NAME¶
Imager::interface.pod - describes the C level virtual image interface
SYNOPSIS¶
DESCRIPTION¶
The Imager virtual interface aims to allow image types to be created for special
purposes, both to allow consistent access to images with different sample
sizes, and organizations, but also to allow creation of synthesized or virtual
images.
This is a C level interface rather than Perl.
Existing Images¶
As of this writing we have the following concrete image types:
- •
- 8-bit/sample direct images
- •
- 16-bit/sample direct images
- •
- double/sample direct images
- •
- 8-bit/sample 8-bit/index paletted images
Currently there is only one virtual image type:
- •
- masked images, where a mask image can control write access
to an underlying image.
Other possible concrete images include:
- •
- "bitmaps", 1 bit/sample images (perhaps limited
to a single channel)
- •
- 16-bit/index paletted images
Some other possible virtual images:
- •
- image alpha combining, where the combining function can be
specified (see the layer modes in graphical editors like the GIMP or
Photoshop.
THE INTERFACE¶
Each image type needs to define a number of functions which implement the image
operations.
The image structure includes information describes the image, which can be used
to determine the structure of the image:
- •
- "channels" - the number of samples kept for each
pixel in the image. For paletted images the samples are kept for each
entry in the palette.
- •
- "xsize", "ysize" - the dimensions of
the image in pixels.
- •
- "bytes" - the number of bytes of data kept for
the image. Zero for virtual images. Does not include the space required
for the palette for paletted images.
- •
- "ch_mask" - controls which samples will be
written to for direct images.
- •
- "bits" - the number of bits kept for each sample.
There are enum values i_8_bits, i_16_bits and i_double_bits (64).
- •
- "type" - the type of image, either i_direct_type
or i_palette_type. Direct images keep the samples for every pixel image,
while i_palette_type images keep an index into a color table for each
pixel.
- •
- "virtual" - whether the image keeps any pixel
data. If this is non-zero then "idata" points to image data,
otherwise it points to implementation defined data, though
"ext_data" is more likely to be used for that.
- •
- "idata" - image data. If the image is 8-bit
direct, non-virtual, then this consists of each sample of the image stored
one after another, otherwise it is implementation defined.
- •
- "tags" - will be used to store meta-data for an
image, eg. tags from a TIFF file, or animation information from a GIF
file. This should be initialized with a call to i_tags_new() in
your image constructor if creating a new image type.
- •
- "ext_data" - for internal use of image types.
This is not released by the standard i_img_exorcise() function. If
you create a new image type and want to store a pointer to allocated
memory here you should point i_f_destroy at a function that will release
the data.
If a caller has no knowledge of the internal format of an image, the caller must
call the appropriate image function pointer. Imager provides macros that wrap
these functions, so it isn't necessary to call them directly.
Many functions have a similar function with an 'f' suffix, these take or return
samples specified with floating point values rather than 8-bit integers
(unsigned char). Floating point samples are returned in the range 0 to 1
inclusive.
- i_f_ppix(im,x,y,color)
- i_f_ppixf(im,x,y,fcolor)
- stores the specified color at pixel (x,y) in the image. If
the pixel can be stored return 0, otherwise -1. An image type may choose
to return 0 under some circumstances, eg. writing to a masked area of an
image. The "color" or "fcolor" always contains the
actual samples to be written, rather than a palette index.
- i_f_plin(im,l,r,y,colors)
- i_f_plinf(im,l,r,y,fcolors)
- stores (r-l) pixels at positions (l,y) ... (r-1, y) from
the array specified by "colors" (or "fcolors").
Returns the number of pixels written to. If l is negative it will return
0. If "r > im->xsize" then only "(im->xsize -
l)" will be written.
- i_f_gpix(im,x,y,color)
- i_f_gpixf(im,x,y,fcolor)
- retrieves a single pixel from position (x,y). This returns
the samples rather than the index for paletted images.
- i_f_glin(im,l,r,y,colors)
- i_f_glinf(im,l,r,y,fcolors)
- retrieves (r-l) pixels from positions (l, y) through (r-1,
y) into the array specified by colors. Returns the number of pixels
retrieved. If l < 0 no pixels are retrieved. If "r >
im->xsize" then pixels "(l, y)" ...
"(im->xsize-1, y)" are retrieved. Retrieves the samples
rather than the color indexes for paletted images.
- i_f_gsamp(im,l,r,y,samples,chans,chan_count)
- i_f_gsampf(im,l,r,y,fsamples,chans,chan_count)
- Retrieves samples from channels specified by
"chans" (for length "chan_count") from pixels at
positions (l,y) ... (r-1, y). If "chans" is NULL then samples
from channels 0 ... "chan_count-1" will be retrieved. Returns
the number of sample retrieved ( not the number of channels). If a
channel in "chans" is not present in the image or l < 0,
returns 0. If "r > im->xsize", then the samples from
"(l,y)" ... "(im->xsize-1, y)" are returned.
The following are for images where type == i_palette_type only.
- i_f_gpal(im,l,r,y,vals)
- Retrieves color indexes from the image for pixels (l, y)
... (r-1, y) into "vals". Returns the number of indexes
retrieved.
- i_f_ppal(im,l,r,y,vals)
- Stores color indexes into the image for pixels (l, y) ...
(r-1, y) from "vals". Returns the number of indexes retrieved.
If indexes are outside the range of the images palette, then you may have
problems reading those pixels with i_gpix() or
i_glin().
- i_f_addcolors(im,colors,count)
- Adds the count colors to the image's palette. Returns the
index of the first color added, or -1 if there is not enough space for
count colors.
- i_f_getcolors(im,index,colors,count)
- Retrieves count colors from the image's palette starting
from entry index in the palette. Returns non-zero on success.
- i_f_colorcount(im)
- Returns the number of colors in the image's palette.
Returns -1 if this is not a paletted image.
- i_f_maxcolors(im)
- Returns the maximum number of colors that can fit in the
image's palette. Returns -1 if this is not a paletted image.
- i_f_findcolor(im,color,entry)
- Searches the image's palette for the specified color,
setting *entry to the index and returning non-zero. Returns zero if the
color is not found.
- i_f_setcolors_t(im,index,colors,count)
- Sets count colors starting from index in the image from the
array colors. The colors to be set must already have entries in the
image's palette. Returns non-zero on success.
Finally, the i_f_destroy function pointer can be set which is called when the
image is destroyed. This can be used to release memory pointed to by ext_data
or release any other resources.
When writing to a paletted image with
i_ppix() or
i_plin() and the
color you are writing doesn't exist in the image, then it's possible that the
image will be internally converted to a direct image with the same number of
channels.
Several functions have been written to simplify creating new image types.
These tools are available by including
imagei.h.
Floating point wrappers¶
These functions implement the floating point sample versions of each interface
function in terms of the integer sample version.
These are:
- i_ppixf_fp
- i_gpixf_fp
- i_plinf_fp
- i_glinf_fp
- i_gsampf_fp
Forwarding functions¶
These functions are used in virtual images where the call should simply be
forwarded to the underlying image. The underlying image is assumed to be the
first pointer in a structure pointed at by ext_data.
If this is not the case then these functions will just crash :)
- i_addcolors_forward
- i_getcolors_forward
- i_colorcount_forward
- i_maxcolors_forward
- i_findcolor_forward
- i_setcolors_forward
Sample macros¶
"Imagei.h" defines several macros for converting samples between
different sizes.
Each macro is of the form "Sample"
size"To"
size where
size is one of 8, 16, or F
(for floating-point samples).
- SampleFTo16(sample)
- Sample16ToF(sample)
- SampleFTo8(sample)
- Sample8ToF(sample)
- Sample16To8(num)
- Sample8To16(num)