GLCONSTANTALPHA

GL_CONSTANT_ALPHA is an enumerant in the OpenGL graphics API used to specify a blend factor for alpha blending. It designates that the alpha component of a constant color, supplied by the glBlendColor function, should be used as the blending multiplier.

In blending operations, the constant alpha value is applied when GL_CONSTANT_ALPHA is selected as a source or destination factor in glBlendFunc or glBlendFuncSeparate. The actual value comes from the alpha component of the constant color set by glBlendColor. When combined with other factors, GL_CONSTANT_ALPHA enables uniform control over the transparency contribution of incoming fragments, independent of their interpolated alpha.

Usage typically involves setting a constant color with glBlendColor(r, g, b, a) and then configuring the blend factors to include GL_CONSTANT_ALPHA. For example, glBlendFunc(GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA) would use the constant alpha as the source factor for the color or alpha channel depending on whether a separate blend function is used. In a separate blend setup, GL_CONSTANT_ALPHA can be used for the alpha channel factors while GL_CONSTANT_COLOR may be used for the color channels.

GL_CONSTANT_ALPHA is part of the broader set of constants for constant blending factors, which also includes GL_CONSTANT_COLOR and their inverses. Its effect is permitted only when GL_BLEND is enabled and the blending equation is valid for the chosen factors. The constant alpha is clamped to the [0,1] range by the OpenGL state machine.