Subsurface Scattering in Toolbag 4

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 4 is now 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 brand new SSS models.

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

Where is my SSS module?

Toolbag 3 users might notice that we’ve shifted things around in this version of Toolbag, and scattering can now be found in the Transmission module. The SSS controls have been revamped to have a single Scatter Depth input. Toolbag’s subsurface shading models take mesh thickness into account, so there is no longer a need to apply a thickness map. You can add thickness information to the Scatter Map if you like; however, it is not expressly necessary. We’ve added a Mask slot as well, which blends between Transmission and Diffusion.

Here’s a comparison of the two new 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.

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.

Ray Tracing On!

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