For each point of the object seen by the observer, a ray of light (line of sight) connects the object to the observer's eye. These lines of sight pass through object points on the picture plane before proceeding onto the retina of the eye. The image to be recorded on paper should be equivalent in angles and proportions to the image made by the object points. Thus, rendering a perspective view from observed reality is simply a matter of copying the proportions and angles as they appear on the imaginary picture plane window.
The proportions of an object can be taken from the picture plane window simply by viewing the object over a straightedge held at arm's length. By marking the lengths on the straightedge with the thumb, the height and width can be compared and transferred to the drawing at whatever scale is desired.
Hold the measuring arm straight so that it will always be the same distance from the eye. Remember that the straightedge represents the picture plane.
Objects diminishing in size as they recede from the observer appear to form angles relative to the picture plane. In order to determine ...