VRayFastSSS is a material that calculates a
sub-surface scattering effect. Since it uses a simplified algorithm, it is
typically faster than the sub-surface scattering option of
VRayMtl and may be easier to control. Note that
VRayFastSSS does not include diffuse and glossy
effects. To add these, create a
VRayBlendMtl material with VRayFastSSS as a
base material and a
VRayMtl
as a coat material.
VRayFastSSS simulates the effect of three
sub-surface layers - a shallow layer over a deep layer over a
back-scattering layer. You can control these separately to achieve different
looks for the surface.
![[VRayFastSSS parameters]](vray_for_3ds_max_manual/vrayfastsss_material/vrayfastsss.png)
Prepass rate -
VRayFastSSS approximates the sub-surface scattering by precomputing
the surface lighting at different points on the surface and then
interpolating between these points. This parameter determines the resolution
at which surface lighting is precomputed during the prepass phase. A value
of 0 means that the prepass will be at the final
image resolution; a value of -1 means half the
image resolution, and so on. For high quality renders it is recommended to
set this to 0.
Interpolation samples - this parameter
determines how many samples will be used when smoothing the surface
lighting. For very translucent surfaces, there must be enough samples to
blur the lighting across a large portion of the surface and so you may need
to increase this parameter. Less translucent surfaces can do with fewer
samples, which also makes the calculations faster.
Diffuse roughness - this parameter
controls the surface roughness for the material. A value of
0.0 produces a diffuse material, while higher
values give the surface a translucent quality.
Shallow radius - this determines the
radius (in scene units) of light scattering for the first (shallow)
sub-surface layer. Smaller values produce less blurring of the surface
lighting and are faster to calculate. Larger values blur the lighting. Note
that increasing this parameter may also require you to increase the
Interpolation samples.
Shallow color - the color for the shallow
sub-surface layer. Note that this color is also used as a filter on top of
the Deep color and the
Back color (e.g. if the
Shallow color is pure white, no deep- or
back-scattering is calculated). The color can also be controlled with a
texture map.
Deep radius - determines the radius (in
scene units) of light scattering for the second (deep) sub-surface layer.
Typically, this value would be several times larger than the
Shallow radius. Note that increasing this
parameter may also require you to increase the
Interpolation samples.
Deep color - the color of the deep layer.
Note that this color is also used as a filter on top of the
Back color (e.g. if the
Deep color is pure white, no back-scattering
will be visible). In a similar manner, the Shallow
color is used as a filter for the Deep one.
The color can also be controlled with a texture map.
Backscatter depth - a maximum depth (in
scene units) at which the backscatter effect will be visible. A value of
0.0 produces no back-scattering.
Back radius - the radius (in scene units)
for the back-scattering effect. Larger values blur the back-scattered light,
but may be slower to calculate. Increasing this parameter may also require
you to increase the Interpolation samples.
Back color - the color for the
back-scattering effect. Note that both the Shallow
color and Deep color are used as
filters on top of the Back color.
![[VRayFastSSS example]](vray_for_3ds_max_manual/vrayfastsss_material/tn_vrayfastsss1.png)
