Parallel projection

A parallel projection is a projection of an object in three-dimensional space onto a fixed plane, known as the projection plane or image plane, where the rays (e.g. lines of sight or projection lines) are parallel to each other. It is a basic tool in descriptive geometry. The projection is called orthographic if the rays are perpendicular (orthogonal) to the image plane, and oblique or skew if they are not.

A parallel projection is a particular case of projection in mathematics, that can be seen as the limit of a central or perspective projection (where the rays pass through a fixed point, called the center or viewpoint), as the center or viewpoint is moved towards infinity. Put differently, a parallel projection corresponds to a perspective projection with an infinite focal length (the distance from the lens to the focal point in photography), or "zoom". In parallel projections, lines that are parallel in three-dimensional space remain parallel in the two-dimensionally projected image.

A central or perspective projection of an object is often considered more realistic than a parallel projection, since human vision and photography work in this fashion. However, parallel projections are often popular in technical applications, since the parallelism of an object's lines and faces is preserved in the resulting image. Among parallel projections, orthographic projections are the most realistic, and are commonly used by engineers. On the other hand, certain types of oblique projections (for example cavalier projection, military projection) are very simple to implement, and are used to create quick pictorials of objects.

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