Am slightly confused about this slide. When we zoom in, aren't we supposed to sample more from the texture because we render more of the image, and vice versa for far away objects? If so, why are the shapes getting bigger as we head from upsampling to downsampling?
@cobaltblue, this is a visualization of the footprint of a pixel in texture space. As we zoom in to the surface a single screen pixel corresponds to a tiny part of the texture. Consider putting your eye right up next to your desk. You can only see a very small fraction of the entire desk (represented by the small purple box above).
Now imagine that you zoom way out. The footprint of a single pixel gets bigger in texture space. Consider a scene where an object is so small that is falls within a single pixel on screen. In this case, a single screen pixel would span all of texture space.