GLALPHA

glAlpha is a function in the OpenGL graphics library used to control the alpha blending behavior of a rendering operation. Alpha blending is a technique used to combine a source color with a destination color based on an alpha value, which represents opacity.

The glAlpha function takes a single argument, a float representing the alpha value. This value is then stored in the alpha test function's reference value. When the alpha test is enabled, fragments (potential pixels) are discarded if their alpha value does not pass the test criteria. The default alpha test function is GL_ALWAYS, meaning all fragments pass. By changing the function and reference value, developers can selectively render or discard fragments based on their opacity.

This function is particularly useful for effects like transparency, fading, and overlaying textures. For instance, to make an object semi-transparent, one might set the alpha value of its fragments to something less than 1.0 and use the alpha test to allow fragments with an alpha greater than a certain threshold to be drawn. However, it's important to note that glAlpha itself doesn't directly perform the blending; it sets up the conditions for the alpha test, which then influences which fragments are passed to the blending stage. The actual blending is controlled by other OpenGL functions like glBlendFunc.