The next operation is called the projection transform. This operation determines how much of the view space will be rendered and how it will be mapped onto the computer screen. This region is known as the frustum and it is defined by six planes (near, far, top, bottom, right, and left planes), as shown in the following diagram:

These six planes are encoded in the Projection matrix. Any vertices lying outside the frustum after applying the transformation are clipped out and discarded from further processing. Therefore, the frustum defines clipping coordinates, and the Projection matrix that encodes the frustum produces clipping coordinates.

The shape and extent of the frustum determines the type of projection from the 3D viewing space to the 2D screen. If the far and near planes have the same dimensions, the frustum will then determine an orthographic projection. Otherwise, it will be a perspective projection, as shown in the following diagram:

Up to this point, we are still working with Homogeneous coordinates, so the clipping coordinates have four components: x, y, z, and w. The clipping is done by comparing the x, y, and z components against the Homogeneous coordinate, w. If any of them is more than, +w, or less than, -w, then that vertex lies outside the frustum and is discarded.