression
is present, it is matched against segment names, and only those segments
with a positive match are parsed (they are nonetheless stored); this
allows for some speed-up if you just need partial information, but be sure
not to miss something necessary; e.g., SOF segments are needed for reading
the picture dimensions. For instance, if you just want to manipulate the
comments, you could set the string to 'COM'.
my $file = new Image::MetaData::JPEG('a_file_name.jpg', 'COM');
The third optional argument is an option string. If it matches the string
'FASTREADONLY', only the segments matching the regular expression are
actually stored; also, everything which is found after a Start Of Scan is
completely neglected. This allows for very large speed-ups, but,
obviously, you cannot rebuild the file afterwards, so this is only for
getting information fast, e.g., when doing a directory scan.
my $file = new Image::MetaData::JPEG('a_file.jpg', 'COM', 'FASTREADONLY');
Nota bene: an old version of "Arles Image Web Page Creator" had a
bug which caused the application to generate JPEG's with illegal comment
segments, reportedly due to a bug in the Intel JPEG library the developers
used at that time (these segments had to 0x00 bytes appended). It is true
that a JPEG file with garbage between segments is to be considered
invalid, but some libraries like IJG's try to forgive, so this module
tries to forgive too, if the amount of garbage isn't too large (only a
warning is printed).
- JPEG::Error
- [arguments: none] If the file reference remains undefined after a call to
new, the file is to be considered not parseable by this module, and one
should issue some error message and go to another file. An error message
explaining the reason of the failure can be retrieved with the
Error method:
die 'Error: ' . Image::MetaData::JPEG::Error() unless $file;
- JPEG::get_segments
- [arguments: "($regex, $do_indexes)"] If the new call is
successful, the returned reference points to an
Image::MetaData::JPEG structure object containing a list of
references to Image::MetaData::JPEG::Segment objects, which can be
retrieved with the get_segments method. This method returns a list
containing the references (or their indexes in the Segment references'
list, if the second argument is the string INDEXES) to those
Segments whose name matches the $regex regular
expression. For instance, if $regex is 'APP', all application Segments
will be returned. If you want only APP1 Segments you need to specify
'^APP1$'. The output can become invalid after adding/removing any Segment.
If $regex is undefined, all references are returned.
my @segments = $file->get_segments($regex, $do_indexes);
- JPEG::drop_segments
- [arguments: "($regex)"] Similarly, if you are only interested in
eliminating some segments, you can use the drop_segments method,
which erases from the internal segment list all segments matching a given
regular expression. If the regular expression is undefined or evaluates to
the empty string, this method throws an exception, because I don't want
the user to erase the whole file just because he/she did not understand
what he was doing. One should also remember that it is not wise to drop
non-meta-data segments, because this in general invalidates the file. As a
special case, if $regex == 'METADATA', all APP* and COM segments are
erased.
$file->drop_segments('^APP1$');
- JPEG::insert_segments
- [arguments: "($segref, $pos, $overwrite)"] Inserting a Segment
into the picture's segment list is done with the insert_segments
method. This method inserts the segments referenced by $segref into the
current list of segments at position $pos. If $segref is undefined, the
method fails silently. If $pos is undefined, the position is chosen
automatically (using find_new_app_segment_position ); if $pos is out of
bound, an exception is thrown; this happens also if $pos points to the
first segment, and it is an SOI. $segref may be a reference to a single
segment or a reference to a list of segment references; everything else
throws an exception. If $overwrite is defined, it must be the number of
segments to overwrite during the splice.
$file->insert_segments([$my_comment_1, $my_comment_2], 3, 1);
- JPEG::get_description
- JPEG::get_dimensions
- [arguments: none] Getting a string describing the findings of the parsing
stage is as easy as calling the get_description method. Those
Segments whose parsing failed have the first line of their description
stating the stopping error condition. Non-printable characters are
replaced, in the string returned by get_description, by a slash followed
by the two digit hexadecimal code of the character. The (x,y) dimensions
of the JPEG picture are returned by get_dimensions from the
Start of Frame (SOF*) Segment:
print $file->get_description();
my ($dim_x, $dim_y) = $file->get_dimensions();
- JPEG::find_new_app_segment_position
- [arguments: "($name)"] If a new comment or application Segment
is to be added to the file, the module provides a standard algorithm for
deciding the location of the new Segment, in the
find_new_app_segment_position method. The argument is the name of
the Segment to be inserted (it defaults to 'COM', producing a warning).
The position is chosen immediately before the first (or after the last)
element of some list, provided that the list is not empty, otherwise the
next list is taken into account: 1) [for COM segments only] after 'COM'
segments; otherwise after APP segments; 2) [for APPx segments only] after
APPy's (trying y = x..0, in sequence); otherwise before APPy's (trying y =
x+1..15, in sequence); 3) before DHP segments; 4) before SOF segments. If
all these tentatives fail, the position immediately after the SOI segment
is returned (i.e., 1).
my $new_position = $file->find_new_app_segment_position('APP2');
- JPEG::save
- [arguments: "($filename)"] The data areas of each Segment in the
in-memory JPEG structure object can be rewritten to a disk file or to an
in-memory scalar, thus recreating the (possibly modified) JPEG picture.
This is accomplished by the save method, accepting a
filename or a scalar reference as argument; if the file name
is undefined, it defaults to the file originally used to create the JPEG
structure object. This method returns "true" (1) if it works,
"false" (undefined) otherwise. Remember that if the file had
initially been opened with the 'FASTREADONLY' option, it is not possible
to save it, and this call fails immediately.
print "Creation of $newJPEG failed!" unless $file->save($newJPEG);
An example of how to proficiently use the in-memory feature to read the
content of a JPEG thumbnail is the following (see later for get_Exif_data,
and also do some error checking!):
my $thumbnail = $file->get_Exif_data('THUMBNAIL');
print Image::MetaData::JPEG->new($thumbnail)->get_description();
Managing a JPEG Segment object¶
- JPEG::Segment::name
- JPEG::Segment::error
- An Image::MetaData::JPEG::Segment object is created for each
Segment found in the JPEG image during the creation of a JPEG object (see
JPEG::new), and a parser routine is executed at the same time. The
name member of a Segment object identifies the "nature"
of the Segment (e.g. 'APP0', ..., 'APP15' or 'COM'). If any error occurs
(in the Segment or in an underlying class), the parsing of that Segment is
interrupted at some point and remains therefore incomplete: the
error member of the relevant Segment object is then set to a
meaningful error message. If no error occurs, the same variable is left
undefined.
printf 'Invalid %s!\n', $segment->{name} if $segment->{error};
- JPEG::Segment::records
- The reference to the Segment object is returned in any case. In this way,
a faulty Segment cannot inhibit the creation of a JPEG structure
object; faulty segments cannot be edited or modified, basically because
their structure could not be fully understood. They are always rewritten
to disk unmodified, so that a file with corrupted or non-standard Segments
can be partially edited without fearing of damaging it. Once a Segment has
successfully been built, its parsed information can be accessed directly
through the records member: this is a reference to an array of
JPEG::Record objects, an internal class modelled on Exif records (see the
subsection about record management for further details).
my $records = $segment->{records};
printf '%s has %d records\n', $segment->{name}, scalar @$records;
- JPEG::Segment::search_record
- JPEG::Segment::search_record_value
- [arguments: "([$dirref], $keys ...)"] If a specific record is
needed, it can be selected with the help of the search_record
method, which searches for a record with a given key (see
"JPEG::Record::key") in a given record directory, returning a
reference to the record if the search was fruitful, the undefined value
otherwise. The algorithm for the search is as follows: 1) a start
directory is chosen by looking at the last argument: if it is an ARRAY ref
it is popped out and used, otherwise the top-level directory is selected;
2) a string is created by joining all remaining arguments on '@', then it
is exploded into a list of keys on the same character (all undefined or
"false" arguments are simply discarded); 3) these keys are used
for an iterative search starting from the initially chosen directory: all
but the last key must correspond to $REFERENCE records. If $key is exactly
"FIRST_RECORD" / "LAST_RECORD", the first/last record
in the current dir is used.
my $segments = $file->get_segments('APP0');
my $segment = $$segments[0];
print "I found it!\n" if $segment->search_record('Identifier');
If you are interested only in the Record's value, you can use the
search_record_value method, a simple wrapper around
search_record(): it returns the record value (with
"JPEG::Record::get_value") if the search is successful, undef
otherwise.
print "Its value is: ", $segment->search_record_value('Identifier');
Nota bene: the returned record is initialised with a "fake"
$REFERENCE record pointing to the records member of the current
segment; this record is therefore returned if search_record is
invoked without arguments. For the same reason, search_record_value
invoked without arguments returns the records member:
$segment->search_record_value() eq $this->{records} || print "error!";
- JPEG::Segment::update
- [arguments: none] If a Segment's content (i.e. its Records' values) is
modified, it is necessary to dump it into the private binary data area of
the Segment in order to have the modification written to disk at
"JPEG::save" time. This is accomplished by invoking the
update method (necessary only if you changed record values "by
hand"; all "high-level" methods for changing a Segment's
content in fact call "update" on their own). However, only
Segments without errors can be updated (don't try to undef the Segment's
error flag, unless you know what you are doing!); trying to update a
segment with errors throws an exception. The same happens when trying to
update a segment without update support or without records (this catches
segments created with the 'NOPARSE' flag). In practise, never use this
method unless you are writing an extension for this module.
Note that this method preliminarly saves a reference to the old segment data
area and restores it if the update process fails (if this happens, a
warning is generated). One wonders wheather there are there cleverer ways
to handle this case (any suggestion is welcome). It is however better to
have a corrupt object in memory, than a corrupt object written over the
original. Currently, this is restricted to the possibility that an updated
segment becomes too large.
$segment->update();
- JPEG::Segment::reparse_as
- [arguments: "($new_name)"] The reparse_as method
re-executes the parsing of a Segment after changing the Segment name. This
is very handy if you have a JPEG file with a "correct"
application Segment exception made for its name. I used it the first time
for a file having an ICC_profile Segment (normally in APP2) stored as
APP13. Note that the name of the Segment is permanently changed, so, if
the Segment is updated and the file is rewritten to disk, it will be
"correct".
for my $segment ($file->get_segments('APP13')) {
$segment->reparse_as('APP2') if $segment->{error} &&
$segment->search_record('Identifier') =~ 'ICC_PROFILE';
$segment->update(); }
- JPEG::Segment::output_segment_data
- [arguments: none] The current in-memory data area of a Segment can be
output to a file through the output_segment_data method (exception
made for entropy coded Segments, this includes the initial two bytes with
the Segment identifier and the two bytes with the length if present); the
argument is a file handle (this is likely to become more general in the
future). If there are problems at output time (e.g., the segment content
is too large), an exception is thrown
eval { $segment->output_segment_data($output_handle) } ||
print "A terrible output error occurred! Help me.\n";
- JPEG::Segment::get_description
- JPEG::Segment::size
- [arguments: none] A string describing the parsed content of the Segment is
obtained through the get_description method (this is the same
string used by the get_description method of a JPEG structure object). If
the Segment parsing stage was interrupted, this string includes the
relevant error. The size method returns the size of the internal
data area of a Segment object. This can be different from the length of
the scalar returned by get_segment_data, because the identifier and the
length is not included.
print $segment->get_description();
print 'Size is 4 + ' . $segment->size();
Managing a JPEG Record object¶
- JPEG::Record::key
- JPEG::Record::type
- JPEG::Record::values
- JPEG::Record::extra
- The JPEG::Record class is an internal class for storing parsed
information about a JPEG Segment, inspired by Exif records. A Record is
made up by four fields: key, type, values and
extra. The key is the record's identifier; it is either
numeric or textual (numeric keys can be translated with the help of the
%JPEG_lookup function in Tables.pm, included
in this package). The type is obviously the type of stored info
(like unsigned integers, ASCII strings and so on ...). extra is a
helper field for storing additional information. Last, values is an
array reference to the record content (almost always there is just one
value). For instance, for a non-IPTC Photoshop record in APP13:
printf 'The numeric key 0x%04x means %s',
$record->{key}, JPEG_lookup('APP13@Photoshop_RECORDS', $record->{key});
printf 'This record contains %d values\n', scalar @{$record->{values}};
A Record's type can be one among the following predefined constants:
0 $NIBBLES two 4-bit unsigned integers (private)
1 $BYTE An 8-bit unsigned integer
2 $ASCII A variable length ASCII string
3 $SHORT A 16-bit unsigned integer
4 $LONG A 32-bit unsigned integer
5 $RATIONAL Two LONGs (numerator and denominator)
6 $SBYTE An 8-bit signed integer
7 $UNDEF A generic variable length string
8 $SSHORT A 16-bit signed integer
9 $SLONG A 32-bit signed integer (2's complement)
10 $SRATIONAL Two SLONGs (numerator and denominator)
11 $FLOAT A 32-bit float (a single float)
12 $DOUBLE A 64-bit float (a double float)
13 $REFERENCE A Perl list reference (internal)
$UNDEF is used for not-better-specified binary data. A record of a numeric
type can have multiple elements in its @{values} list ($NIBBLES
implies an even number); an $UNDEF or $ASCII type record instead has only
one element, but its length can vary. Last, a $REFERENCE record holds a
single Perl reference to another record list: this allows for the
construction of a sort of directory tree in a Segment.
- JPEG::Record::get_category
- [arguments: none] The category of a record can be obtained with the
get_category method, which returns 'p' for Perl references, 'I' for
integer types, 'S' for $ASCII and $UNDEF, 'R' for rational types and 'F'
for floating point types.
for my $record (@{$segment->{records}}) {
print "Subdir found\n" if $record->get_category() eq 'p'; }
- JPEG::Record::get_description
- [arguments: "($names)"] A human-readable description of a
Record's content is the output of the get_description method. Its
argument is a reference to an array of names, which are to be used as
successive keys in a general hash keeping translations of numeric tags. No
argument is needed if the key is already non-numeric (see the example of
get_value for more details). In the output of get_description
unreasonably long strings are trimmed and non-printing characters are
replaced with their hexadecimal representation. Strings are then enclosed
between delimiters, and null-terminated $ASCII strings have their last
character chopped off (but a dot is added after the closing delimiter).
$ASCII strings use a " as delimiter, while $UNDEF strings use '.
print $record->get_description($names);
- JPEG::Record::get_value
- [arguments: "($index)"] In absence of "high-level"
routines for collecting information, a Record's content can be read
directly, either by accessing the values member or by calling the
get_value method: it returns the $index-th value in the value list;
if the index is undefined (not supplied), the sum/concatenation of all
values is returned. The index is checked for out-of-bound errors. The
following code, an abridged version of get_description, shows how to
proficiently use these methods and members.
sub show_directory {
my ($segment, $records, $names) = @_;
my @subdirs = ();
for my $record (@$records) {
print $record->get_description($names);
push @subdirs, $record if $record->get_category() eq 'p'; }
foreach my $subdir (@subdirs) {
my $directory = $subdir->get_value();
push @$names, $subdir->{key};
printf 'Subdir %s (%d records)', $names, scalar @$directory;
show_directory($segment, $directory, $names);
pop @$names; } }
show_directory($segment, $segment->{records}, [ $segment->{name} ]);
- JPEG::Record::get
- [arguments: "($endianness)"] If the Record structure is needed
in detail, one can resort to the get method; in list context this
method returns (key, type, count, dataref). The data reference points to a
packed scalar, ready to be written to disk. In scalar context, it returns
the dereferenced dataref. This is tricky (but handy for other routines).
The argument specifies an endianness (this defaults to big endian).
my ($key, $type, $count, $dataref) = $record->get();
- JPEG::get_number_of_comments
- JPEG::get_comments
- [arguments: none] Each " COM" Segment in a JPEG file
contains a user comment, whose content is free format. There is however a
limitation, because a JPEG Segment cannot be longer than 64KB; this limits
the length of a comment to $max_length = (2^16 - 3)
bytes. The number of comment Segments in a file is returned by
get_number_of_comments, while get_comments returns a list of
strings (each string is the content of a COM Segment); if no comments are
present, they return zero and the empty list respectively.
my $number = $file->get_number_of_comments();
my @comments = $file->get_comments();
- JPEG::add_comment
- [arguments: "($string)"] A comment can be added with the
add_comment method, whose only argument is a string. Indeed, if the
string is too long, it is broken into multiple strings with length smaller
or equal to $max_length, and multiple comment Segments are added to the
file. If there is already at least one comment Segment, the new Segments
are created right after the last one. Otherwise, the standard position
search of find_new_app_segment_position
is applied.
$file->add_comment('a' x 100000);
- JPEG::set_comment
- [arguments: "($index, $string)"] An already existing comment can
be replaced with the set_comment method. Its two arguments are an
$index and a $string: the
$index-th comment Segment is replaced with one or more new Segments based
on $string (the index of the first comment Segment is 0). If $string is
too big, it is broken down as in add_comment. If $string is undefined, the
selected comment Segment is erased. If $index is out-of-bound a warning is
printed out.
$file->set_comment(0, 'This is the new comment');
- JPEG::remove_comment
- JPEG::remove_all_comments
- [arguments: "($index)" for remove_comment] However, if
you only need to erase the comment, you can just call
remove_comment with just the Segment $index. If you want to remove
all comments, just call remove_all_comments.
$file->remove_comment(0);
$file->remove_all_comments();
- JPEG::join_comments
- [arguments: "($separation, @selection)"] It is known that some
JPEG comment readers out there do not read past the first comment. So, the
join_comments method, whose goal is obvious, can be useful. This
method creates a string from joining all comments selected by the
@selection index list (the
$separation scalar is a string inserted at each
junction point), and overwrites the first selected comment while deleting
the others. A exception is thrown for each illegal comment index. Similar
considerations as before on the string length apply. If no separation
string is provided, it defaults to \n. If no index is provided in
@selection, it is assumed that the method must join all the comments into
the first one, and delete the others.
$file->join_comments('---', 2, 5, 8);
JFIF data ("APP0" segments)¶
APP0 Segments are written by older cameras adopting the
JFIF (JPEG
File Interchange Format), or one of its extensions, for storing images. JFIF
files use the APP0 application Segment for inserting configuration data and a
JPEG or RGB packed
thumbnail image. The format is described in the
appendix about the APP0 structure, including the names of all possible tags.
It is of course possible to access each APP0 Segment individually by means of
the get_segments and search_record_value methods. A snippet of code for doing
this is the following:
for my $segment ($file->get_segments('APP0')) {
my $iden = $segment->search_record_value('Identifier');
my $xdim = $segment->search_record_value('Xthumbnail');
my $ydim = $segment->search_record_value('Ythumbnail');
printf 'Segment type: %s; dimensions: %dx%d\n',
substr($iden, 0, -1), $xdim, $ydim;
printf '%15s => %s\n', $_->{key}, $_->get_value()
for $segment->{records}; }
- JPEG::get_app0_data
- [arguments: none] However, if you want to avoid to deal directly with
Segments, you can use the get_app0_data method, which returns a
reference to a hash with a plain translation of the content of the first
interesting APP0 segment (this is the first 'JFXX' APP0 segment, if
present, the first 'JFIF' APP0 segment otherwise). Segments with errors
are excluded. An empty hash means that no valid APP0 segment is present.
my $data = $file->get_app0_data();
printf '%15s => %s\n', $_, (($_=~/..Thumbnail/)?'...':$$data{$_});
Exif data ("APP1" segments)¶
The DCT Exif (Exchangeable Image File format) standard provides photographic
meta-data in the APP1 section. Various tag-values pairs are stored in groups
called IFDs (Image File Directories), where each group refers to a different
kind of information; one can find data about how the photo was shot, GPS data,
thumbnail data and so on ... (see the appendix about the APP1 segment
structure for more details). This module provides a number of methods for
managing Exif data without dealing with the details of the low level
representation. Note that, given the complicated structure of an Exif APP1
segment (where extensive use of "pointers" is made), some digital
cameras and graphic programs decide to leave some unused space in the JPEG
file. The dump routines of this module, on the other hand, leave no unused
space, so just calling
update() on an Exif APP1 segment even without
modifying its content can give you a smaller file (some tens of kilobytes can
be saved).
- JPEG::retrieve_app1_Exif_segment
- [arguments: "($index)"] In order to work on Exif data, an Exif
APP1 Segment must be selected. The retrieve_app1_Exif_Segment
method returns a reference to the $index-th such
Segment (the first Segment if the index is undefined). If no such Segment
exists, the method returns the undefined reference. If $index is (-1), the
routine returns the number of available APP1 Exif Segments (which is non
negative).
my $num = $file->retrieve_app1_Exif_segment(-1);
my $ref = $file->retrieve_app1_Exif_segment($num - 1);
- JPEG::provide_app1_Exif_segment
- [arguments: none] If you want to be sure to have an Exif APP1 Segment, use
the provide_app1_Exif_segment method instead, which forces the
Segment to be present in the file, and returns its reference. The
algorithm is the following: 1) if at least one Segment with this
properties is already present, we are done; 2) if [1] fails, an APP1
segment is added and initialised with a big-endian Exif structure (its
position is chosen by find_new_app_segment_position, as usual). Note that
there is no $index argument here.
my $ref = $file->provide_app1_Exif_segment();
- JPEG::remove_app1_Exif_info
- [arguments: "($index)"] If you want to eliminate the $index-th
Exif APP1 Segment from the JPEG file segment list use the
remove_app1_Exif_info method. As usual, if $index is (-1), all Exif
APP1 Segments are affected at once; if $index is undefined, it defaults to
-1, so both (-1) and undef cause all Exif APP1 segments to be removed. Be
aware that the file won't be a valid Exif file after this.
$file->remove_app1_Exif_info(-1);
How to inspect your Exif data
- JPEG::Segment::get_Exif_data
- JPEG::get_Exif_data
- [arguments: "($what, $type)"] Once you have a Segment reference
pointing to your favourite Exif Segment, you may want to have a look at
the records it contains, by using the get_Exif_data method: it
accepts two arguments ( $what and
$type) and returns the content of the APP1 segment
packed in various forms. Error conditions (invalid $what's and $type's)
manifest themselves through an undefined return value.
All Exif records are natively identified by numeric tags (keys), which can
be "translated" into a human-readable form by using the Exif
standard docs; only a few fields in the Exif APP1 preamble (they are not
Exif records) are always identified by this module by means of textual
tags. The $type argument selects the output format for the record keys
(tags):
* NUMERIC: record tags are native numeric keys
* TEXTUAL: record tags are human-readable (default)
Of course, record values are never translated. If a numeric Exif tag is not
known, a custom textual key is created with "Unknown_tag_"
followed by its numerical value (this solves problems with non-standard
tags). The subset of Exif tags returned by this method is determined by
the value of $what, which can be one of:
$what returned info returned type
---------------------------------------------------------------------
ALL (default) everything but THUMBNAIL ref. to hash of hashes
IMAGE_DATA a merge of IFD0_DATA and SUBIFD_DATA ref. to flat hash
THUMB_DATA this is an alias for IFD1_DATA ref. to flat hash
THUMBNAIL the actual (un)compressed thumbnail ref. to scalar
ROOT_DATA header records (TIFF and similar) ref. to flat hash
IFD0_DATA primary image TIFF tags ref. to flat hash
SUBIFD_DATA Exif private tags ref. to flat hash
MAKERNOTE_DATA MakerNote tags (if struct. is known) ref. to flat hash
GPS_DATA GPS data of the primary image ref. to flat hash
INTEROP_DATA interoperability data ref. to flat hash
IFD1_DATA thumbnail-related TIFF tags ref. to flat hash
Setting $what equal to 'ALL' returns a reference to a hash of hashes, whose
top-level hash contains the following keys: ROOT_DATA, IFD0_DATA,
SUBIFD_DATA, GPS_DATA, INTEROP_DATA, MAKERNOTE_DATA and IFD1_DATA; each
key corresponds to a second-level hash containing a copy of all Exif
records present in the IFD (sub)directory corresponding to the key (if
this directory is not present or contains no records, the second-level
hash exists and is empty). Note that the Exif record values' format is not
checked to be valid according to the Exif standard. This is, in some
sense, consistent with the fact that also "unknown" tags are
included in the output. This complicated structure is more easily
explained by showing an example (see also the section about valid Exif
tags for details on possible records):
my $hash_ref = $segment->get_Exif_data('ALL', 'TEXTUAL');
can give
$hash_ref = {
'ROOT_DATA' =>
{ 'Signature' => [ 42 ],
'Endianness' => [ 'MM' ],
'Identifier' => [ "Exif\000\000" ],
'ThumbnailData' => [ ... image ... ], },
'IFD1_DATA' =>
{ 'ResolutionUnit' => [ 2 ],
'JPEGInterchangeFormatLength' => [ 3922 ],
'JPEGInterchangeFormat' => [ 2204 ],
'Orientation' => [ 1 ],
'XResolution' => [ 72, 1 ],
'Compression' => [ 6 ],
'YResolution' => [ 72, 1 ], },
'SubIFD_DATA' =>
{ 'ApertureValue' => [ 35, 10 ],
'PixelXDimension' => [ 2160 ],
etc., etc. ....
'ExifVersion' => [ '0210' ], },
'MAKERNOTE_DATA' => {},
'IFD0_DATA' =>
{ 'Model' => [ "KODAK DX3900 ZOOM DIGITAL CAMERA\000" ],
'ResolutionUnit' => [ 2 ],
etc., etc. ...
'YResolution' => [ 230, 1 ], },
'GPS_DATA' => {},
'INTEROP_DATA' =>
{ 'InteroperabilityVersion' => [ '0100' ],
'InteroperabilityIndex' => [ "R98\000" ], }, };
Setting $what equal to '*_DATA' returns a reference to a flat hash,
corresponding to one or more IFD (sub)dirs. For instance, 'IMAGE_DATA' is
a merge of 'IFD0_DATA' and 'SUBIFD_DATA': this interface is simpler for
the end-user, because there is only one dereference level; also, he/she
does not need to be aware of the partition of records related to the main
image into two IFDs. If the (sub)directory is not present or contains no
records, the returned hash exists and is empty. With reference to the
previous example:
my $hash_ref = $segment->get_Exif_data('IMAGE_DATA', 'TEXTUAL');
gives
$hash_ref = {
'ResolutionUnit' => [ 2 ],
'JPEGInterchangeFormatLength' => [ 3922 ],
'JPEGInterchangeFormat' => [ 2204 ],
'Orientation' => [ 1 ],
'XResolution' => [ 72, 1 ],
'Compression' => [ 6 ],
'YResolution' => [ 72, 1 ],
'ApertureValue' => [ 35, 10 ],
'PixelXDimension' => [ 2160 ],
etc., etc. ....
'ExifVersion' => [ '0210' ], };
Last, setting $what to 'THUMBNAIL' returns a reference to a copy of the
actual Exif thumbnail image (this is not included in the set returned by
'THUMB_DATA'); if there is no thumbnail, a reference to the empty string
is returned (the undefined value cannot be used, because it is assumed
that it corresponds to an error condition here). Note that the pointed
scalar may be quite large (~ 10^1 KB). If the thumbnail is in JPEG format
(this corresponds to the 'Compression' property, in IFD1, set to 6), you
can create another JPEG picture object from it, like in the following
example:
my $data_ref = $segment->get_Exif_data('THUMBNAIL');
my $thumb = new Image::MetaData::JPEG($data_ref);
print $thumb->get_description();
If you are only interested in reading Exif data in a standard
configuration, you can skip the segment-search calls and use directly
JPEG::get_Exif_data (a method of the JPEG class, so you only need a
JPEG structure object). This is an interface to the method with the same
name in the Segment class, acting on the first Exif APP1 Segment (if no
such segment is present, the undefined value is returned) and passing the
arguments through. Note that most JPEG files with Exif data contain at
most one Exif APP1 segment, so you are not going to loose anything here. A
snippet of code for visualising Exif data looks like this:
while (my ($d, $h) = each %{$image->get_Exif_data('ALL')}) {
while (my ($t, $a) = each %$h) {
printf '%-25s\t%-25s\t-> ', $d, $t;
s/([\000-\037\177-\377])/sprintf '\\%02x',ord($1)/ge,
$_ = (length $_ > 30) ? (substr($_,0,30) . ' ... ') : $_,
printf '%-5s', $_ for @$a; print "\n"; } }
How to modify your Exif data
- JPEG::Segment::set_Exif_data
- JPEG::set_Exif_data
- [arguments: "($data, $what, $action)"]
Similarly to the getter case, there is a set_Exif_data method
callable from a picture object, which does nothing more than looking for
the first Exif APP1 segment (creating it, if there is none) and invoke the
method with the same name in the Segment class, passing its arguments
through. So, the remaining of this section will concentrate on the Segment
method. The problem of setting a new thumbnail or erasing it is dealt with
in the last paragraphs of this section. (The APP1 Exif structure is quite
complicated, and the number of different possible cases when trying to
modify it is very large; therefore, designing a clean and intuitive
interface for this task is not trivial. Fell free to suggest improvements
and cleaner interfaces).
Exif records are usually characterised by a numeric key (a tag); this
was already discussed in the "getter" section. Since these keys,
for valid records, can be translated from numeric to textual form and
back, the end user has the freedom to use whichever form better fits his
needs. The two forms can even be mixed in the same "setter"
call: the method will take care to translate textual tags to numeric tags
when possible, and reject the others; then, it will proceed as if all tags
were numeric from the very beginning. Records with unknown textual or
numeric tags are always rejected.
The arguments to set_Exif_data are $data, $what and $action. The
$data argument must be a hash reference to a flat
hash, containing the key - record values pairs supplied by the user. The
"value" part of each hash element can be an array reference
(containing a list of values for the record, remember that some records
are multi-valued) or a single scalar (this is internally converted to a
reference to an array containing only the supplied scalar). If a record
value is supposed to be a null terminated string, the user can supply a
Perl scalar without the final null character (it will be inserted
automatically).
The $what argument must be a scalar, and it selects the
portion of the Exif APP1 segment concerned by the set_Exif_data
call. So, obviously, the end user can modify only one section at a time;
this is a simplification (for the developer of course) but also for the
end user, because trying to set all Exif-like values in one go would
require an offensively complicated data structure to specify the
destination of each record (note that some records in different sections
can have the same numerical tag, so a plain hash would not trivially
work). Valid values for $what are (MakerNote data are not currently
modifiable):
$what modifies ... $data type
--------------------------------------------------------------------
IMAGE_DATA as IFD0_DATA and SUBIFD_DATA ref. to flat hash
THUMB_DATA this is an alias for IFD1_DATA ref. to flat hash
THUMBNAIL the actual (un)compressed thumbnail ref. to scalar/object
ROOT_DATA header records (endianness) ref. to flat hash
IFD0_DATA primary image TIFF tags ref. to flat hash
SUBIFD_DATA Exif private tags ref. to flat hash
GPS_DATA GPS data of the primary image ref. to flat hash
INTEROP_DATA interoperability data in SubIFD ref. to flat hash
IFD1_DATA thumbnail-related TIFF tags ref. to flat hash
The $action argument controls whether the setter adds
($action = 'ADD') records to a given data directory or replaces ($action =
'REPLACE') them. In the first case, each user-supplied record replaces the
existing version of that record if present, and simply inserts the record
if it was not already present; however, existing records with no
counterpart in the user supplied $data hash remain untouched. In the
second case, the record directory is cleared before inserting user data.
Note that, since Exif and Exif-like records are non-repeatable in nature,
there is no need of an 'UPDATE' action, like for IPTC (see the IPTC
section).
The set_Exif_data routine first checks that the concerned segment is
of the appropriate type (Exif APP1), that $data is a hash reference (a
scalar reference for the thumbnail), and that $action and $what are valid.
If $action is undefined, it defaults to 'REPLACE'. Then, an appropriate
(sub)IFD is created, if absent, and all user-supplied records are checked
for consistency (have a look at the appendixes for this). Last, records
are set in increasing (numerical) tag order, and mandatory data are added,
if not present. The return value of the setter routine is always a hash
reference; in general it contains records rejected by the specialised
routines. If an error occurs in a very early stage of the setter, this
reference contains a single entry with key='ERROR' and value set to some
meaningful error message. So, returning a reference to an empty hash means
that everything was OK. An example, concerning the much popular task of
changing the DateTime record, follows:
$dt = '1994:07:23 12:14:51';
$hash = $image->set_Exif_data({'DateTime' => $dt}, 'IMAGE_DATA', 'ADD');
print "DateTime record rejected\n" if %$hash;
Depending on $what, some of the following notes apply:
- ROOT_DATA
- The only modifiable item is the 'Endianness' (and it can only be set to
big-endian, 'MM', or little-endian, 'II'); everything else is rejected
(see the APP1 structure for further details). This only influences how the
image is written back to disk (the in-memory representation is always
native).
- IMAGE_DATA
- By specifying this target one can address the IFD0_DATA and SUBIFD_DATA
targets at once. First, all records are tried in the IFD0, then, rejected
records are tried into SubIFD (then, they are definitively rejected).
- IFD0_DATA
- See the canonical, additional and company-assigned tags' sections in the
appendixes (this target refers to the primary image). The 'XResolution',
'YResolution', 'ResolutionUnit', and 'YCbCrPositioning' records are forced
if not present (to [1,72], [1,72], 2 and 1 respectively). Note that the
situation would be more complicated if we were dealing with uncompressed
(TIFF) primary images.
- SUBIFD_DATA
- See the private Exif section in the appendixes. The 'ExifVersion',
'ComponentsConfiguration', 'FlashpixVersion', 'ColorSpace', and
'Pixel[XY]Dimension' records are forced if not present (to '0220', '1230',
'0100', 1 and 0x0 respectively). Image dimensions can be retrieved from
the SOF segment with the JPEG structure object's method
get_dimensions() and set explicitly by the user if necessary (this
cannot be done from within the APP1 segment, because it does not link back
to its parent); however, the horizontal field in the SubIFD should not
include data padding, while that in the SOF segment does, so the meaning
is slightly different and these fields cannot be automatically
calculated.
- THUMB_DATA (or its alias IFD1_DATA)
- See the canonical, additional and company-related tag lists' sections in
the appendixes (this target refers to thumbnail properties). The
'XResolution', 'YResolution', 'ResolutionUnit', 'YCbCrSubSampling',
'PhotometricInterpretation' and 'PlanarConfiguration' records are forced
if not present (to [1,72], [1,72], 2, [2,1], 2 and 1 respectively). Note
that some of these records are not necessary for all types of thumbnails,
but JPEG readers will probably skip unnecessary information without
problems.
- GPS_DATA
- See the GPS tags section in the appendixes. The 'GPSVersionID' record is
forced, if it is not present at the end of the process, because it is
mandatory (ver 2.2 is chosen). There are some record inter-correlations
which are still neglected here (for instance, the 'GPSAltitude' record can
be inserted without providing the corresponding 'GPSAltitudeRef'
record).
- INTEROP_DATA
- JPEG::forge_interoperability_IFD
- [arguments: none] See the Interoperability directory section in the
appendixes. The 'InteroperabilityIndex' and 'InteroperabilityVersion'
records are forced, if they are not present at the end of the process,
because they are mandatory ('R98' and ver 1.0 are chosen). Note that an
Interoperability subIFD should be made as standard as possible: if you
just want to add it to the file, it is better to use the
forge_interoperability_IFD method, which takes care of all values
('RelatedImageFileFormat' is set to 'Exif JPEG Ver. 2.2', and the
dimensions are taken from get_dimensions()).
- MAKERNOTE_DATA
- See the appendix on MakerNotes for a detailed discussion on how the
content of a MakerNote is managed. If there is an error during the parsing
of the MakerNote, only those tags which could be fully decoded before the
error are returned. Note that MakerNote structures are often partially
known, so many tags will likely be translated as 'Unknown_tag_...'.
MakerNotes cannot be currently modified.
- THUMBNAIL
- $data must be a reference to a scalar containing the new thumbnail or to a
valid Image::MetaData::JPEG object; if it points to an empty string, the
thumbnail is erased (the undefined value DOES NOT erase the thumbnail, it
generates instead an error). All thumbnail specific records (see the
canonical tags section) are removed, and only those corresponding to the
newly inserted thumbnail are calculated and written back. Currently, it is
not possible to insert an uncompressed thumbnail (this will probably
happen in the form of a TIFF image), only JPEG ones are accepted
(automatic records contain the type, length and offset). The following
code shows how to set and delete a thumbnail.
my $image = new Image::MetaData::JPEG('original_image.jpg');
my $thumb = new Image::MetaData::JPEG('some_thumbnail.jpg');
$image->set_Exif_data($thumb, 'THUMBNAIL');
$image->save('modified_image.jpg');
$image->set_Exif_data(\ '', 'THUMBNAIL');
$image->save('thumbless_image.jpg');
XMP data ("APP1" segments)¶
XMP (eXtensible Metadata Platform) is a technology, conceived by Adobe Systems,
to tag graphic files with metadata, and to manage them during a lifetime made
of multiple processing steps. Its serialisation (the actual way metadata are
saved in the file) is based on RDF (Resource Description Framework)
implemented as an application of XML. Its flexibility allows to accomodate
existing, future and private metadata schemas. In a JPEG file, XMP information
is included alongside Exif and IPTC data, and is stored in an APP1 segment on
its own starting with the XMP namespace URI and followed by the actual XMP
packet (see XMP APP1 segment structure for more details).
XMP was introduced in 2001 as part of Adobe Acrobat version 5.01. Adobe has a
trademark on XMP, and retains control over its specification. Source code for
the XMP software-development kit was released by Adobe, but with a custom
license, whose compatibility with the GNU public license and open-source
nature altogether is questioned.
Photoshop and IPTC data ("APP13" segments)¶
Adobe's Photoshop program, a de-facto standard for image manipulation, has,
since long, used the APP13 segment for storing non-graphical information, such
as layers, paths, ecc..., including editorial information modelled on
IPTC/NAA recommendations. This module provides a number of methods for
managing Photoshop/IPTC data without dealing with the details of the low level
representation (although sometimes this means taking some decisions for the
end user ....). The structure of the IPTC data block(s) is managed in detail
and separately from the rest, although this block is a sort of
"sub-case" of Photoshop information. The interface is intentionally
similar to that for Exif data.
All public methods have a
$what argument selecting which
part of the APP13 segment you are working with. The default is 'IPTC'. If
$what is invalid, an exception is always raised. The kind of information you
can access with different values of $what is explained in the following (have
a look at the appendices about valid Photoshop-style and IPTC tags for further
details):
$what: Concerned pieces of information:
----------- --------------------------------
'IPTC' or Editorial information like caption, abstract, author,
'IPTC_2' copyright notice, byline, shot site, user defined keywords,
and many more; in practise, all what is covered by the IPTC
Application Record 2. This is the most common option; the
default value of $what, 'IPTC', is a synonym for 'IPTC_2'
for backward compatibility (NOT a merge of 'IPTC_1/2').
'IPTC_1' This refers to more obscure pieces of information, contained
in the IPTC Envelope Record 1. One is rarely interested by
this, exception made for the "Coded Character Set" tag,
which is necessary to define a character set different
from ASCII (i.e., when you don't write or read in English).
'PHOTOSHOP' Alpha channels, colour information, transfer functions,
or 'PS_8BIM' and many other details concerning the visual rendering of
or 'PS_8BPS' the picture. These fields are most often only modified by
or 'PS_PHUT' an image manipulation program, and not directly by the user.
Recent versions of Photoshop (>= 4.0) use a resource data
block type equal to '8BIM', and this is the default in
this module (so, 'PHOTOSHOP' and 'PS_8BIM' are synonyms).
However, some other older or undocumented resource data
block types are also allowed.
- JPEG::retrieve_app13_segment
- [arguments: "($index, $what)"] In order to work on
Photoshop/IPTC data, a suitable Photoshop-style APP13 Segment must first
be selected. The retrieve_app13_segment method returns a reference
to the $index-th Segment (the first Segment if the
$index is undefined) which contains information matching the $what
argument. If such Segment does not exist, the method returns the undefined
reference. If $index is (-1), the routine returns the number of available
suitable APP13 Segments (which is non negative). Beware, the meaning of
$index is influenced by the value of $what.
my $num_IPTC = $file->retrieve_app13_segment(-1, 'IPTC');
my $ref_IPTC = $file->retrieve_app13_segment($num - 1, 'IPTC');
- JPEG::provide_app13_segment
- [arguments: "($what)"] If you want to be sure to have an APP13
Segment suitable for the kind of information you want to write, use the
provide_app13_segment method instead, which forces the Segment to
be present in the file, and returns its reference. If at least one segment
matching $what is already present, the first one is returned. Otherwise,
the first Photoshop-like APP13 is adapted by inserting an appropriate
subdirectory record (update is called automatically). If no such segment
exists, it is first created and inserted (the "Photoshop
3.0\000" identifier is used). Note that there is no $index argument
here.
my $ref_Photoshop = $file->provide_app13_segment('PHOTOSHOP');
- JPEG::remove_app13_info
- [arguments: "($index, $what)"] If you want to remove all traces
of some flavour of APP13 information from the $index-th APP13
Photoshop-style Segment, use the remove_app13_info method with
$what set to the appropriate value. If, after this, the segment is empty,
it is eliminated from the list of segments in the file. If $index is (-1),
all APP13 Segments are affected at once. Beware, the meaning of $index is
influenced by the value of $what.
$file->remove_app13_info(3, 'PHOTOSHOP');
$file->remove_app13_info(-1, 'IPTC');
$file->remove_app13_info(0, 'IPTC_1');
How to inspect and modify your IPTC data
- JPEG::Segment::get_app13_data
- [arguments: "($type, $what)"]
Once you have a Segment reference pointing to your favourite IPTC-enabled
APP13 Segment, you may want to have a look at the records it contains. Use
the get_app13_data method for this: its behaviour is controlled by
the $type and $what argument
(here, $what is 'IPTC_1' or 'IPTC_2' alias 'IPTC', of course). It returns
a reference to a hash containing a copy of the list of the appropriate
IPTC records, if present, undef otherwise: each element of the hash is a
pair (key, arrayref), where arrayref points to an array with the real
values (some IPTC records are repeatable so multiple values are possible).
The record keys can be the native numeric keys ($type eq 'NUMERIC') or
translated textual keys ($type eq 'TEXTUAL', default); in any case, the
record values are untranslated. If a numeric key stored in the JPEG file
is unknown, and a textual translation is requested, the name of the key
becomes "Unknown_tag_$tag". Note that there is no check on the
validity of IPTC records' values: their format is not checked and one or
multiple values can be attached to a single tag independently of its
repeatability. This is, in some sense, consistent with the fact that also
"unknown" tags are included in the output. If $type or $what is
invalid, an exception is thrown out. An example of how to extract and
display IPTC data is given here:
my $hash_ref = $segment->get_app13_data('TEXTUAL', 'IPTC');
while (my ($key, $vals) = each %$hash_ref) {
printf "# %20s =", $key; print " '$_'" for @$vals; print "\n"; }
### This could print:
# DateCreated = '19890207'
# ByLine = 'Interesting picture' 'really'
# Category = 'POL'
# Keywords = 'key-1' 'key-2' 'key-99'
# OriginatingProgram = 'Mapivi'
- JPEG::Segment::set_app13_data
- [arguments: "($data, $action, $what)"] The hash returned by
get_app13_data can be edited and reinserted with the set_app13_data
method, whose arguments are $data,
$action and, as usual, $what.
If $action or $what is invalid, an exception is generated. This method
accepts IPTC data in various formats and updates the corresponding
subdirectory in the segment. The key type of each entry in the input hash
can be numeric or textual, independently of the others (the same key can
appear in both forms, the corresponding values will be put together). The
value of each entry can be an array reference or a scalar (you can use
this as a shortcut for value arrays with only one value). The $action
argument can be:
- ADD : new records are added and nothing is deleted; however, if you
try to add a non-repeatable record which is already present,
the newly supplied value ejects (replaces) the pre-existing value.
- UPDATE : new records replace those characterised by the same tags,
but the others are preserved. This makes it possible to modify
some repeatable IPTC records without deleting the other tags.
- REPLACE : all records present in the IPTC subdirectory are deleted
before inserting the new ones (this is the default action).
If, after implementing the changes required by $action, any mandatory
dataset (according to the IPTC standard), is still undefined, it is added
automatically. This often concerns version datasets, with numeric index 0.
The return value is a reference to a hash containing the rejected key-values
entries. The entries of %$data are not modified. An entry in the %$data
hash can be rejected for various reasons (you might want to have a look at
appendix about valid IPTC tags for further information): a) the tag is
undefined or not known; b) the entry value is undefined or points to an
empty array; c) the non-repeatability constraint is violated; d) the tag
is marked as invalid; e) a value is undefined f) the length of a value is
invalid; g) a value does not match its mandatory regular expression.
$segment->set_app13_data($additional_data, 'ADD', 'IPTC');
A snippet of code for changing IPTC data looks like this:
my $segment = $file->provide_app13_segment('IPTC');
my $hashref_1 = { CodedCharacterSet => "\033\045G" }; # UTF-8
my $hashref_2 = { ObjectName => 'prova',
ByLine => 'ciao',
Keywords => [ 'donald', 'duck' ],
SupplementalCategory => ['arte', 'scienza', 'diporto'] };
$segment->set_app13_data($hashref_2, 'REPLACE', 'IPTC');
$segment->provide_app13_subdir('IPTC_1');
$segment->set_app13_data($hashref_1, 'ADD', 'IPTC_1');
- JPEG::get_app13_data
- [arguments: "($type, $what)"] If you are only interested in
reading IPTC data in a standard configuration, you can skip most of
the previous calls and use directly JPEG::get_app13_data (a method
in the JPEG class, so you only need a JPEG structure object). This is an
interface to the method with the same name in the Segment class, acting on
the first relevant APP13 Segment (if no such segment is present, the
undefined value is returned) and passing the arguments through. Note that
most JPEG files with Photoshop/IPTC data contain at most one APP13
segment, so you are not going to "loose" anything here. A
snippet of code for visualising IPTC data looks like this:
my $hashref = $file->get_app13_data('TEXTUAL', 'IPTC');
while (my ($tag, $val_arrayref) = each %$hashref) {
printf '%25s --> ', $tag;
print "$_ " for @$val_arrayref; print "\n"; }
- JPEG::set_app13_data
- [arguments: "($data, $action, $what)"] There is, of course, a
symmetric JPEG::set_app13_data method, which writes data to the
JPEG object without asking the user to bother about Segments: it uses the
first available suitable Segment; if this is not possible, a new Segment
is created and initialised (because the method uses
"JPEG::provide_app13_segment" internally, and not
"JPEG::retrieve_app13_segment" as
"JPEG::get_app13_data" does).
$file->set_app13_data($hashref, 'UPDATE', 'IPTC');
How to inspect and modify your Photoshop data
The procedure of inspecting and modifying Photoshop data (i.e., non-IPTC data in
a Photoshop-style APP13 segment) is analogous to that for IPTC data, but with
$what set to 'PHOTOSHOP' (alias 'PS_8BIM'), or to the
seldom used 'PS_8BPS' and 'PS_PHUT'. The whole description will not be
repeated here, have a look at the IPTC section for it: this section takes only
care to point out differences. If you are not acquainted with the structure of
an APP13 segment and its terminology (e.g., "resource data block"),
have a look at the Photoshop-style tags' section.
About get_app13_data, it should only be pointed out that resource block names
are appended to the list of values for each tag (even if they are undefined),
so the list length is alway even. Things are more complicated for
set_app13_data: non-IPTC Photoshop specifications are less uniform than IPTC
ones, and checking the correctness of user supplied data would be an
enumerative task. Currently, this module does not perform any syntax check on
non-IPTC data, but this could change in the future (any contribution is
welcome); only tags (or, how they are called in this case, "resource
block identifiers") are checked for being in the allowed tags list (see
the Photoshop-style tags' table for details). The IPTC/NAA tag is of course
rejected: IPTC data must be inserted with $what set to 'IPTC' or its siblings.
Although not explicitly stated, it seems that non-IPTC Photoshop tags are
non-repeatable (let me know if not so), so two resource blocks with the same
tag shouldn't exist. For this reason, the 'UPDATE' action is changed
internally to 'ADD'. Moreover, since the resource block structure is not
explored, all resource blocks are treated as single-valued and the value type
is $UNDEF. So, in the user-supplied data hash, if a tag key returns a data
array reference, only the first element (which cannot be undefined) of the
array is used as resource block value: if a second element is present, it is
used as resource block name (which is otherwise set to the null string).
Suppling more than two elements is an error and causes the record to be
rejected.
my $segment = $file->provide_app13_segment('PHOTOSHOP');
my $hashref = {
GlobalAngle => pack('N', 0x1e),
GlobalAltitude => pack('N', 0x1e),
CopyrightFlag => "\001",
IDsBaseValue => [ pack('N', 1), 'Layer ID Generator Base' ] };
$segment->set_app13_data($hashref, 'ADD', 'PHOTOSHOP');
NOTES¶
On the subject of year specification in a date¶
There are currently eight fields whose purpose is to store a
date in a
JPEG picture, namely 'DateTime', 'DateTimeOriginal' and 'DateTimeDigitized'
(in IFD0/1 or SubIFD), 'GPSDateStamp' (in the GPS section), and 'ReleaseDate',
'ExpirationDate', 'DateCreated' and 'DigitalCreationDate' (in the IPTC
section). Most of these dates refer to some electronic treatment of images, a
kind of process which was not available before the late twentieth century. Two
of them refer to release and expiration dates in the IPTC standard, and should
therefore not be set to a date before the introduction of the standard itself.
However, there exist users who want to use some of these fields in a
non-conventional way to refer to dates when analog photography but not digital
photography was available. For this reason, all tags (but one) can be written
with a year starting from 1800 (and not from 1900 as in earlier releases).
Users are however advised to check the "specifications" for these
tags before setting the date and take responsibility for their
non-conventionality.
There is one notable exception to the previous considerations, that is the IPTC
'DateCreated' dataset, which should explicitly refer to the creation date of
the object represented in the picture, which can be many centuries in the
past. For this dataset a special regular expression is provided which allows a
date in the full ISO-8601 YYYY-MM-DD format (however, it should be noted that
even ISO-8601 does not allow a date before 0AD, so not all masterworks from
ancient Greece can be tagged in this way ... let me know if I am wrong). I am,
of course, still open to suggestions and reconsiderations on this subject.
On the problem of MakerNote corruption and ways to overcome it¶
A widespread problem with Exif
maker notes is that there is no common
standard for how to parse and rewrite the information in the
MakerNote
data area. This is the reason why most programs dealing with Exif JPEG files
corrupt the MakerNote on saving, or decide to drop it altogether (be aware
that there existed programs known to hang when they try to read a corrupt
maker note).
In fact, many maker notes contain a non-standard
IFD structure, with some
tags storing file offsets (see the documentation page describing the IFD
structure). Therefore, saving a maker note without regard for internal
offsets' adjustment reduces the note mostly to garbage. Re-dumping a maker
note after changing the Exif APP1 segment endianness incurs the same problem,
because no internal byte-swap is performed.
A few countermeasures have been introduced in this package to try to cure some
maker note problems. The first one concerns the correct byte order (the
endianness, which is not always the same used in the Exif segment), which
needs not to be known in advance; it is in fact determined by using the fact
that, if the note is IFD-like (even non-standard), the number of tags is
always in the range [1,255], so the two-bytes tag count has always the most
significant byte set to zero, and the least significant byte set to non-zero.
There is also a prediction and correction mechanism for the offsets in the
interoperability arrays, based on the simple assumption that the absolute
value of offsets can be wrong, but their differences are always right, so, if
one can get the first one right ... a good bet is the address of the byte
immediately following the next_IFD link (or the tag list, if this link is
absent). If the parsing process does not end successfully, this mechanism is
enabled and its "corrected" findings are stored instead of the
original ones if it is able to cure the problems (i.e., if the second try at
parsing the note is successful).
CURRENT STATUS¶
A lot of other routines for modifying other meta-data could be added in the
future. The following is a list of the current status of various meta-data
Segments (only APP and COM Segments).
Segment Possible content Status
* COM User comments parse/read/write
* APP0 JFIF data (+ thumbnail) parse/read
* APP1 Exif or XMP data parse/read[Exif]/write[Exif]
* APP1 Maker notes parse/read
* APP2 FPXR data or ICC profiles parse
* APP3 additional Exif-like data parse
* APP4 HPSC nothing
* APP12 PreExif ASCII meta parse
* APP13 IPTC and PhotoShop data parse/read/write
* APP14 Adobe tags parse
KNOWN BUGS¶
USE WITH CAUTION! THIS IS EXPERIMENTAL SOFTWARE!
This module is still
experimental, and not yet finished. In particular,
it is far from being well tested, and some interfaces could change depending
on user feedback. The ability to modify
maker notes is not yet
implemented (moreover, have a look at the MakerNote appendix for a general
note on the problem of MakerNote corruption). APP13 data spanning
multiple
Segments are not correctly read/written. Most of APP12 Segments do not fit
the structure parsed by
parse_app12(), probably there is some standard
I don't know.
OTHER PACKAGES¶
Other packages are available in the free software arena, with a feature set
showing a large overlap with that found in this package; a probably incomplete
list follows. However, none of them is (or was) completely satisfactory with
respect to the package's objectives, which are: being a single package dealing
with all types of meta-information in read/write mode in a JPEG (and possibly
TIFF) file; depending on the least possible number of non standard packages
and/or external programs or libraries; being open-source and written in Perl.
Of course, most of these objectives are far from being reached ....
- "Image::ExifTool" by Phil Harvey
- ExifTool is a Perl module with an included command-line application
for reading and writing meta information in image files. It recognizes
EXIF, GPS, IPTC, XMP, JFIF, GeoTIFF, ICC Profile, Photoshop IRB and ID3
meta information as well as the maker notes of many digital cameras
including Canon, Casio, FujiFilm, Kodak, Leaf, Minolta/Konica-Minolta,
Nikon, Olympus/Epson, Panasonic/Leica, Pentax/Asahi, Ricoh, Sanyo and
Sigma/Foveon. It was started as a highly customisable, read-only report
tool, capable of organising the results in various ways. Since version
4.10 (beginning of 2005) it added the ability to modify and rewrite JPEG
tags. So sad there are now two projects with such a large overlap.
- "Image::IPTCInfo" by Josh Carter
- This is a CPAN module for for extracting IPTC image meta-data. It allows
reading IPTC data (there is an XML and also an HTML output feature) and
manipulating them through native Perl structures. This library does not
implement a full parsing of the JPEG file, so I did not consider it as a
good base for the development of a full-featured module. Moreover, I don't
like the separate treatment of keywords and supplemental categories.
- "JPEG::JFIF" by Marcin Krzyzanowski, "Image::EXIF" by
Sergey Prozhogin and "exiftags" by Eric M. Johnston
- JPEG::JFIF is a very small CPAN module for reading meta-data in
JFIF/JPEG format files. In practice, it only recognises a subset of the
IPTC tags in APP13, and the parsing code is not suitable for being reused
for a generic JPEG segment. Image::Exif is just a Perl wrapper
around exiftags, which is a program parsing the APP1 section in
JPEG files for Exif meta-data (it supports a variety of MakerNotes).
exiftags can also rewrite comments and date and time tags.
- "Image::Info" by Gisle Aas
- This CPAN module extracts meta information from a variety of graphic
formats (including JPEG and TIFF). So, it is not specifically about JPEG
segments: reported information includes media type, extension, width,
height, colour type, comments, Interlace, Compression, Gamma, and
LastModificationTime. For JPEG files, it additionally reports from JFIF
(APP0) and Exif (APP1) segments (including MakerNotes). This module does
not allow for editing.
- "exif" by Martin Krzywinski and "exifdump.py" by
Thierry Bousch
- These are two basic scripts to extract Exif information from JPEGs. The
first script is written in Perl and targets Canon pictures. The second one
is written in Python, and it only works on JPEG files beginning with an
APP1 section after the SOI. So, they are much simpler than all other
programs/libraries described here. Of course, they cannot modify Exif
data.
- "jhead" by Matthias Wandel
- The jhead program (written in C) is used to display JPEG comments and Exif
data, and to perform limited manipulation of Exif headers (such as
changing the internal time-stamps, removing the thumbnail, or transferring
headers back into edited images) and comments. Exif header data
modification is very limited, as jhead's internal implementation of the
file system contained in the Exif header is read-only; there, for
instance, no way to replace the thumbnail in the Exif header with
another.
- "exifprobe" by Duane H. Hesser
- This is a C program which examines and reports the contents and structure
of JPEG and TIFF image files. It recognises all standard JPEG markers and
reports the contents of any properly structured TIFF IFD encountered, even
when entry tags are not recognised. Camera MakerNotes are included. GPS
and GeoTIFF tags are recognised and entries printed in "raw"
form, but are not expanded. The output is nicely formatted, with
indentation and colouration; this program is a great tool for inspecting a
JPEG/TIFF structure while debugging.
- "libexif" by Lutz Mueller
- This is a library, written in C, for parsing, editing, and saving Exif
data. All Exif tags described in Exif standard 2.1 are supported. Libexif
can only handle some maker notes, and even those not very well. It is used
by a number of front-ends, including: Exif (read-only command-line
utility), gexif (a GTK+ front-end for editing Exif data), gphoto2
(command-line front-end to libgphoto2, a library to access digital
cameras), gtkam (a GTK+ front-end to libgphoto2), thirdeye (a digital
photos organiser and driver for eComStation).
- "jpegrdf" by Norman Walsh
- This is a Java application for manipulating (read/write) RDF meta-data in
the comment sections of JPEG images (is this the same thing which can be
found in APP1 segments in XMP format?). It can also access and convert
into RDF the Exif tags and a few other general properties. However, I
don't want to rely on a Java environment being installed in order to be
able to access these properties.
- "OpenExif" by Eastman Kodak Company
- This is an object-oriented interface written in C++ to Exif formatted JPEG
image files. It is very complete and sponsored by a large company, so it
is to be considered a sort of reference. The toolkit allows creating,
reading, and modifying the meta-data in the Exif file. It also provides
means of getting and setting the main image and the thumbnail image.
OpenExif is also extensible, and Application segments can be added.
AUTHOR¶
Stefano Bettelli,
bettelli@cpan.org
COPYRIGHT AND LICENSE¶
Copyright (C) 2004,2005,2006 by Stefano Bettelli
This library is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License. See the COPYING and LICENSE file
for the license terms.
SEE ALSO¶
Have a look at the technical appendixes of the
Image::MetaData::JPEG
module [
M in the following], packaged as separate documents: they
contain a description of segment structures [
M::Structures], and lists
of valid tags [
M::TagLists], including a tentative description of some
MakerNote formats [M::MakerNotes]. See also your current
perl(1)
documentation, an explanation for the General Public License and the manual
pages of the following optional Perl modules:
Image::ExifTool(3pm),
Image::IPTCInfo(3pm),
JPEG::JFIF(3pm),
Image::EXIF(3pm)
and
Image::Info(3pm).