A flat sheet is a 2D plane. A fitted sheet, however, is topologically distinct because of its elastic corners. It behaves like a 3D "bag."
If you try to fold it like a piece of paper, the elastic resists.
Standard algorithms fail on non-planar geometry.
To solve this, we must transform the 3D "bag" into a 2D rectangle. We do this by mapping the corners on top of each other to neutralize the curvature.
Interact: Drag the slider to fold the Right Side (C & D) all the way onto the Left Side (A & B).
Once aligned, the sheet enters the "Salsa Skirt" state: a long, flat rectangle. The geometry problem is solved.
Now it is merely an area reduction problem. We apply a recursive function: Area_new = Area_current / 2.