### Tuning

Properties of the algorithm control the quality and complexity
of the rendering. The calculation of subsurface scattering is
broken into three components: single scatter, multiple scatter, and
diffusion. Depending on the optical properties of the material, the
contributions of each component may dominate or be negligible. An
important factor in determining this is the mean free path length
of the material. This is the average distance a photon will travel
in the material without scattering or absorption and is equal to
the reciprocal of the sum of the scattering and absorption
coefficients.

- depth
- The shader uses two different ways to scatter photons. After a
certain number of scatterings a photon is scattered less precisely,
yet faster. The depth determines how many scattering events are
done using the precise scattering. The precise scattering is also
called "shallow layer". The depth determines the end of the shallow
layer and the begin of the deep layer.
- max_samples
- is the maximum number of samples evaluated by the single and
multiple scatter components. Typical values range between 10 and
30.
- max_photons
- is the maximum number of photons to sample per lookup. It is
similar to the global illumination accuracy parameter. Typical
values range between 100 to 1000. Values up to 5000 are useful for
scenes with uneven photon distribution.
- max_radius
- is the maximum radius of a sphere centered at a sample point
from which to collect photons. It is similar to the global
illumination accuracy radius. Typical values are a small percentage
of the total size of the object.
- approx_diffusion
- this switch turns the diffusion component of the calculation on
or off. Usually it should be on.
- approx_single_scatter
- this switch turns the single scatter component of the
calculation on or off. Usually it should be on.
- approx_multiple_scatter
- this switch turns the multiple scatter component of the
calculation on or off. Usually it should be on.
- lights
- list of lights directly linked to this shader.
- mode
- determines which lights should be used, see light lists.

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