Regarding graphics pipeline

Question

In graphics pipeline after vertex shader comes, primitive assembly-> Clipping to view frustum-> normalized device coordinates -> viewport transformation.

Now in vertex shader we multiply object cordinates by modelview and projection matrix. " The Projection Matrix transforms the vertices in view coordinates into the canonical view volume (a cube of sides 2 2 2, centered at the origin, and aligned with the 3 coordinate axes). Typically, this will be either by an orthographic projection or a perspective projection. This transform includes multiplication by the projection transformation matrix followed by a normalization of each vertex, calculated by dividing each vertex by its own w coordinate. "

Now, if this is done in vertex shader only why it comes after the vertex shader part in pipeline shouldn't it just be a part of vertex shader.? If not what is the output of projection matrix multiplied by vertex coordinates?

Answer 1

I'm not sure I understand your question, but after you multiply your points by modelview and projection matrix in vertex shader, your points will be in clip coordinates. This is done, because now, the graphics hw can determine which objects can be visible and which not. This is called clipping and it is a separate step after the vertex shader. After this, it does the perspective division (divides xyz coordinates by homogenous coordinate w, this is hard coded inside the gpu) to get normalized device coordinates [-1, 1].