The standard shadow buffer (now called "Classical") required the user to carefully set a "Bias" value. This bias is an offset value in shadow calculations, to prevent faces from self-shadowing. This self-shadowing inaccuracy is most noticeable when a shadowbuffer is relatively small (or far away), because 1 pixel in a shadowbuffer (one Z value) maps to a large region in the rendering.

The disadvantage of a large bias value however, is that precision gets lost. Faces that have a short distance don't cast shadow anymore. Click in the images to the side and use back and forward commands of your browser to see the differences between them.

A proven method to solve this error is by rendering a Shadowbuffer with two values; the nearest Z value, and the 2nd nearest Z value. In the end these two Z values then gets averaged, efficiently removing the biasing errors. Only truly extreme cases will still need bias adjustments.

However, this method only works properly when a 3d scene has been created without faces that overlap, in that case the '2nd nearest Z' is identical to the nearest Z, and biasing will occur again.

For that reason, and for backwards compatibility, this enhancement is a new option in the "Shadow and Spot" Panel, named as buffer type "Classic-Halfway" (known in other software as "midpoint"). Because it gives much better results, it has been made the default shadow method.

Also note that for the "Halo Step" option, this method doesn't always work properly. Especially when using planes (not volumes) you will notice errors due to using the halfway z value.