Clipping

Learn about clipping!

clipper

decides which parts of primitives are rasterised, avoids wasting resources on pixels that won’t be visible

3 cases of 2d line clipping

lines are fully inside, fully outside or may need to be shortened

cohen-sutherland algorithm

divide the viewpoint coords into 9 regions represented by a 4 bit outcode. each bit encodes whether the region is outside of the image edges

why is clipping polygons insufficient?

we still need to assemble new points into a new polygon - they have surface, direction and interior

clipping pipeline order

top bottom right left

reason clipping flat polygons is not too hard 1

easy to determine whether a straight line intersects a clipping plane

reason clipping flat polygons is not too hard 2

easy to calculate where this intersection is

reason clipping flat polygons is not too hard 3

easy to calculate new vertices where needed and update polygons

what changes when we move to 3d clipping?

we need to calculate plane-line intersection

clipping for orthographic viewing

we have a cubic clipping volume where each side corresponds to a plane in 3d space

clipping for arbitrary projections

clip in pre-warped clip coordinates