The Z coordinates are treated in the same fashion as the x and y coordinates. After transformation, clipping and perspective division, they occupy the range -1.0 through 1.0. The glDepthRange() mapping specifies a transformation for the z coordinate similar to the viewport transformation used to map x and y to window coordinates. The glDepthRange() mapping is somewhat different from the viewport mapping in that the hardware resolution of the depth buffer is hidden from the application. The parameters to the glDepthRange() call are in the range [0.0, 1.0]. The z or depth associated with a fragment represents the distance to the eye. By default the fragments nearest the eye (the ones at the near clip plane) are mapped to 0.0 and the fragments farthest from the eye (those at the far clip plane) are mapped to 1.0. Fragments can be mapped to a subset of the depth buffer range by using smaller values in the glDepthRange() call. The mapping may be reversed so that fragments furthest from the eye are at 0.0 and fragments closest to the eye are at 1.0 simply by calling glDepthRange1.0,0.0(1.0,0.0). While this reversal is possible, it may not be practical for the implementation. Parts of the underlying architecture may have been tuned for the forward mapping and may not produce results of the same quality when the mapping is reversed.
To understand why there might be this disparity in the rendering quality, its important to understand the characteristics of the window coordinate z coordinate. The z value does specify the distance from the fragment to the plane of the eye. The relationship between distance and z is linear in an orthographic projection, but not in a perspective projection. In the case of a perspective projection, the amount of the non-linearity is proportional to the ratio of far to near in the Frustum call (or zFar to zNear in the gluPerspective call). Figure 9 plots the window coordinate z value as a function of the eye-to-pixel distance for several ratios of far to near. The non-linearity increases the resolution of the z-values when they are close to the near clipping plane, increasing the resolving power of the depth buffer, but decreasing the precision throughout the rest of the viewing frustum, thus decreasing the accuracy of the depth buffer in this part of the viewing volume. Empirically it has been observed that ratios greater than 1000 have this undesired result.
The simplest solution is to improve the far to near ratio by moving the near clipping plane away from the eye. The only negative effect of doing this is that objects rendered close to the eye may be clipped away, but this is seldom a problem in typical applications. The position of the near clipping plane has no effect on the projection of the x and y coordinates and therefore has minimal effect on the image.
In addition to depth buffering, the z coordinate is also used for fog computations. Some implementations may perform the fog computation on a per-vertex basis using eye z and then interpolate the resulting colors whereas other implementations may perform the computation for each fragment. In this case, the implementation may use the window z to perform the fog computation. Implementations may also choose to convert the computation into a cheaper table lookup operation which can also cause difficulties with the non-linear nature of window z under perspective projections. If the implementation uses a linearly indexed table, large far to near ratios will leave few table entries for the large eye z values. This can cause noticeable Mach bands in fogged scenes.