Note that the glEnablei/glDisablei functions are not part of this functionality. So, given the function glBlendFuncSeparate, there is a glBlendFuncSeparatei whose first parameter is a draw buffer index. This function takes one additional parameter at the beginning, an unsigned integer representing the draw buffer index whose blending data this function call is intended to change. If this functionality is available, for every function described below, there is an equivalent function that has an "i" at the end of the name ("iARB" for the extension version). No pre-4.0 hardware from NVIDIA implements this.
Note: All AMD/ATI 3.x-class hardware, the HD2000, 3000, and 4000 series, implement ARB_draw_buffers_blend. Given the availability of this feature, the user can specify different blending parameters or equations for each buffer. When blending is active, the input colors from the fragment processor does not merely overwrite the corresponding value currently in the output buffer, but instead the two colors are combined together based on a function. Once the matching is arranged, the color can be written to that buffer. Each fragment output color is matched up with a particular buffer (or discarded) via a call to glDrawBuffers or glDrawBuffer. These colors must be written to zero or more of the color buffers in the current framebuffer. There are other outputs from fragment processing, but blending only deals with colors. The result of the fragment processing stage of the OpenGL Pipeline, whether using Fragment Shaders or not, are zero or more color values. Doing this requires that you take certain steps, some of them quite difficult to deal with. This is not its only use, but it is one of its main uses.
Blending can be used to make objects appear transparent.
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