TIP #98: Adding Transparency Compositing Rules to Photo Images

Title:Adding Transparency Compositing Rules to Photo Images
Version:$Revision: 1.5 $
Author:Donal K. Fellows <fellowsd at cs dot man dot ac dot uk>
Created:Saturday, 09 June 2001


This TIP adds compositing rules to Tk's photo images to give programmers better control over what happens when two transparent images are combined. This TIP also allows for several frames of an animated GIF file to be correctly displayed in an image even when the transparent area is not constant.


This is a TIP that is inspired by the tkchat application in Tcllib, and in particular by the image used to represent the LOL smiley. The problem with this image is that its transparent area changes over time, and this is caused by the fact that Tk_PhotoPutBlock() only allows one way of compositing a block with an image; it behaves as if the data being added was on a sheet of cel (the material used to make hand-drawn animated cartoons) allowing for sophisticated layering effects. Unfortunately, for many applications (and animated GIF images are definitely among these) this sophistication works against us. In a GIF image, transparency is treated not as extra information that is added to each pixel's colour, but rather as a special colour; a pixel cannot be, for example, red and transparent at the same time. Support for this requires a different (and indeed simpler) kind of compositing rule. And of course, once you have such a facility in the underlying C code, it should be exposed to scripts.

There are other kinds of compositing rule (for example, acting like the added block is placed under the image, and many others) but this TIP does not propose adding anything other than a way to chose between the current behaviour and the behaviour required for supporting GIF animation, in the belief that those two compositing rules are the ones most useful to programmers, and that once the general facility is there, the other rules will be relatively easy to add in the future.


This TIP adds a compositingRule argument to Tk_PhotoPutBlock (and Tk_PhotoPutZoomedBlock) to allow selection between the current behaviour (overlaying) and the other one I wish to support (setting/overriding.) The permitted values of this argument will be TK_PHOTO_COMPOSITE_OVERLAY (the currently implemented behaviour) and TK_PHOTO_COMPOSITE_SET (the behaviour required to support GIF file animation.)

At the Tcl level, when copying from one image to another (the other photo image subcommands do not currently support transparency at all) the photo get will take an extra option -compositingrule to allow selection of the compositing rule. The permitted values of this option will be overlay and set by analogy with the values described above.

Implementation Notes

Proposed implementation patch: https://sourceforge.net/tracker/index.php?func=detail&aid=566765&group_id=12997&atid=312997

The proposed implementation of this TIP naturally includes backward-compatibility functions that allow pre-compiled extensions to continue to operate without recompilation (provided they use Stubs for linking.) Furthermore, extension authors can also define the symbol USE_COMPOSITELESS_PHOTO_PUT_BLOCK when compiling and have source-level compatibility with the old functions.

The proposed implementation also creates TkSubtractRegion as a new internal function; it is the analogue of XSubtractRegion as TkIntersectRegion is the analogue of XIntersectRegion. It might be useful to other parts of the core that manipulate regions.

Both the PPM and GIF file readers use the set compositing rule, PPM because the format does not support transparency (and set should at least theoretically be faster) and GIF because it is required semantically. Other image formats are not required to do this, of course. The $img put $data photo image subcommand uses set compositing because it does not support transparency.


This document is placed in the public domain.

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