The alpha test discards fragments depending on the outcome of a comparison between an incoming fragment's alpha value and a constant reference value. glAlphaFunc specifies the reference value and the comparison function. The comparison is performed only if alpha testing is enabled. By default, it is not enabled. (See glEnable and glDisable of GL_ALPHA_TEST.) func and ref specify the conditions under which the pixel is drawn. The incoming alpha value is compared to ref using the function specified by func. If the value passes the comparison, the incoming fragment is drawn if it also passes subsequent stencil and depth buffer tests. If the value fails the comparison, no change is made to the frame buffer at that pixel location. The comparison functions are as follows: glAlphaFunc operates on all pixel write operations, including those resulting from the scan conversion of points, lines, polygons, and bitmaps, and from pixel draw and copy operations. glAlphaFunc does not affect screen clear operations.
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glAlphaFunc: man2/glAlphaFunc.xml
The alpha test discards fragments depending on the outcome of a comparison between an incoming fragment's alpha value and a constant reference value. glAlphaFunc specifies the reference value and the comparison function. The comparison is performed only if alpha testing is enabled. By default, it is not enabled. (See glEnable and glDisable of GL_ALPHA_TEST.) func and ref specify the conditions under which the pixel is drawn. The incoming alpha value is compared to ref using the function specified by func. If the value passes the comparison, the incoming fragment is drawn if it also passes subsequent stencil and depth buffer tests. If the value fails the comparison, no change is made to the frame buffer at that pixel location. The comparison functions are as follows: glAlphaFunc operates on all pixel write operations, including those resulting from the scan conversion of points, lines, polygons, and bitmaps, and from pixel draw and copy operations. glAlphaFunc does not affect screen clear operations.
Alpha testing is performed only in RGBA mode.