PERSPECTIVE MEASUREMENTS
Method of Triangles
Because any line parallel to the picture plane (PP) can be subdivided
proportionately to scale, we can use such a parallel line as a
measuring line (ML) to subdivide any intersecting line into equal or
unequal parts.
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From one end (A) of the receding line to
be subdivided, we draw ameasuring line
(ML) parallel to PP.Ifthe receding line is
horizontal in space, then ML will be a
horizontal line in the drawing.
• At an appropriate scale w mark off the
, e
desired subdivisions on ML.
• We define atriangle by connecting the
end (B) of ML and the end (C) of the
receding line.
-. From each of the scaled subdivisions, we
draw lines that are parallel to BC and
therefore converge at the same
vanishing point. These lines subdivide
the receding line into the same
proportional segments.
Extending a Depth Measurement
If the forward edge of arectangular plane is parallel to the picture
plane (PP), we can extend and duplicate its depth in perspective.
• Note that it is usually better to subdivide alarger measurement
into equal parts than it is to multiply a smaller measurement to
arrive at a Ia rger whole. The reason for this is t hat, in the latter
procedure, even minute errors can accumulate and become visible
in the overall measurement.
---. First, we establish the midpoint of the
rear edge opposite the forward edge of
the rectangle.
Then we extend adiagonal from a
forward corner through this midpoint to
meet an extended side of the rectan
gle.
• From this point, we draw aline parallel
to the forward edge. The distance from
the first to the second edge is identical
to the distance from the second to the
third edge, but the equa l spaces are
foreshortened in perspective.
• We can repeat this procedure as often
as necessary to produce the desired
number of equal spaces in the depth of
aperspective drawing.
P RSPECTIVE DRAWINGS I 117
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