A demonstration of the basic principles.

The word perspective is derived from the Latin for to look (spectare) and through (per). If you look through a transparent sheet to an object, the shape traced on the sheet is the perspective view of the object. It turns out, however, that the shape on the sheet has its own geometric properties. In particular, lines which are parallel on the object (such as the top and bottom of a cube) create lines on the traced shape which converge on a single point - the vanishing point. This program illustrates how these lines relate and interact.

The diagram shows a simple perspective setup in 3d view, elevation, and plan. It includes a viewpoint (the spherical eyeball), an object (the cube), and a transparent sheet between them. The top three buttons on the right control the visibility of lines - the connecting lines between eye and object (red), the traced drawing lines on the transparent sheet (yellow), and the vanishing lines extending the drawing lines to the vanishing points (green). You can move the eye or the cube by clicking on them in the 3d view (they will turn red) and dragging them. You can also adjust the cube using the middle buttons. You can swivel the scene by dragging anywhere in it.

The perspective picture drawn on the transparent sheet can sometimes look a little odd, especially with a very oblique viewing angle. The proof that it is correct lies in whether it corresponds with the actual view of the object. Clicking the move view position to eyeball button will show you that it does, as long as you are looking from the viewpoint itself.