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Yes so short question: Is it possible to use any of the RGB components as alpha in Opengl ES 1.1 (fixed pipeline)?

It seems that GL_COMBINE does not allow one to use the RGB as a source for alpha.

Why is this limit imposed, and what are alternatives? I've tried quite a few things but I am unable to find a solution.

genpfault
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Tony
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  • How should RGB be used as source for alpha, how should GL derive a single value from multiple ones? Use the A component (alpha) for alpha. – Christian Rau Oct 03 '11 at 19:44
  • Can you tell why do you need this and maybe someone suggests another way to solve the problem ? – Max Oct 03 '11 at 20:07
  • I am rendering a white mask by alpha testing. The alpha is only useful for this test, and the final written alpha result can be discarded. Any of the RGB components should be used as alpha in another pass. Another way is of course to use the z test which is faster, but I am kind of surprised this is just not possible. A to RGB works fine for example. – Tony Oct 12 '11 at 18:37
  • Actually I am on the iPhone and writing into the z buffer from a texture is impossible, so I have to do it cpu side and then copy it over (even though the server shares the client memory!). This just isn't cute. – Tony Oct 12 '11 at 18:39

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