gl FragDepth, then it is responsible for always writing it.

3.12Antialiasing Application

If antialiasing is enabled for the primitive from which a rasterized fragment was produced, then the computed coverage value is applied to the fragment. In RGBA mode, the value is multiplied by the fragment’s alpha (A) value to yield a final alpha value. In color index mode, the value is used to set the low order bits of the color index value as described in section 3.2. The coverage value is applied separately to each fragment color.

3.13Multisample Point Fade

Finally, if multisampling is enabled and the rasterized fragment results from a point primitive, then the computed fade factor from equation 3.2 is applied to the fragment. In RGBA mode, the fade factor is multiplied by the fragment’s alpha value to yield a final alpha value. In color index mode, the fade factor has no effect. The fade factor is applied separately to each fragment color.

Version 2.0 - October 22, 2004

Chapter 4

Per-Fragment Operations and the

Framebuffer

The framebuffer consists of a set of pixels arranged as a two-dimensional array. The height and width of this array may vary from one GL implementation to another. For purposes of this discussion, each pixel in the framebuffer is simply a set of some number of bits. The number of bits per pixel may also vary depending on the particular GL implementation or context.

Corresponding bits from each pixel in the framebuffer are grouped together into a bitplane; each bitplane contains a single bit from each pixel. These bitplanes are grouped into several logical buffers. These are the color, depth, stencil, and accumulation buffers. The color buffer actually consists of a number of buffers: the front left buffer, the front right buffer, the back left buffer, the back right buffer, and some number of auxiliary buffers. Typically the contents of the front buffers are displayed on a color monitor while the contents of the back buffers are invisible. (Monoscopic contexts display only the front left buffer; stereoscopic contexts display both the front left and the front right buffers.) The contents of the auxiliary buffers are never visible. All color buffers must have the same number of bitplanes, although an implementation or context may choose not to provide right buffers, back buffers, or auxiliary buffers at all. Further, an implementation or context may not provide depth, stencil, or accumulation buffers.

Color buffers consist of either unsigned integer color indices or R, G, B, and, optionally, A unsigned integer values. The number of bitplanes in each of the color buffers, the depth buffer, the stencil buffer, and the accumulation buffer is fixed and window dependent. If an accumulation buffer is provided, it must have at least as many bitplanes per R, G, and B color component as do the color buffers.

The initial state of all provided bitplanes is undefined.

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