3D graphics eBook - Course Materials Repository

Z-fighting 252
Z-fighting
Z-fighting is a phenomenon in 3D rendering that occurs when two or
more primitives have similar values in the z-buffer. It is particularly
prevalent with coplanar polygons, where two faces occupy essentially
the same space, with neither in front. Affected pixels are rendered with
fragments from one polygon or the other arbitrarily, in a manner
determined by the precision of the z-buffer. It can also vary as the
scene or camera is changed, causing one polygon to "win" the z test,
then another, and so on. The overall effect is a flickering, noisy
rasterization of two polygons which "fight" to color the screen pixels.
This problem is usually caused by limited sub-pixel precision and
floating point and fixed point round-off errors.
The effect seen on two coplanar polygons
Z-fighting can be reduced through the use of a higher resolution depth buffer, by z-buffering in some scenarios, or by
simply moving the polygons further apart. Z-fighting which cannot be entirely eliminated in this manner is often
resolved by the use of a stencil buffer, or by applying a post transformation screen space z-buffer offset to one
polygon which does not affect the projected shape on screen, but does affect the z-buffer value to eliminate the
overlap during pixel interpolation and comparison. Where z-fighting is caused by different transformation paths in
hardware for the same geometry (for example in a multi-pass rendering scheme) it can sometimes be resolved by
requesting that the hardware uses invariant vertex transformation.
The more z-buffer precision one uses, the less likely it is that z-fighting will be encountered. But for coplanar
polygons, the problem is inevitable unless corrective action is taken.
As the distance between near and far clip planes increases and in particular the near plane is selected near the eye,
the greater the likelihood exists that you will encounter z-fighting between primitives. With large virtual
environments inevitably there is an inherent conflict between the need to resolve visibility in the distance and in the
foreground, so for example in a space flight simulator if you draw a distant galaxy to scale, you will not have the
precision to resolve visibility on any cockpit geometry in the foreground (although even a numerical representation
would present problems prior to z-buffered rendering). To mitigate these problems, z-buffer precision is weighted
towards the near clip plane, but this is not the case with all visibility schemes and it is insufficient to eliminate all
z-fighting issues.