On Tuesday 12 April 2005 13:06, JS wrote:
> Hi,
>     I'm trying to locate the alpha compositions code in Gimp and
> cannot find it. I've found C/MMX/Altivec versions of many
> image-image composition operations but they don't seem to use the
> alpha channel at all.
>     For example, I wonder wether the "overlay" composition
> corresponds to the Porter-Duff "over" operand. I've seen that some
> of the Porter-Duff operations are now part of GGGL
> (http://pippin.gimp.org/gggl/). I'm a little lost in all this. So I
> guess my questions are:
> 1) Does Gimp implement the 12 Porter-Duff operations
> (http://www.w3.org/TR/2002/WD-SVG11-20020215/masking.html)? If so,
> I guess they must have different names.
> 2) Where are the specifications for the compositing functions found
> in Gimp (like blend, screen, overlay, etc)? None of these functions
> seem to make use of the alpha channel, but is there still a link I
> do not see with the Porter-Duff ops?
> 3) Where is Gimp heading wrt composition ops? Is GGGL the new
> standard?
>     Thanks,
Hi! The
app/paint-funcs/paint-funcs-generic.h. file Bill pointed too is not 
used anymore - it is there in case someone builds the GIMP disabling 
aceleration with an option I forgot which is.

For the composting functions, take a look in the app/composite 
directory, in the "gimp-composite-generic.c" file.
Some of the functions are acelerated for MMX or SSE in the 
corresponding files, others are not.

As for your question  (reading about Porter Duff) wow...
comparing 12 kindso oif operations is a lot - but AFAIK the 
compositing models on the GIMP were not created thinking on these 
specific Ops. Some may be the same, others certainly are not.

2) There is not a proper Spec. This area of the current Docuemtnation 
is finding lacking, and there is even a bug openned against the docs. 
A better description can be found on the older "groking the GIMP" 
book for some of the modes. Currently, teh best way to know what 
eachmode does, if the explanation in the above sources does nto 
suffice, is checking the code in the above file.

3) No. GEGL is not in use yet. As I told, the funciotns are in the 
app/composite directory.

4) Please, take a look at 
for a way  to check a cheap way of implementing these new combination 
modes in a way that would not cluter the GIMP ui. If there is people 
wanting it working (custom layer modes) , I could go backk to work on 
it, even making it available to the GIMP if GEGL use is further 

5) Nice name this of yours!

        Joćo Sebastićo de Oliveira Bueno Calligaris
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