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gimp/devel-docs/parasites.txt

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PARASITE REGISTRY - 2007-10-18
=================
This document is designed for the convenience of GIMP developers.
It does not need to concern users.
>>>> If your plugin or script writes parasites, please
>>>> amend this file in SVN or submit patches to
>>>> gimp-developer@scam.xcf.berkeley.edu
------------------------------------------------------------------
*** NAMESPACE
Plug-in-specific data should be prefixed by the plug-in function name and
a slash, i.e. private data of plug_in_displace should be named like:
plug_in_displace/data1
plug_in_displace/data2
etc.
Global data follows no strict rules.
------------------------------------------------------------------
*** KNOWN PREFIXES:
"tiff" : The standard GIMP TIFF plugin
"jpeg" : The standard GIMP JPEG plugin
"png" : The standard GIMP PNG plugin
"gimp" : For common and standard parasites
------------------------------------------------------------------
*** KNOWN GLOBAL PARASITES:
"jpeg-save-defaults" (GLOBAL, PERSISTENT)
Default save parameters used by the JPEG plug-in.
"png-save-defaults" (GLOBAL, PERSISTENT)
Default save parameters used by the PNG plug-in.
"<plug-in>/_fu_data" (GLOBAL, IMAGE, DRAWABLE, PERSISTENT)
The Gimp::Fu module (Perl) might store the arguments of the
last plug-in invocation. It is usually attached to images,
but might also be found globally. The data format is either
pure character data (Data::Dumper) or a serialized data
stream created by Storable::nfreeze.
"exif-orientation-rotate" (GLOBAL, PERSISTENT)
Whether a load plug-in should automatically rotate the image
according to the orientation specified in the EXIF data. This
has values "yes" or "no". If the parasite is not set, the
plug-in should ask the user what to do. This parasite may be
removed in a future version (assuming always yes).
------------------------------------------------------------------
*** KNOWN IMAGE PARASITES:
"gimp-comment" (IMAGE, PERSISTENT)
Standard GIF-style image comments. This parasite should be
human-readable text in UTF-8 encoding. A trailing \0 might
be included and is not part of the comment. Note that image
comments may also be present in the "gimp-metadata" parasite.
"gimp-brush-name" (IMAGE, PERSISTENT)
A string in UTF-8 encoding specifying the name of a GIMP brush.
Currently, the gbr plug-in uses this parasite when loading and
saving .gbr files. A trailing \0 might be included and is not
part of the name.
"gimp-brush-pipe-parameters" (IMAGE, PERSISTENT)
This is all very preliminary:
A string, containing parameters describing how an brush pipe
should be used. The contents is a space-separated list of
keywords and values. The keyword and value are separated by a
colon.
This parasite is currently attached to an image by the psp
plug-in when it loads a .tub file (Paint Shop Pro picture
tube). It is used (first attached with values asked from the
user, if nonexistent) by the gpb plug-in when it saves a .gih
file. The .gih file contains the same text in it.
The keywords are:
ncells: the number of brushes in the brush pipe
step: the default spacing for the pipe
dim: the dimension of the pipe. The number of cells
in the pipe should be equal to the product
of the ranks of each dimension.
cols: number of columns in each layer of the image,
to be used when editing the pipe as a GIMP image
rows: ditto for rows. Note that the number of columns and rows
not necessarily are identical to the ranks of the
dimensions of a pipe, but in the case of two-
and three-dimensional pipes, it probably is.
rank0, rank1, ...: (one for each dimension): the index range
for that dimension
placement: "default", "constant" or "random". "constant" means
use the spacing in the first brush in the pipe.
"random" means perturb that with some suitable
random number function. (Hmm, would it be overdoing it
if the pipe also could specify what random function
and its parameters...?)
sel0, sel1, ...: "default", "random", "incremental", "angular",
"pressure", "velocity", and whatever else suitable we might
think of ;-) Determines how one index from each dimension is
selected (until we have pinpointed the brush to use).
"gimp-image-grid" (IMAGE, PERSISTENT)
The GimpGrid object serialized to a string. Saved as parasite
to keep the XCF files backwards compatible. Although gimp-1.2
does not know how to handle the image grid, it keeps the grid
information intact.
"gimp-pattern-name" (IMAGE, PERSISTENT)
A string in UTF-8 encoding specifying the name of a GIMP pattern.
Currently, the pat plug-in uses this parasite when loading and
saving .pat files. A trailing \0 might be included and is not
part of the name.
"tiff-save-options" (IMAGE)
The TiffSaveVals structure from the TIFF plugin.
"jpeg-save-options" (IMAGE)
The JpegSaveVals structure from the JPEG plugin.
"jpeg-exif-data" (IMAGE) (deprecated)
The ExifData structure serialized into a uchar* blob from
libexif. This is deprecated in favor of "exif-data".
"jpeg-original-settings" (IMAGE, PERSISTENT)
The settings found in the original JPEG image: quality (IJG),
color space, component subsampling and quantization tables.
These can be reused when saving the image in order to minimize
quantization losses and keep the same size/quality ratio.
"gamma" (IMAGE, PERSISTENT)
The original gamma this image was created/saved. For JPEG; this is
always one, for PNG it's usually taken from the image data. GIMP
might use and modify this. The format is an ascii string with the
gamma exponent as a flotingpoint value.
Example: for sRGB images this might contain "0.45454545"
"chromaticity" (IMAGE, PERSISTENT)
This parasite contains 8 floatingpoint values (ascii, seperated by
whitespace) specifying the x and y coordinates of the whitepoint, the
red, green and blue primaries, in this order.
Example: for sRGB images this might contain
"0.3127 0.329 0.64 0.33 0.3 0.6 0.15 0.06"
wx wy rx ry gx gy bx by
"rendering-intent" (IMAGE, PERSISTENT)
This specifies the rendering intent of the image. It's a value
between 0 and 3, again in ascii:
0 - perceptual (e.g. for photographs)
1 - relative colorimetric (e.g. for logos)
2 - saturation-preserving (e.g. for business charts)
3 - absolute colorimetric
"hot-spot" (IMAGE, PERSISTENT)
Use this parasite to store an image's "hot spot". Currently
used by the XBM plugin to store mouse cursor hot spots.
Example: a hot spot at coordinates (5,5) is stored as "5 5"
"exif-data" (IMAGE, PERSISTENT)
The ExifData structure serialized into a character array by
libexif (using exif_data_save_data). If a "gimp-metadata"
parasite is present, it should take precedence over this one.
"gimp-metadata" (IMAGE, PERSISTENT)
The metadata associated with the image, serialized as one XMP
packet. This metadata includes the contents of any XMP, EXIF
and IPTC blocks from the original image, as well as
user-specified values such as image comment, copyright,
license, etc.
"icc-profile" (IMAGE, PERSISTENT | UNDOABLE)
This contains an ICC profile describing the color space the
image was produced in. TIFF images stored in PhotoShop do
oftentimes contain embedded profiles. An experimental color
manager exists to use this parasite, and it will be used
for interchange between TIFF and PNG (identical profiles)
"icc-profile-name" (IMAGE, PERSISTENT | UNDOABLE)
The profile name is a convenient name for referring to the
profile. It is for example used in the PNG file format. The
name must be stored in UTF-8 encoding. If a file format uses
a different character encoding, it must be converted to UTF-8
for use as a parasite.
"decompose-data" (IMAGE, NONPERSISTENT)
Starting with GIMP 2.4, this is added to images produced by
the decompose plug-in, and contains information necessary to
recompose the original source RGB layer from the resulting
grayscale layers. It is ascii; a typical example would be
"source=2 type=RGBA 4 5 6 7". This means that layer 2 was
decomposed in RGBA mode, giving rise to layers 4, 5, 6, and 7.
"print-settings" (IMAGE, NONPERSISTENT)
This parasite is stored by the Print plug-in and holds settings
done in the Print dialog. It also has a version field so that
changes to the parasite can be done. GIMP 2.4 used version 0.3.
The format is GKeyFile. A lot of the contents are identical to
what is stored in ~/.gimp-2.x/print-settings but the parasite
has some additional image-related fields.
"print-page-setup" (IMAGE, NONPERSISTENT)
This parasite is stored by the Print plug-in and holds settings
done in the Page Setup dialog. The format is GKeyFile as created
from GtkPageSetup. The content is identical to what is stored in
~/.gimp-2.x/print-page-setup.
------------------------------------------------------------------
*** KNOWN LAYER/DRAWABLE PARASITES:
"gimp-text-layer" (LAYER, PERSISTENT)
The associated GimpText object serialized to a string. For
convenience the string is terminated by a trailing '\0'.
The idea of using a parasite for text layers is to keep the XCF
files backward compatible. Although gimp-1.2 doesn't know how
to handle the text layer, it keeps the parasite intact.
"gfig" (LAYER, PERSISTENT)
As of GIMP 2.2, the gfig plug-in creates its own layers, and
stores a representation of the figure as a layer parasite.
The parasite contains a GFig save file, in an ascii format.
If gfig is started while the active layer contains a "gfig"
parasite, the contents of the parasite are loaded at startup.
------------------------------------------------------------------
*** PARASITE FORMAT:
The parasite data format is not rigidly specified. For non-persistant
parasites you are entirely free, as the parasite data does not survive the
current gimp session. If you need persistant data, you basically have to
choose between the following alternatives (also, having some standard for
non-persistant data might be fine as well):
- Cook your own binary data format
You can invent your own data format. This means that you will either
loose totally (consider endian-ness or version-ness issues) or you will
get yourself into deep trouble to get it "right" in all cases.
- Use character (string) data
Obvious to perl people but less so to C programmers: just sprintf your
data into a string (e.g. "SIZE 100x200 XRES 300 YRES 300") and store
that in the parasite, and later sscanf it again. This often solves most
of the problems you might encounter, makes for easier debugging and
more robustness (consider the case when you add more entries to your
persistant data: older plug-ins might be able to read the relevant
parts and your application can detect missing fields easily). The
drawback is that your data is likely to be larger than a compact binary
representation would be. Not much a problem for most applications,
though.
You could also use one parasite per field you store, i.e. foo-size,
foo-offset-x, foo-offset-y etc...
- Use the libgimpconfig serialize functions
This is a special case of the previous one, using the convenience
functions provided by libgimpconfig. If you are not concerned about
the size of the string representation of your data, you can use
gimp_config_serialize_to_string() and other functions to easily
convert your data to/from a character string.
- Obsolete: Use the libgimp serialize functions
NOTE: libgimp/gserialize.[ch] has been excluded from the build since
gimp-1.2. This decision was made since noone seemed to use it so
far. The files can still be pulled out of SVN, so if you decide
to use them, you will have to include a copy into your plugin
source or resurrect the functionality in libgimp.
Look at the stuff in libgimp/gserialize.h. These functions allow for
relatively easy serializing/deserializing of structs. The advantages
are that the gimp-developers have already taken care of endian-ness
issues and other hazzles. The drawback is that you might encounter
problems when you want to extend your structures later, as you have to
be prepared for images saved with parasites form a very old version of
your plug-in, and the gserialize functions do not handle different data
formats nicely itself.
One way out around this is to prefix your data with a version identifier
(remember to use a guchar, i.e. something without endian-ness problems).
Remember to skip it before deserializing.
Another very easy way is to add a version tag to your parasite name,
i.e. "foo-bar-v1", "foo-bar-v2". Your plug-in could then check for older
versions and act accordingly and/or attach the new parasite or both the
new and the old version of your data.
The gserialize stuff also makes it possible to just append more fields
(i.e. more gserialized structs) to your data. You could check the length
of the parasite data ("anything left?") to decide wether to decode more
fields. Here's some example:
data = parasite_data(p);
size = parasite_data_size(p);
length = gdeserialize(...);
tlength += length;
data += length;
if (tlength != size)
gdeserialize the next one, etc.
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