Occlusion [[View-Frustum Culling|Culling]] is a *per-pixel* operation to not render a part of an object. Only pixels closest to the eye point are to be rendered. Occlusion Culling utilises a [[Depth Buffer]] that stores the z values of the pixels that are closest to the eye. When trying to render a fragment, if that fragments depth is farther than the [[Depth Buffer]] recorded for that pixel, that pixel is culled / 'occluded'.