Flatness problem

The flatness problem has to do with shapes. Imagine that you draw a picture of a mountain. This mountain has points that are tallest (the peaks) and points that are lowest (the valleys). Now if you press down on all the points of the mountain, it wouldn't be a mountain anymore; it would be a flat piece of paper! That's the flatness problem. It means that when you try to draw a shape like a mountain, it gets flattened out and doesn't look like the shape you wanted.