EXT_primitive_bounding_box : how does Primitive Generator process gl_BoundingBoxEXT[]

In the definition of Tessellation Primitive Generator, it processes gl_TessLevelOuter[] and gl_TessLevelInner[] and generates u,v,w of the relative value within [0…1]. It is not a real vertex location. How does PG process gl_BoundingBoxEXT[]? What is the function or formula related to gl_BoundingBoxEXT[]?

Note gl_BoundingBoxEXT is the output of tesselation evaluation stage (not control stage), after the tesselator. So gl_Postion-s are already known when gl_BoundingBoxEXT[] is considered.

Regarding your question gl_BoundingBoxEXT takes clip coordinates - homogeneus x,y,z,w.

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