The subject matter of the theory of images. Drawing a plane figure is not geometrically difficult because the image drawn is either an exact copy of the original or a similar figure, e.g. the drawing of a circle looks like the original circle. Drawing geometric solids is quite a different matter. Unfortunately, there are no "spatial pencils" which can trace an object in the air. Such a pencil would "draw" a cube by tracing along its edges. Hence, we have to sketch a cube on paper with an ordinary pencil. A plane image will never be an exact copy of a solid and, therefore, a certain routine ought to be followed in drawing a solid that would create an image of the original in the best way.

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