In this small article, I would like to give some clear technical rules on drawing objects in perspective.
The depth is an optical effect that makes the objects appear to us smaller as they become farther from our eyes and run into the horizon. Let's see, for example, in this tree-lined road how the trees become smaller and the branches seem to create an arch above the road.
As a consequence of this effect, lines which we know being parallel, like the furrows on a ploughed field, shorten more and more the distance between each others as they run towards infinite, towards an imaginary point on the horizon. To create a drawing which suggests depth and perspective effect means finding a geometrical symbology which makes our mind understand the drawing, which is 2-D, with the visual signals of reality, which is 3-D.
We unconsciously try to see what we already know, and to draw as our mind sees, which means we use abstract models that in their simple structure might be easily brought back to reality.
Let's see an example: the lines we see in the figure on the left resume the spatial structure of the castle. Our mind's habit to interpret reality leads us to see significance in every abstract scheme.
Linear perspective was invented to realize geometric spatial imagines seen from a certain fixed point with eyes still in a certain position. Therefore it is a conventional graphic procedure which our mind is able to understand. We must decide first of all the horizon line (HL), the imaginary line which passes at eyes level. Then we must decide the ground line (GL), which is conventional too, and represents the base line. On the horizon line there is the main flight point P, also called point of view, which is the perpendicular projection of the visual ray to the observer's eye, and is the point toward which all lines convert.
When we draw any view, the horizon line will stay at the observer's eyes level. So if the observer is looking from the ground level, the horizon line will appear near to him, and his view will be limited (see the drawing on the left), if the observer is looking from a higher level the horizon line will be further away and his view will be wider (see the drawing in the center) and if the observer is looking from a even higher position, then the horizon line will go further and further, and his view will get wider and wider (see the drawing on the right).
In central perspective, with one single point of view, lines which are parallel to each other but perpendicular to observer (like the side of a road, a tree-lined road, telephone wires...) will converge ideally towards one single point on the horizon, and they are called flight lines. Vertical lines appear to get shorter, while lines parallel to the ground remain horizontal and parallel, but appear to get nearer one to the other untill they mix with the horizon line.
So a man walking in the foreground may appear higher than a car in second ground, higher than a house in the background.
And the clouds too, which are set on the imaginary roof of the sky, do suffer a perspective deformation. Each small cloud leads our glance to the sense of depth. Everything appear to be framed in a grid of lines and geometric levels which are sided by the horizon line, the ground line and the main point of view, which is the projection of the observer position.