Introduction

The three stages of the viewing transformation

There are three conceptual stages in the 3D viewing process: clipping, projection and the transformation to 2D screen coordinates. This tutorial will cover the projection process.

Projection is the transformation of an n-dimensional coordinate system to an m-dimensional system (where m is less than n). In computer graphics applications (such as CAD and virtual reality) the most common type of projection is the transformation of 3D dimensional data to 2D data. This involves defining straight lines or `rays' called projectors which through points on the object being projected and through the projection plane to form the 2D image.

The most commonly used projections are the parallel projection and the perspective projection.

Parallel projection

In this, the projection lines are all parallel to each other. This means that the image formed on the projection plane has the same relative dimensions as that of the original object (i.e. there is no concept of distance from the viewer). This feature is important in applications such as CAD where relative size must be apparent in the projected image.

Perspective projection

This type of projection includes the concept of distance from the viewer by scaling the projected image of objects by an amount relative to their distance from the view reference point.

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