Very fast 3D distance check?

Question

Is there a way to do a quick and dirty 3D distance check where the results are rough, but it is very very fast? I need to do depth sorting. I use STL sort like this

Answer 1

You could compare squares of distances instead of the actual distances, since d² = (x₁-x₂)² + (y₁-y₂)²+ (z₁-z₂)². It doesn't get rid of the multiplication, but it does eliminate the square root operation.