The goal of this PR is to remove any user facing parameter
bias that fixes issues that are caused by inherent nature of
the shadow map (aliasing or discretization).
Compute shadow bias in the normal direction to avoid
both shadow leaking at certain angles or shadow
acnee.
This bias is computed automatically based on the minimum
bias amount required to remove all errors. The render
setting is removed as of no use for now.
#### Normal bias
We do the bias in world space instead of shadow space
for speed and simplicity. This requires us to bias using
the upper bound of biases for the same location in space
(using biggest texel world radius instead of UVZ bias).
This isn't much of an issue since the bias is still less
than 2 texel. The bias is still modulated by facing
ratio to the light so surfaces facing the light have no
biase.
This fixes both self shadowing and flat occluders aliasing
artifacts at the cost of moving the shadow a bit on the
side. This is the blue arrow in the diagram.
We always bias toward the normal direction instead of the
light direction. This is alike Cycles geometric offset for
the shadow terminator fix. This is better since it does'nt
modify the shading at all.
#### Slope bias
To avoid aliasing issue on zero slope receiver, we still
have to use the slope bias with a size of 1 pixel.
#### PCF filtering
We now parametrize the filter around the normal instead of
using the shadow map local space. This requires to use
a disk filter instead of box, which is also more pleasant
for most light shapes (all except rectangle lights).
Setting the filter around normal avoid overshadowing from
zero slope occluders. This cannot be fixed by more slope bias
in light space PCF. We could fix it in light space by projecting
onto the normal plane but that gives an unbounded bias when `N.L`
is near 0 which causes either missing shadows or self shadow if
using an arbitrary max offset value.
To avoid overshadowing from any surface behind the shading
point, we reflect the offset to always face the light.
Doing so instead of using the perpendicular direction
is better for very sharp geometric angles, has less
numerical precision issue, is symetrical and is cheaper.
To avoid any self shadowing artifact on zero slope receivers
with angled neighbors (like a wall and the floor), we have
to increase the slope bias according to the filter size.
This might be overkill in most situation but I don't feel
this should become a setting and should be kept in sync
with the filter. If it has to become an option, it should
simply a factor between unbiased filter and best bias.
#### Shadow terminator
The remaining artifacts are all related to shadow terminator
one way or another. It is always caused by the shading
normal we use for biasing and visibility computation not
being aligned with the geometric normal.
This is still something we need a setting for somewhere.
Pull Request: https://projects.blender.org/blender/blender/pulls/121088
bb44bce95c