Introduction to SSS

Subsurface Scattering (SSS) is a rendering technique used to simulate how light travels through translucent surfaces, such as skin, cloth, hair, and wax, to name a few. SSS has been integral to Toolbag since day one and has now since received a powerful upgrade.

SSS in Toolbag is split up into two shading models: Subsurface Scattering and Subsurface Volumetric. Scatter, Fuzz, and Mask maps can also be authored in Texture Projects. Learn more below about Toolbag 4’s SSS models.

For this demonstration, we’ll be using a character by Vadim Sorici.

Where is my SSS module?

Subsurface scattering can be found in the Transmission module and the SSS controls have a single Scatter Depth input. Toolbag’s subsurface shading models take mesh thickness into account, so there is no need to apply a thickness map. You can add thickness information to the Scatter Map if you like; however, it is not expressly necessary. You’ll notice a Mask slot as well, which blends between Transmission and Diffusion.

Here’s a comparison of the two scattering models’ ray-traced results at different Scatter Depth settings. Generally, 1mm produces a realistic result. But you can use higher values for a more exaggerated or waxy effect. You can also author a Scatter Depth texture and vary the brightness if you want more or less scattering in different areas. For instance, you may want less scattering in areas with heavy makeup.

Note: If the Scatter Depth slider doesn’t seem to do much, check your scene scale. You can show the Scale Reference guide in the per-Viewport options (gear icon in each VP). To adjust scale globally, go to Scene, and adjust the Imported Units. If your 3D app is set up to model in CM, setting Imported Units to CM usually sorts this out.

Subsurface Scattering and Volumetric are approximated in Raster mode (Ray Tracing off). This uses the old Toolbag 3 SSS model with some tweaks to accommodate the new shading parameters.

Subsurface Volumetric uses the Random Walk model, which is more of a brute force method. This shading model tends to be very accurate but slower to render and can cause complications with low polygon meshes and thin surfaces like hair cards.

Raster (RT Off)Ray Tracing

Here’s a closer look at comparing Scattering VS Volumetric. Note how Subsurface Volumetric causes faceting on low-density geometry. This will be more apparent on very low poly meshes as well.

Subsurface ScatteringSubsurface Volumetric

Ray Tracing On!

Here’s an example lit with a studio-lighting Sky. SSS models use Sky lighting for scattering effects (previously, translucency only worked with direct lights). Ray Traced Sky occlusion/shadowing also makes a huge difference, and now it’s easy to create nice-looking lighting with HDRI Skies and no additional lights.

Ray Tracing OffRay Tracing On

Subsurface Scattering in Video

Get an in-depth view of SSS in Toolbag in our Advanced Shaders episode of Learn Toolbag 4.

Thanks for reading. For more tutorials, check out our Tutorials & Resources section and follow along using our 30-day trial of Toolbag.