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A Fragment is a collection of values produced by the Rasterizer. Each fragment represents a sample-sized segment of a rasterized Primitive. The size covered by a fragment is related to the pixel area, but rasterization can produce multiple fragments from the same triangle per-pixel, depending on various multisampling parameters and OpenGL state. There will be at least one fragment produced for every pixel area covered by the primitive being rasterized.

Fragments are processed in a manner similar to vertices in a Vertex Shader. For them, an input vertex, built from Vertex Attributes defined during Vertex Specification, enters the Vertex Shader. After arbitrary processing, the vertex shader writes a number of values. These values represent the output vertex, and this output vertex is passed along to the next stages of the pipeline.

Fragments work the same way. An input fragment, built by the rasterizer, enters the Fragment Shader. After arbitrary processing, the fragment shader writes a number of values. These values represent the output fragment, and this output fragment is passed along to the next stage of the pipeline.

Fragment shader inputs

Initially, the data for a fragment consists of the following:

Not all of these are accessible from the fragment shader.

Fragment shader outputs

After processing, the output fragment from a fragment shader consists of the following:

  • a depth value, either written by the fragment shader or passed through from the screen-space fragment's Z value.
  • a stencil value.
  • An array of zero or more color values, as written by the fragment shader.

The color value array elements are routed to different buffers based on the framebuffer's draw buffer state.

If no fragment shader is active, then the fragment has only stencil and depth (the screen-space fragment's Z value). It has no defined color values in its array.

Fragment discard

Fragments generated by the rasterizer may be discarded. This can happen due to the failure of various per-fragment tests or by command from the Fragment Shader.

No matter what, a discarded fragment will never affect:

Normally, a discarded fragment will also not affect the current depth or stencil buffers. However, there are certain cases where such updates can occur, even for a discarded fragment.

If a fragment shader specifies that depth/stencil tests should happen before the shader, then this also means that the depth/stencil buffers will be updated before the shader (in accord with the Stencil Test operation and Write Masks). So with early fragment tests turned on, if the fragment shader explicitly discards the fragment, the depth and stencil buffers will have already been updated. This also means that Occlusion Query sample counts are updated even from such discarded fragments.

The other way is for the stencil buffer. If Stencil Testing is active, then discarded fragments can still affect the stencil buffer. This would be either due to enforced early tests as above, or if the discarding is due to the Depth Test. The stencil test has the ability to modify the stencil buffer due to a failure of the depth test.

Outside of these cases, a discarded fragment will have no effects.

See also