Pixel Shader Stage
The pixel-shader stage (PS) enables rich shading techniques such as per-pixel lighting and post-processing. A pixel shader is a program that combines constant variables, texture data, interpolated per-vertex values, and other data to produce per-pixel outputs. The rasterizer stage invokes a pixel shader once for each pixel covered by a primitive, however, it is possible to specify a NULL shader to avoid running a shader.
The Pixel Shader
When multisampling a texture, a pixel shader is invoked once per-covered pixel while a depth/stencil test occurs for each covered multisample. Samples that pass the depth/stencil test are updated with the pixel shader output color.
The pixel shader intrinsic functions produce or use derivatives of quantities with respect to screen space x and y. The most common use for derivatives is to compute level-of-detail calculations for texture sampling and in the case of anisotropic filtering, selecting samples along the axis of anisotropy. Typically, a hardware implementation runs a pixel shader on multiple pixels (for example a 2x2 grid) simultaneously, so that derivatives of quantities computed in the pixel shader can be reasonably approximated as deltas of the values at the same point of execution in adjacent pixels.
When the pipeline is configured without a geometry shader, a pixel shader is limited to 16, 32-bit, 4-component inputs. Otherwise, a pixel shader can take up to 32, 32-bit, 4-component inputs.
Pixel shader input data includes vertex attributes (that can be interpolated with or without perspective correction) or can be treated as per-primitive constants. Pixel shader inputs are interpolated from the vertex attributes of the primitive being rasterized, based on the interpolation mode declared. If a primitive gets clipped before rasterization, the interpolation mode is honored during the clipping process as well.
Vertex attributes are interpolated (or evaluated) at pixel shader center locations. Pixel shader attribute interpolation modes are declared in an input register declaration, on a per-element basis in either an argument or an input structure. Attributes can be interpolated linearly, or with centroid sampling. Centroid evaluation is relevant only during multisampling to cover cases where a pixel is covered by a primitive but a pixel center may not be; centroid evaluation occurs as close as possible to the (non-covered) pixel center.
Inputs may also be declared with a system-value semantic, which marks a parameter that is consumed by other pipeline stages. For instance, a pixel position should be marked with the SV_Position semantic. The IA stage can produce one scalar for a pixel shader (using SV_PrimitiveID); the rasterizer stage can also generate one scalar for a pixel shader (using SV_IsFrontFace).
A pixel shader can output up to 8, 32-bit, 4-component colors, or no color if the pixel is discarded. Pixel shader output register components must be declared before they can be used; each register is allowed a distinct output-write mask.
Use the depth-write-enable state (in the output-merger stage) to control whether depth data gets written to a depth buffer (or use the discard instruction to discard data for that pixel). A pixel shader can also output an optional 32-bit, 1-component, floating-point, depth value for depth testing (using the SV_Depth semantic). The depth value is output in the oDepth register, and replaces the interpolated depth value for depth testing (assuming depth testing is enabled). There is no way to dynamically change between using fixed-function depth or shader oDepth.
A pixel shader cannot output a stencil value.