Debug graphics

When you transfer a beautiful and slender mathematical model to the real world of the code, unpleasant surprises is waiting for you. Such, for example, as unprecise calculations with floating point. Ways to fight this problem have long been known, but to choose the right one, you need to formulate the problem in specific conditions. And sometimes it's better to build a visual debugging model than to study an array of digits in the debugger.

This is how my model looks. This is for debugging the coordinates of the subpixel grid to the nearest pixel. The centers of the red circles are points on the straight line with subpixel precision, the white segments from the centers are the direction and size of the rounding.