The output of Vertex post processing is in Window space, here after (based on standard pipeline ) primitive assembling comes, why?
I know primitive assembly happens in many stages like before clipping, but why do we need PA at this stage.
Primitive assembly happens before clipping, but the specification isn't clear on exactly where before it. In fact:
After a primitive is formed, it is clipped to a clip volume.
That's all it says about its location (outside of needing to do it for tessellation or GS's).