NAME¶
c44 - DjVuPhoto encode.
SYNOPSIS¶
c44 [options] inputfilename
[outputfilename]
DESCRIPTION¶
Produces a DjVuPhoto encoded image. The input image file
inputfilename
can be either a portable gray-map (
PGM) or a portable pix-map
(
PPM). Input images compressed with
JPEG are
also accepted. It is however suggested to only use high quality
JPEG files (low compression ratio, large size) because the
wavelet compression will increase the defects already present in highly
compressed
JPEG files.
The program produces a DjVuPhoto file
outputfilename. If the output file
name is not specified, a default file name will be generated by replacing the
input file name suffix by suffix
djvu.
The main design objective for the DjVu wavelets consisted of allowing
progressive rendering and smooth scrolling of large images with limited memory
requirements. Decoding functions process the compressed data and update a
memory efficient representation of the wavelet coefficients. Imaging function
then can quickly render an arbitrary segment of the image using the available
data. Both process can be carried out in two threads of execution. This design
plays an important role in the DjVu system. We investigated various
state-of-the-art wavelet compression schemes. Although these schemes may
achieve slightly smaller file sizes, the decoding functions did not even
approach our requirements. The
IW44 wavelets reach these
requirements today and may in the future implement more modern refinements if
these refinements can be implemented within our constraints.
QUALITY SELECTION OPTIONS¶
DjVuPhoto files are logically composed of a sequence of "slices"
containing successive image refinements. Slices are grouped in
"chunks" defining the progressive rendering sequence. The viewer is
able to display an intermediate image after processing each chunk. A typical
DjVuPhoto files contains 80 to 120 slices grouped into 1 to 4 chunks.
The quality selection options provide various ways to specify the number of
chunks and the number of slices per chunk. The
c44 program adds slices
to the current chunk until exceeding a target number of slices, a target file
size, or a target quality specification. The following options define targets
for each chunk. The option argument contain several numerical values (one per
chunk) separated by either commas or pluses.
- -slice n+...+n
- Specify the number of slices in each chunk. The option argument contains
plus-separated numerical values (one per chunk) indicating the number of
slices per chunk. Option -slice 74+13+10, for instance, would be
appropriate for compressing a photographic image with three progressive
refinements. More quality and more refinements can be obtained with option
-slice 72+11+10+10.
- -slice n,...,n
- Specify the cumulative number of slices for each chunk. Since the final
quality is determined by the total number of slices, it is often more
convenient to use comma-separated values (one per chunk) indicating the
cumulative number of slices for each chunk (i.e. including those encoded
in all previous chunks). The values suggested above can also be expressed
as -slice 74,87,97 and -slice 72,83,93,103.
- -size n,...,n
- Specify size targets for each chunk expressed in bytes. The option
argument can be either a plus-separated list specifying a size for each
chunk, or a comma separated list specifying cumulative sizes for each
chunk and all previous chunks. Size targets are approximates. Slices will
be added to each chunk until exceeding the specified target.
- -bpp n,...,n
- Specify size targets for each chunk expressed in bits-per-pixel. Both
comma-separated and plus-separated specifications are accepted. Option
-bpp 0.25,0.5,1 usually provides good results.
- -percent n,...,n
- Specify size targets for each chunk expressed as a percentage of the input
file size. Both comma-separated and plus-separated specifications are
accepted. Results can be drastically different according to the format of
the input image (raw or JPEG compressed).
- -decibel n,...,n
- Specify quality targets for each chunk expressed as a comma-separated list
of increasing decibel values. Decibel values range from 16 (very low
quality) to 48 (very high quality). This criterion should not be relied
upon when re-encoding an image previously compressed by another
compression scheme. Selecting this option significantly increases the
compression time.
- -dbfrac frac
- Indicate that the decibel values specified in option -decibel
should be computed by averaging the mean squared errors of only the
fraction frac of the most mis-represented blocks of 32 x 32 pixels.
This option is useful with composite images containing solid color
features (e.g. an image with a large white border).
Providing no quality specification options automatically selects a default
quality specification
-slice 74,89,99. Multiple quality specification
options are allowed. The program outputs a file whose total number of chunks
is the largest number of chunks of all quality specifications. Slices are
added to each chunk until reaching any of the quality target for this chunk.
OTHER OPTIONS¶
The following additional options are supported:
- -dpi n
- Specify the resolution information encoded into the output file expressed
in dots per inch. The resolution information encoded in DjVu files
determine how the decoder scales the image on a particular display.
Meaningful resolutions range from 25 to 1200. The default value, 100 dpi,
should be suitable for most photographic images.
- -gamma n
- Specify the gamma correction information encoded into the output file. The
argument n specified the gamma value of the device for which the
input image was designed. The default value is 2.2. This is appropriate
for images designed for a standard computer monitor.
- -mask pbmfilename
- The design of the IW44 wavelets allows for compressing
partially masked images. This option can be used when certain pixels of a
background image are going to be covered by foreground objects like text
or drawings. File pbmfile must be a PBM file whose
size matches the size of the input file. Each black pixel in
pbmfile means that the value of the corresponding pixel in the
input file is irrelevant. The IW44 encoder will replace the
masked pixels by a color value whose coding cost is minimal (see
http://www.djvuzone.org/djvu/techpapers/mask/index.djvu for
technical details.)
- -crcbnormal
- Select normal chrominance encoding. Chrominance information is encoded at
the same resolution as the luminance. This is the default.
- -crcbhalf
- Selects half resolution chrominance encoding. Chrominance information is
encoded at half the luminance resolution.
- -crcbdelay n
- This option can be used with -crcbnormal and -crcbhalf to
modify the quality of the chrominance information. The option arguments
specifies a parameter n, expressed in slices, that reduces the
bit-rate associated with the chrominance. The default chrominance encoding
delay is 10 slices.
- -crcbfull
- Select the highest possible quality for encoding the chrominance
information. This is equivalent to specifying -crcbnormal and
-crcbdelay 0.
- -crcbnone
- Disable the encoding of the chrominance. Only the luminance information
will be encoded. The resulting image will show in shades of gray.
The default quality setting of the DjVuLibre version of
c44 has been
increased. It produces larger files with a better quality. Quality can be
lowered using the quality selection options!
BUGS¶
The encoder requires more memory than necessary.
The rechunking capability is currently broken.
CREDITS¶
This program was written by Léon Bottou
<leonb@users.sourceforge.net> and was then improved by Andrei Erofeev
<andrew_erofeev@yahoo.com>, Bill Riemers <docbill@sourceforge.net>
and many others.
SEE ALSO¶
djvu(1),
pnm(5),
cjpeg(1).