Hi all,
I wonder if anyone has any thoughts about this? Is my solution, to define
a new glBlendFunc for software renderer, the only one for this problem or
are there less invasive solutions?
Any thoughts on if this extension would be acceptable to Mesa code? :-)
Jaakko
If OpenGL is used for creating images with transparency, and images and
drawing primitives are rendered on transparent background, the OpenGL
Blending functions in standard do not blend the colours correctly. In case
the background is completely transparent, and a semitransparent
(translucent) colour or image is drawn on top of that, the usual
Blending Function behaves incorrectly:
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
The extended version of this common function fixes the problem with
incorrect Alpha values in the resulting image, but fails to fix the
color channels:
glBlendFuncSeparateEXT (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA,
GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
The color channels are mixed with these settings with this formula:
(Example for Red)
Rd = destination Red
Ad = destination Alpha
Rs = source Red
As = source Alpha
R' = result destination Red
A' = result destination Alpha
A' = As + (1 - As) * Ad
R' = As * Rs + (1 - As) * Ad
Example:
As = 0.5
Rs = 0.8
Ad = 0
Rd = 0.2
R' = As * Rs + (1 - As) * Ad = 0.5 * 0.8 + 0.5 * 0.2
= 0.5
This is incorrect, the correct R' is obviously 0.8, because Ad = 0, and Rd
should not affect the resulting colour at all! The solution is to compute
the R' by weighing the Rs and Rd with their alphas relative to resulting
alpha:
R' = As/A' * Rs + (1-As/A1) * Rd
R' = 0.5/0.5 * 0.8 + (1-0.5/0.5) * 0.2
= 1 * 0.8 + 0 * 0.2
= 0.8
This is correct. These patches create extension GL_FORECA_blend_relative
that introduces Blending Functions GL_SRC_ALPHA_RELATIVE_FORECA and
GL_ONE_MINUS_SRC_ALPHA_RELATIVE_FORECA that can be used to implement
the latter formula:
http://www.iki.fi/hyvatti/sw/Mesa-6.4.2-ext-foreca.diff
http://www.iki.fi/hyvatti/sw/Mesa-6.5-ext-foreca.diff
Use this to set the correct blending function:
glBlendFuncSeparateEXT
(/* sfactorRGB */ GL_SRC_ALPHA_RELATIVE_FORECA,
/* dfactorRGB */ GL_ONE_MINUS_SRC_ALPHA_RELATIVE_FORECA,
/* sfactorA */ GL_ONE,
/* dfactorA */ GL_ONE_MINUS_SRC_ALPHA);
Both generic and reasonably optimized version of the function
is implemented.
--
Foreca Ltd [EMAIL PROTECTED]
Pursimiehenkatu 29-31 B, FIN-00150 Helsinki, Finland http://www.foreca.com
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