The position of the station point is shown in the figure by its two projections SPY and SPid. SPY must always be in VH. The distance between SPH and IIPP shows the distance of the observer's eye in front of the picture plane (§ 43).
el' and v'd may be found as in the preceding problems.
The position of the plane on which the object is to rest should next be established by drawing VII, the distance between VH and VH, showing the height of the observer's eye above the ground (§ 44).
In addition to the plane of the ground represented by VH„ a second ground plane, represented by has been chosen some distance below VII, In the figure, two perspective projections have been found, one resting on each of these two ground planes. The perspective which rests upon the plane represented by VH, shows the house as though seen by a man standing with his eyes nearly on a level with the tops of the windows (§ 29). The view which rests on the plane represented by shows a bird's-eye view of the house, in which the eye of the observer (always in VII) is at a distance above the plane on which the view rests, equal to about two and one-half dines the height of the ridge of the house above the ground.
The 3 two perspective projections illustrate the effect of 'changing the distance between VII and the vertical trace (§ 34, note) of the plane on which the perspective projection is supposed to rest. The construction of both views is exactly the same. The following explanation applies to both equally well, and the student may consider either in studying the problem.
61. We will first neglect the roof of the house, and of the porch. The remaining portion of the house will be seen to con sist of two rectangular blocks, one representing the main body of the house, and the other representing the porch.
The block representing the main part of the house occupies a position exactly similar to that of the block shown in Fig. 19. First consider this block irrespective of the remainder of the house. A vertical line dropped from the corner of the diagram that lies in IIPP will be a measure line for the block, and will establish, by its intersection with (or the position of the point eP," in exactly the same way that the point el' in Fig. 19 was established. shows the true height of the part of the house under consideration, and should be made equal to the Cor responding height as shown by the elevations. The rec
tangular block representing the main part of the house may now be drawn exactly as was the block in Fig. 19; Problem IV.
62. Having found the perspective of the main part of the house, the porch (without its roof) may be considered as a second rectangular block, no vertical edge of which lies in the picture plane. It may be treated in a manner exactly similar to that of the block shown in Fig. 21, § 59. We may consider that the rear vertical face of the block, which forms the porch of the house (q, q), has been extended until it intersects the picture plane in the line ae, giving a line of measures for this face, just as in Fig. 21 the nearest vertical face of the block was extended until it intersected the picture plane in the line of measures nat.
On eraP, make ere equal to the true height of the vertical wall of the porch, as given by the elevation. A line through vanishing at v", will be the perspective of the upper horizontal edge of the rear face of the block which forms the porch. The line through er, vanishing at vab, which forms the lower edge of the front face of the main body of the house, also forms the lower edge of the rear face of the porch. Through the point lc in the diagram, draw at visual ray, and through the intersection of this visual ray with IIPP drop a vertical line. Where this vertical line crosses the upper and lower horizontal edgeg of the rear face of the porch, will establish the points gr and hP respectively. Having found the vertical edge ?IP, the remainder of the per spective of the porch (except the roof) can be found without difficulty, the horizontal edges of the porch vanishing at either v" or v", according to the system to which they belong. Each vertical edge of the porch will be vertically below the point where HPP is crossed by a visual ray drawn through the point in the diagram which represents that edge. The fact that the porch projects, in part, in front of the picture plane, as indicated by the relation between the positions of the diagram and HPP, makes absolutely no difference in the construction of the perspective Projection.