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A fragment shader is a programmable stage of the rasterization pipeline that computes values for a fragment, such as its output color and optionally its depth. It receives interpolated data from rasterized primitives and can use texture mapping to sample an image. A fragment is a candidate contribution; later tests and blending determine what reaches the final digital image.^[ref]

For example, a shader can multiply a surface's interpolated color by a sampled image to decorate a triangle. A fragment hidden behind a nearer opaque surface may fail the depth buffer test. With multiple samples per pixel, fragments, shader invocations and final pixels need not correspond one to one.

Texture mapping associates surface locations with values sampled from an image or other texture data. In a common two-dimensional case, each vertex carries coordinates $(u,v)$ identifying a location in the texture. Rasterization interpolates the coordinates, and a fragment shader uses them to obtain a color or another surface parameter.^[ref]

For example, a square wall can use $(0,0)$, $(1,0)$, $(1,1)$ and $(0,1)$ at its corners so a rectangular image covers it once. Repeating coordinates from zero to four can tile the same image when the wrapping mode allows repetition. The mapping is separate from sampling: nearest-sample selection and interpolation can give visibly different results from identical coordinates.

When a tilted surface is viewed through perspective projection, equal distances on the surface need not occupy equal distances on screen. Correct interpolation must account for that projection. A texture also supplies data rather than an automatic physical material model: a stored color and the light reflected toward the camera are different quantities.