METHOD OF THE REVOLVED PLAN.
To find the perspective of a rectangular block resting upon a horizontal plane 1" below the level of the eye, and turned so that the long side of the block makes an angle of 30° with the picture plane.
The block is shown in plan and elevation at the left of the figure. The first step will be to make an auxiliary horizontal pro jection of the block on the plane of the horizon, showing the exact position of the block as it is to be seen in the perspective projec tion. This auxiliary horizontal projection is really a revolved plan of the object, and is called a Diagram. It is the general rule, in making a perspective projection, to place the object behind the picture plane with one of its principal vertical lines lying in the picture plane (24 h). HPP is usually drawn near the upper edge of the paper, leaving just room enough behind to place the auxiliary plan or diagram. In the figure the diagram is shown
in the required position, i.e., with one of its long sides (able) making an angle of 30° with the picture plane. The vertical edge (ae) of the block is supposed to lie in the picture plane. VH may now be drawn parallel to HPP at any convenient distance from it, as indicated. VH„ the vertical trace of the plane on which th'e block is supposed to rest, should be assumed in accord ance with the given data, i.e., 1" below VH (§ 44).
The position of the observer's eye should next be established. SPH is its horizontal projection, and shows by its distance from HPP the distance in front of the picture plane at which the ob•• server is supposed to stand. SO` is its vertical projection, and must always be found in VH. In this problem the station point is in front of the picture plane..