Previously, I went at some lengths to explain a concrete method of drawing a laying square at any perspective. The reason this was done was in order to establish a groundwork for the student to begin to visualize the square as the base of a cube. A cube can me made from the laying square by drawing vertical lines up from the corners, and by adding an identical laying square as the lid. Alter the height and width dimensions, and one can easily draw various Rectangular Prisms in perspective.
The Rectangular Prism in 3-Dimensional space is the primary shape Loomis encourages us to become familiar with from any angle. The reason being, is that this shape will enclose any other shape which exists. Even the perfect sphere will fit in the perfect cube. Some shapes are long, some tall, many are irregular; but when one visualizes a Rectangular Prism bounding what one is trying to draw around the object, the artist takes the first step to being able to draw that object with mass and perspective.
If one does not use a beginning framework of Rectangular Prisms to work out the perspective of the various objects within a composition, the shortcomings will be evident to viewers on account of, again, ‘Intelligent Perception’. Stated differently, there is no way to “fake” correct perspective, so it does one best to work it out properly at the beginning.
Setting up a a geometric shape as a shorthand for an organic shaped object has further uses beyond perspective and understanding mass. An additional use is in aid of the artist seeking to establish values. When an artist is trying to work out his values (lights and darks) within a picture, it is much easier -and more appealing visually- if the gradation of light to dark is done in a series of planes, rather than rendered as one continuous blend. The ability to see form as a series of interconnected geometric shapes is essential to rendering transition of values in planes. This treatment of light and dark creates an aesthetic which the photograph cannot duplicate, and thus, it remains the domain of visual artists.