Use the black points to change the vertices of the quadrilateral.
Known bugs: NOTE: FIXED a) If you superimpose points, you will no longer be able to make a quadrilateral unless you click the green flag. You can however, make triangles, line segments or move a single point across the screen :P b) Not exactly a bug but the quadrilateral doesn't really follow the vertex when you drag it. Turbo mode can fix this issue.
