Docs @ Psychtoolbox Wiki : GLDSTALPHA

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Return the string 'GL_DST_ALPHA', which specifies an alpha blending
factor to Screen('BlendFunction', ...).

"Alpha" is a factor which weights RGB values when combining pixels by
drawing or copying. Alpha values weight the pixels drawn, the "source
surface". Separate alpha values weight the pixels drawn onto, the
"destination surface". A given pixel's alpha factor is not necessarily
the alpha component of that pixel's [rgba] color vector, but may instead
be the other combined surface's alpha component, a function of either or
both both alpha compoenents, a constant, or it may derive from RGB values
of the other combined surface. The Psychtoolbox command
Screen('BlendFunction') selects which. It implements the OpenGL function

In MATLAB, Screen('BlendFunction') accepts strings named for C constants
passed to the OpenGL function glBlendFunc().Enter "Help
PsychAlphaBlending" in the MATLAB command window for a list of blending
constants (and other functions related to alpha blending).

GL_DST_ALPHA may be used as either a source or a destination factor.

see also: PsychAlphaBlending, AlphaDemo, AlphaBlendingTest.

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