Parameter Settings

The shader decouples subsurface scattering into three types of calculations:

single scattering
multiple scattering
diffusion

The figure below shows how each of the three above types contributes to the overall effect:

Simplified diagram of single, diffuse and multiple approximation

Any of these three computations can have a dominant influence under differing optical parameters. For example, single scattering has dominance under low scattering conditions, whereas multiple scattering and diffusion approximation dominate under high scattering conditions, for example, where the scattering coefficient is much larger than the absorption coefficient.

Many parameters of the shader affect all three components, while few parameters act on specific components only. The table below shows a summary.

Parameter	Diffuse	Single	Multi
Depth	yes	no	yes
Max_samples	no	yes	yes
Max_radius	no	no	yes

The other two parameters which are correlated are the max_photons and max_radius. Photon averaging can be forced by increasing lookup radius. The figure below shows a series of images from the milk tutorial, where max_radius and max_photons parameters are increased to force photon averaging.

max_radius and max_photons increase from left to right.

Each instance of the misss_physical shader reports information about its rendered results. A sample information report is as follows:

    PHEN 0.2 info : "SkimMilk|Shader" channels 0  1  2
    PHEN 0.2 info : albedo: 99.80 99.80 99.26
    PHEN 0.2 info : coefficient of extinction: 0.70 1.22 1.91
    PHEN 0.2 info : mean free path length: 1.43 0.82 0.52
    PHEN 0.2 info : reduced albedo: 99.01 98.99 96.40
    PHEN 0.2 info : reduced coefficient of extinction: 0.14 0.25 0.39
    PHEN 0.2 info : reduced mean free path length: 7.07 4.06 2.54
    PHEN 0.2 info : critical depth: 0.00 0.00 0.00
    
    PHEN 0.2 info : "SkimMilk|Shader" photon statistics
    PHEN 0.2 info : received: 41328
    PHEN 0.2 info : channels 0 1 2:
    PHEN 0.2 info : stored: 44335 44932 43903
    PHEN 0.2 info : stored: 2155 2412 2566 isotropic
    PHEN 0.2 info : propagated: 2526 2266 2101
    PHEN 0.2 info : scatter depth:
    PHEN 0.2 info : min: 1.00 1.00 2.00
    PHEN 0.2 info : max: 69.00 82.00 64.00
    PHEN 0.2 info : mean: 12.75 12.78 12.66
    PHEN 0.2 info : variance: 59.85 64.61 57.77
    
    PHEN 0.2 info : "SkimMilk|Shader" multi-scatter component
    PHEN 0.2 info : samples: 79364
    PHEN 0.2 info : channels 0 1 2:
    PHEN 0.2 info : minimum: 0.0000, 0.0000, 0.0000
    PHEN 0.2 info : maximum: 19.9112, 31.2088, 53.3636
    PHEN 0.2 info : mean: 4.4254, 5.9017, 6.6080
    PHEN 0.2 info : variance: 12.2979, 28.7988, 50.9151
    
    PHEN 0.2 info : "SkimMilk|Shader" multi-scatter photons per sample
    PHEN 0.2 info : photons: 476184
    PHEN 0.2 info : minimum: 0.0000
    PHEN 0.2 info : maximum: 512.0000
    PHEN 0.2 info : mean: 503.7983
    PHEN 0.2 info : variance: 3104.0889
    
    PHEN 0.2 info : "SkimMilk|Shader" isotropic scatter component
    
    PHEN 0.2 info : samples: 39682
    PHEN 0.2 info : channels 0 1 2:
    PHEN 0.2 info : minimum: 0.0000, 0.0000, 0.0000
    PHEN 0.2 info : maximum: 6.4335, 8.1329, 5.3061
    PHEN 0.2 info : mean: 3.1183, 3.1236, 1.5807
    PHEN 0.2 info : variance: 2.1284, 3.0800, 1.3180
    
    PHEN 0.2 info : "SkimMilk|Shader" results
    PHEN 0.2 info : minimum: 0.1085, 0.1059, 0.1020
    PHEN 0.2 info : maximum: 32.8014, 35.6229, 29.9977
    PHEN 0.2 info : mean: 4.0739, 4.4873, 3.7535
    PHEN 0.2 info : variance: 6.6836, 9.3765, 10.9592
    PHEN 0.2 info : "SkimMilk|Shader" sampled 79364 single scatter sites

The seven main categories contained in the report are:

Optical properties of the medium that are derived from the shader parameters.
Photon statistics:
- Number of photons received from all sources, global illum and caustics.
- Number of photons and photon type stored per color channel.
- Number of photons propagated back into the scene.
- Scatter chain depth.
QMC scatter component statistics and irradiance.
Isotropic scatter component irradiance.
Final results.

The list below provides explanation for some of the photon statistics.

PHEN 0.2 info : "SkimMilk|Shader" photon statistics:

PHEN 0.2 info : received: 41328

Provides the number of photons that hit the object and invoked the shader.
PHEN 0.2 info : stored: 44335 44932 43903

The number of photons that were stored in the shallow layer for each color channel.
PHEN 0.2 info : stored: 2155 2412 2566 isotropic

The number of photons that were stored in the deep (isotropic) layer for each color channel.
PHEN 0.2 info : propagated: 2526 2266 2101

The number of photons that re-emerged from the object back into the scene for each color channel.