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PARALLEL OR ONE-POINT PERSPECTIVE.

79. When the diagram of an object is placed with one of its principal systems of horizontal lines parallel to the picture plane, it is said to be in Parallel Perspective. This is illustrated in Fig. 24, by the rectangular block there shown. One system of horizontal lines in the block being parallel to the picture plane, the other system of horizontal lines must be perpendicular to the picture plane. The vanishing point for the latter system will be coincident with SPv (§ 52). The horizontal system that is parallel to the picture plane will have no vanishing point within finite limits (§ 54, with note ; also last paragraph of note under § 69). The third system of lines in the object is a vertical one, and will have no vanishing point within finite limits (§ 54). Thus, of the three systems of lines that form the edges of the block, only one will have a vanishing point within finite limits. This fact has led to the term One-Point Perspective, which is often applied to an object in the position shown in Fig. 24. As will be seen, this is only a special case of the problems already studied, and the construction of the perspective of an object in parallel perspective is usually simpler than when the diagram is turned at an angle with HPP.

80. The vertical face (able) of the block lies in the picture plane. It will thus show in its true size and shape (§ 24 h). The points eP and fr will be found•on VH, vertically below the points e and f in the diagram.

81. Both the edges ePaP and fPbP are lines of measures, and will show the true height of the block, as given by the elevation.

82. The two lines arbP and ePfP , since they are formed by the intersection of the bases of the block with the picture plane, will also be lines of measures (§ 55), and will show the true length of the block, as given by the plan and elevation.

83. The perspective of the front face of the block, which is coincident with the picture plane, can be drawn immediately. From eg,br, er, , and f", the horizontal edges, which are perpendicu lar to the picture plane, will vanish at v" (coincident with SPv).

The rear vertical edges of the block may be found in the usual manner.

84. The lines eV, dPcP, Or, and l,"y", which form the hori zontal edges parallel to the picture plane, will all be drawn paral lel to one another (§ 54, note) ; and since the lines in space which they represent are horizontal, ce , dr, cr , On and kV will all be horizontal (see last paragraph of note under § 69).

All of the principles that have been stated in connection with the other problems will apply equally well to an object in parallel perspective.

85. Interior views are often shown in parallel perspective. One wall of the interior is usually assumed coincident with the picture plane, and is not shown in the drawing. For illustration, the rectangular blOck in Fig. 24 may be considered to represent a hollow box, the interior of which is to be shown in perspective. Assume the face (aPbreP) that lies in the picture plane to be removed. The resulting perspective projection would show the interior of the box. In making a parallel perspective of an interior, however, VH is usually drawn lower than is indicated in Fig. 24, in order to show the inside of the upper face, or ceiling, of the interior. With such an arrangement, three walls, the ceiling, and the floor of the interior, may all be shown in the perspective projection.

86. Fig. 25 shows, an example of interior parallel perspective. The plan of the •room is shown at the top of the plate. This has been placed so that it may be used for the diagram, and save the necessity of making a separate drawing. The elevation of the. room is shown at the left of the plate, and for convenience it has been placed with its lower horizontal edge in line with VH,. In this position. all vertical dimensions in the object may be carried by horizontal construction lines directly from the elevation to the vertical line of. measures (a" el or bPP) in the perspective projection.