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Pixel processing is performed by pixel shaders on individual pixels. Pixel shaders work in concert with vertex shaders; the output of a vertex shader provides the inputs for a pixel shader. Other pixel operations (fog blending, stencil operations, and render-target blending) occur after execution of the shader.
A pixel shader completely replaces the pixel-blending functionality specified by the multi-texture blender including operations previously defined by the texture stage states. Texture sampling and filtering operations which were controlled by the standard texture stage states for minification, magnification, mip filtering, and the wrap addressing modes, can be initialized in shaders. The application is free to change these states without requiring the regeneration of the currently bound shader. Setting state can be made even easier if your shaders are designed within an effect.
For pixel shader versions ps_1_1 - ps_2_0, diffuse and specular colors are saturated (clamped) to the range 0 through 1 before use by the shader because this is the range of valid inputs to the shader.
Color values input to the pixel shader are assumed to be perspective correct, but this is not guaranteed in all hardware. Colors generated from texture coordinates by the address shader are always iterated in a perspective correct manner. However, they are also clamped to the range 0 to 1 during iteration.
For pixel shader versions ps_1_1 - ps_1_4, the result emitted by the pixel shader is the contents of register r0. Whatever it contains when the shader completes processing is sent to the fog stage and render-target blender.
For pixel shader versions ps_2_0 and above, output color is emitter from oC0 - oC4.