On Oct 20, 2007, at 1:58 PM, Red Marble Games wrote:
This is more a general OpenGL question, but it is for a Mac
project, so I'm hopeful it is permissible here: I'm drawing a
number of textures with alpha to context 1 using (GL_SRC_ALPHA,
GL_ONE_MINUS_SRC_ALPHA), and in context 1 things look great. But
then I'm using context 1 as a source texture for drawing to context
2, and am getting some artifacts where the original drawing in
context 1 involved alpha values other than 1 or 0. I assume this
is because, as the spec says, blending affects the alpha component
of the destination pixels as well as the color components, so that
when context 1 becomes a texture, it has non-1.0 alpha values in
it. And indeed, if I turn off blending for the draw from context 1
-> context 2, everything is OK. But for a variety of reasons (I'm
simplifying the situation to describe it here) I can't adopt that
solution. So, my question is: once drawing into context 1 is
complete, is there a way to force all of the alpha values in
context 1 to be 1.0 before drawing it into context 2, so that it
will draw correctly even with blending enabled?
What you are effectively winding up with in your context 1
destination buffer is what is known as "premultiplied alpha", where
the color have already been multipled by alpha. If you multiply
them by alpha again, things will look wrong.
Assuming that you want blending enabled in context 2 and you know you
are texturing from a texture that came from context 1, you can try
using a blend func of (GL_ONE, GL_ONE_MINUS_SRC_ALPHA).
This is what the WindowServer does when rendering window backing
store textures which are also premultiplied.
-Ken
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