How to put a Given Point into Perspective
Let us place the given point P on a geometrical plane, to show how far it is from the base line, and indeed in the exact position we wish it to be in the picture. The geometrical plane is supposed to face us, to hang down, as it were, from the base line AB, like the side of a table, the top of which represents the perspective plane. It is to that perspective plane that we now have to transfer the point P.
Fig. 107.
From P raise perpendicular Pm till it touches the base line at m. With centre m and radius mP describe arc Pn so that mn is now the same length as mP. As point P is opposite point m, so
must it be in the perspective, therefore we draw a line at right angles to the base, that is to the point of sight, and somewhere on this line will be found the required point P·. We now have to find how far from m must that point be. It must be the length of mn, which is the same as mP. We therefore from n draw nD to the point of distance, which being at an angle of 45°, or half a right angle, makes mP· the perspective length of mn by its intersection with mS, and thus gives us the point P·, which is the perspective of the original point.
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A Perspective Point being given, Find its Position on the Geometrical Plane
| Fig. 108. |