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Table Of Contents

Overview

Sub-Surface Scattering

This section covers Sub-Surface Scattering (SSS) across your entire scene, with options allowing you to quickly override the SSS mode and quality settings from one area.

Override Mode

Allows you to override all Sub-Surface Scattering effects in your scene from the following options:

  • None - No override, use the sub-surface scattering mode specified at the material level for each shader.
  • Ray-Traced - Override all materials to use Ray-Traced sub-surface scattering.
  • Point-Based - Override all materials to use Point-Based sub-surface scattering.

You can use either point-based or ray-traced multiple sub-surface scattering in Redshift and both have their own advantages and disadvantages. In quite a few cases the techniques should look similar to each other but there are situations where the differences will be readily apparent, this is primarily due to the normalization of light that occurs when using point-based mode. Ray-traced mode does not normalize the light which can lead to differences that are most notable on thin objects or objects with more surface detail.

Please note that due to the differences in the two modes the final result can differ when comparing progressive ray-traced SSS to bucket rendered point-based SSS. A comparison of the pros and cons for each SSS type is provided below:


Ray-Traced SSS

  • Pros
    • More detailed and accurate
    • Works in Progressive Mode
    • Ability to isolate SSS effect between objects
  • Cons
    • Slower and noisier
    • More samples needed for clean results


Point-Based SSS

  • Pros
    • Much faster and smoother
  • Cons
    • Less detailed and accurate
    • Does not work in Progressive Mode
    • Requires a "prepass" stage
    • Higher chance of flickering in difficult lighting situations
    • Not possible to isolate SSS effect between objects which can result in unnecessary "light bleeding" artifacts.


Mode: Point-Based

Render Time: 13s

Ray-Traced

1m 11s

Point-Based SSS with Progressive Rendering

If your scene is setup to use point-based SSS shaders and you render in progressive mode it will automatically use ray-traced SSS during progressive renders.

This is done so you can actually preview the SSS effect in progressive mode (and not just the diffuse texture) and tweak settings interactively - Point-Based SSS will still be used for the final rendering when set to the Bucket render mode.

The enclosed parameters only apply to Point-Based Sub-Surface Scattering.
Ray-Traced sub-surface scattering quality is bound exclusively by unified sampling parameters and material level ray-traced SSS settings.

Mode

Specifies the sub-surface scattering computation Mode to be used from the following options:

  • Rebuild (don't save) - Redshift will compute new SSS points from scratch (for each frame) but will not save it to disk. The frame will be rendered to completion.
  • Rebuild (prepass only) - Redshift will compute new SSS points from scratch (for each frame) and will save it to the user-specified file. The final rendering pass will be skipped.
  • Rebuild - Redshift will compute new SSS points from scratch (for each frame) and will save it to the user-specified file. The frame will be rendered to completion.
  • Load - Redshift skips the SSS computation stage and the data is loaded from the user-specified file in the parameter below. The frame will be rendered to completion.

Point-Based sub-surface scattering data is view-dependant which means that it has to be re-generated when either the camera or any objects move. It also has to be regenerated if lights change position or intensity and if materials are adjusted. However, there are a few settings that do not affect the Point-Based SSS points:

  • All antialiasing settings
  • Any parameter that has to do with "number of samples."

If you are making any last-minute adjustments to your frame and tweaking these kinds of parameters you can save some time by re-using the sub-surface scattering you computed last time.

File

Specifies where to save or load the sub-surface scattering data depending on the chosen sub-surface scattering mode as covered below.

  • Rebuild (prepass only) - Save sub-surface scattering data to the user-specified file.
  • Rebuild - Save sub-surface scattering data to the user-specified file.
  • Load - Load sub-surface scattering data from the user-specified file.

Rate

Controls the spacing of the Point-Based SSS points by setting the number of samples used per pixel.

Similar to antialiasing, the more samples-per-pixel you use the more points will be created. More points can help catch small details but they also take longer to compute and use more memory.

It is recommended that, if you get visual artifacts you try a setting of "Medium (1 sample per pixel)" first and then raise it to "High (4 samples per pixel)".

Please note that for the example images below the radius of the SSS effect was set low to exacerbate the issues of setting the Point-Based SSS Rate value too low.

Rate: Low

Dark and suffers from splotchiness.

Medium

The cones look brighter (this is a more correct result) but there are still splotches in the thinner parts of the geometry (the tips of the cones).

High

The splotches are improved – especially near the tips of the cones.

Interpolation Quality

Controls the the quality of the final render by determining how many SSS points are used to produce the final lighting at each pixel.

The higher the setting, the more points are used and the smaller the chance of any interpolation artifacts appearing on the surfaces of objects at the expense of slower render times.

We recommend leaving this setting to its default "Low" value and only use "Medium" or "High" if you observe SSS artifacts that look like 'lighting rings' or excessive flickering in animations.


GI Rays

Controls how many GI rays are scattered through objects using point-based SSS if global illumination is enabled.

If Global Illumination is enabled, GI lighting will also be computed for each SSS point. The "GI Rays" parameter is similar to the Brute-Force GI or Irradiance Cache GI "Rays" settings.

Since subsurface scattering is generally a soft effect, it can often work with relatively few GI rays whereas GI on hard surfaces might require several hundred (or even thousands) rays. It is, therefore, recommended that you start with fairly low numbers such as the default 128 and raise this only if you get persistent lighting artifacts on subsurface scattering surfaces.


Cut-off Thresholds

For advanced users only! Incorrect settings can easily generate very noisy images or long rendering times!

The Cut-off Threshold set of parameters covered below allow the user to specify minimum values that are considered "black" and allow the renderer to terminate tracing earlier. 

As rays go through reflections, refractions or shadows, they get tinted and become dimmer. For example, a ray coming from the camera might hit a very faint mirror. Whatever is reflected on that mirror will be dim. When the rays become dim, they contribute very little to the final image so there is no point for the renderer to keep bouncing them around. 

The defaults should work fine for most scenes but, in some extreme cases, a very dark mirror might be reflecting an extremely bright piece of geometry. If the renderer "cuts off" the mirrored rays early, grain will appear. If you suspect this is happening to your scene, you can try lowering these numbers. To disable this optimization altogether, set the values to 0.0.

Diffuse

Sets the minimum value for diffuse ray cut-offs.

Higher values result in more dim rays being cut-off while lower values result in fewer dim rays being cut-off.

Reflection

Sets the minimum value for reflection ray cut-offs.

Higher values result in more dim rays being cut-off while lower values result in fewer dim rays being cut-off.

Refraction

Sets the minimum value for refraction ray cut-offs.

Higher values result in more dim rays being cut-off while lower values result in fewer dim rays being cut-off.

Shadow

Sets the minimum value for shadow ray cut-offs.

Higher values result in more dim rays being cut-off while lower values result in fewer dim rays being cut-off.

Direct Lighting

Sets the minimum value for direct lighting cut-offs.

Higher values result in more dim rays being cut-off while lower values result in fewer dim rays being cut-off.

Russian Roulette

Importance Threshold

Falloff

Ray Tracing Acceleration

Complete Construction Before Rendering

Max Leaf Primitives

Fast Preprocessing