OpenGL 1.1 only allows simple additive combinations of the source and destination color components during blending. Two ways in which the blending operations have been extended by vendors include the ability to blend with a constant color and the ability to use other blending equations. The blending color extension (EXT_blend_color) adds a constant RGBA color state variable which can be used as a blending factor in the blend equation. This capability can be very useful for implementing blends between two images without needing to specify the individual source and destination alpha components on a per pixel basis.
The blend equation extension (EXT_blend_minmax) provides the framework for specifying alternate blending equations. For example, in OpenGL 1.1, the accumulation buffer is the only mechanism which allows pixel values to be subtracted, but there is no easy method to include a per-pixel scaling factor such as alpha, so a subtractive blending equation has been implemented as an extension to 1.1 and is part of the imaging subset in OpenGL 1.2. Min and max functions are useful in image processing algorithms (e.g., for computing maximum intensity projections) and are also implemented as an extension to 1.1 and as part of the 1.2 imaging subset.