Let's go outdoors.
The blue line now becomes the 'horizon line'.
add a few visual clues ... try it
... a railway line, telegraph posts and suggestions of clouds ...
Let us look at the structure again ... and draw a cube ... and ...
... indicate a light source and plot a shadow (it gets complicated very quickly doesn't it? To plot the shadow drop a vertical line from the light source. From where it hits the ground radiate lines from that point to intersect lines from the light itself.) Note the method of drawing ellipses on the surfaces of the cube. When you feel comfortable try drawing and plotting the shadow of an archway using two point perspective. Let's stay outdoors...
