In diagonal perspectives the sides of buildings are often so distorted that you cannot grasp the visual centre point of the building without be-ing irritated by this distortion. To determine where the visual centre point of a building seen in perspective lies, take the pencil with your thumb at the centre of its length and hold it in front of your eyes. Now move the pencil towards or away from you until the length of the pencil exactly covers the building that you want to draw. If you look again at your thumb and note the point of the building that is hidden behind it, you obtain the centre point of the building and of the drawing you are about to make and can position it better on the sheet.

For instance, in order to depict regularly spaced street lamps in perspective you must first of all choose a distance (D) between two street lamps. Then you determine the middle line (M) of the two lamps and connect this with the vanishing point (VP). From the top of the first lamp you now draw a diagonal through the middle point of the second lamp and extend it to meet a bottom point on the lower vanishing line. This is the point at which the third lamp stands.

You find the middle point of vertical lines in a perspective by measuring the actual length or height of the lines. In contrast, the middle point (M)of rectangular areas is found by drawing an “X” through a rectangular area in perspective. Centrally positioned building elements (flags, towers, windows) can then be more easily constructed from this middle point.

Circles are always found within squares, ellipses within a rectangle. From the square or rectangle with an “X” drawn through it we arrive at the circle or the ellipse through the intermediate step of the octagon.