glPrimitiveBoundingBox

glPrimitiveBoundingBox specifies a clip space bounding box which may allow implementations to optimize performance if the application provides bounds of primitives that will be generated by the tessellation primitive generator or the geometry shader prior to executing those stages. If the provided bounds are incorrect and primitives extend beyond them, the rasterizer may or may not generate fragments for the portions of primitives outside the bounds.

The bounding box in clip space is composed of 16 vertices formed by all combinations of the minimum and maximum values for each dimension. This bounding box is clipped against ${\mathit{w}}_{\mathit{c}}>0$ and projected to three dimensions by dividing ${\mathit{x}}_{\mathit{c}}$ , ${\mathit{y}}_{\mathit{c}}$ , and ${\mathit{z}}_{\mathit{c}}$ by ${\mathit{w}}_{\mathit{c}}$ for each vertex. The viewport transform is then applied to each vertex to produce a three-dimensional bounding volume in window coordinates. The window space bounding volume is expanded in the X and Y dimensions to accommodate the rasterization rules for the primitive type, and to fall on fragment boundaries.

If tessellation is active, each invocation of the tessellation control shader may re-specify the bounding box by writing to the built-in gl_BoundingBox [] variable. If the shader statically assigns a value to any part of this variable, then gl_BoundingBox[0] is used instead of minX , minY , minZ , minW , and gl_BoundingBox[1] is used instead of maxX , maxY , maxZ , maxW . If the shader contains a static assignment to gl_BoundingBox[] and there is an execution path through the shader that does not write all components of gl_BoundingBox[], the value of unwritten components and corresponding bounding box coordinates is undefined for executions of the shader that take that path.

glGet with argument GL_PRIMITIVE_BOUNDING_BOX

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