问题
I'm looking at SurfaceFlinger, i.e. the the code that is doing the composition in Android, and I have trouble understanding some of the OpenGL ES 1.0 calls as I've only programmed ES 2.0 code.
Here goes the piece of code that interests me:
glTexEnvx(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
glColor4x(0x10000, 0x10000, 0x10000, 0x10000);
glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
What it is supposed to be doing is blending a texture into the buffer.
Why does one need the glTextEnvx and glColor4x calls? Isn't glBlendFunc enough for achieving blending?
I know my question is naive, but I still don't understand what is glTexEnvx for even after reading the docs.
回答1:
glTexEnv() sets the texture environment mode. GL_REPLACE tells the renderer to skip the current color (for example, from glColor4()) and instead tells the renderer to use your texture's colors for every corresponding pixel. If you, instead of GL_REPLACE, use GL_MODULATE, then your glColor4() call will be included together with the texture's colors when the renderer sets a pixel's color.
Your glColor4() call shouldn't be doing anything (when using GL_REPLACE) that can be seen on your object.
About your glBlendFunc() arguments:
GL_ONE is using the current color that comes from your incoming primitive, which we call source. GL_ONE_MINUS_SRC_ALPHA is multiplying the destination (which is the currently stored pixel in the frame buffer) by (1 - source alpha value).
Normally, when you're not using textures, you achieve a transparent effect from color primitives when the glColor4() contains an alpha value where 1 equals fully opaque and 0 fully transparent.
来源：https://stackoverflow.com/questions/6832612/how-are-gltexenv-glcolor4x-and-glblendfunc-related-in-opengl-es-1-0