NOW A QUICK REVIEW: We see things, as shown on pages 18 and 19, by means of a central visual ray surrounded by a cone of vision. The central visual ray focuses upon the center of interest, while the cone of vision defines the roughly circular area within which we can see things clearly. Perpendicular to the central visual ray is the picture plane, which may be thought of as a piece of glass or the drawing paper or canvas itself. The observer's face will also be considered perpendicular to the central visual ray, hence always parallel to the picture plane. Keep this schema in mind.
Also recall the following points:
Lines and planes parallel to the observer's face (and consequently to the picture plane) undergo no distortion, but maintain their true directions and shapes.
Lines and planes which are not parallel to the observer's face (picture plane) appear to converge and foreshorten. (Such lines are sometimes described as "receding.")
The vanishing point for any set of receding parallel lines is the point at which the observer's sight line parallel to the set intersects the picture plane. To locate this point, the observer merely "points" in the same direction as the lines.
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