Calculating rectangle 3D coordinate with coordinate its shadow?

Question

Sometimes was a problem what is the rectangle 3D rotated and be perspective transition (for example in CSS) draw as the tetragon. But we want obtain the rectangle (width, le

Answer 1

figure fig.1 points a,c diagonal rectangle(yellow) points A,C diagonal tetragon(shadow) (red)

fig.2 a,b,c,d rectangle points(yellow) A,B,C,D shadow(tetragon) (red)