ALL PARALLEL LINES CONVERGE TO THE SAME VANISHING POINT (VP)
Pencil drawing by Antonio Lopez Garcia
Cast Shadows work in the same way as anything else in perspective, they have geometric laws that guide them. Once your object or figure is drawn in the space, determine where you want the light source to be positioned. Draw a straight line down from the light source to the GL. Cross a line from the where the light source line meets the GL to the bottom corners of the object, then cross a line from the light itself to the top corners of the object. Where the lines meet will be the spot where the shadow ends. Connect these points and you will have you cast shadows.