At a very high level, data enters the graphics pipeline as a stream of primitives and is processed by up to as many as three shader stages: A vertex shader performs per-vertex processing such as transformations, skinning, vertex displacement, and calculating per-vertex material attributes. Tessellation of higher-order primitives should be done before the vertex shader executes. As a minimum, a vertex shader must output vertex position in homogeneous clip space. Optionally, the vertex shader can output texture coordinates, vertex color, vertex lighting, fog factors, and so on. A geometry shader performs per-primitive processing such as material selection and silhouette-edge detection, and can generate new primitives for point sprite expansion, fin generation, shadow volume extrusion, and single pass rendering to multiple faces of a cube texture. A pixel shader performs per-pixel processing such as texture blending, lighting model computation, and per-pixel normal and/or environmental mapping. Pixel shaders work in concert with vertex shaders; the output of a vertex shader provides the inputs for a pixel shader. In Direct3D 9 some pixel operations (such as fog blending, stencil operations, and render-target blending) occur after the pixel shader is finished.