What is a Thickness Map and Why Does It Matter for Subsurface Scattering?
Thickness maps are one of the less well-known baked maps in game art — but they're essential for any asset that needs subsurface scattering. Without a thickness map, skin looks like painted plastic, leaves look like flat paper, and wax looks like stone. This guide explains why.
What is a Thickness Map?
A thickness map is a greyscale texture that stores how deep a mesh is at any given surface point — measured from that point straight through to the opposite surface. Think of it as measuring the material's depth: how much stuff light has to travel through to get out the other side.
The encoding is straightforward: white areas are thin — like the edge of a leaf or the tip of a finger — and dark areas are thick — like the centre of a character's torso or the solid body of a rock. The brighter the value, the less material there is between the front and back surfaces at that point.
What is Subsurface Scattering?
Subsurface scattering (SSS) is the way light behaves inside translucent materials. Rather than reflecting cleanly off the surface, light enters the material, bounces around inside it, and exits at a different point — slightly offset from where it entered, tinted by the material's internal color.
This is why human skin has a warm, soft glow that looks nothing like painted plastic. Light enters the skin, scatters through the tissue and blood vessels underneath, and exits nearby with a warm reddish tint. It's why ears look red when backlit, why fingertips glow orange when pressed against a torch, and why wax candles have that characteristic translucent warmth.
Without SSS
Light bounces directly off the surface. Skin looks like painted rubber — hard, opaque, flat. No warmth or translucency. Immediately recognisable as wrong at any normal viewing distance.
With SSS
Light enters the material and scatters internally before exiting. Skin has warmth and depth. Thin areas (ears, fingers) glow in backlit conditions. The material looks physically alive.
Any material that allows some light to pass through it benefits from SSS — skin, leaves, wax, marble, fabric, jellyfish, fruit, mushrooms. The thickness map is what tells the engine where that effect should be strong and where it should be absent.
How Thickness and SSS Work Together
The thickness map is the primary input that drives where subsurface scattering is visible and how intense it is at each point on the surface.
Strong SSS effect
Where the mesh is thin — earlobes, leaf edges, fingertips, nose tip — light can scatter all the way through the material relatively easily. These areas show the strongest translucency and glow most intensely when backlit. White values in the thickness map = maximum SSS.
Weak or no SSS effect
Where the mesh is thick — the torso, palm of a hand, thick trunk of a tree — light can't scatter through. These areas appear opaque even under strong backlighting. Dark values in the thickness map = little to no SSS.
The result is physically correct: the parts of a character that are anatomically thin glow in backlit conditions, and the parts that are anatomically thick don't. The thickness map produces this behaviour automatically, without any hand-painting of SSS intensity.
Common Use Cases
Thickness maps are most valuable for organic and semi-translucent materials. Here are the situations where you'll reach for one most often.
Ears, nose, fingertips, lips — thin anatomical areas show warm SSS. The thickness map identifies these automatically, no hand-painting needed.
Leaves and petals are thin throughout. High SSS makes them glow warmly in backlit conditions — essential for convincing trees and plants.
Translucent wax shows warm orange-red SSS near heat sources. Thin edges of a candle flame glow; the thick base doesn't.
Thin marble panels show subtle translucency — the characteristic property that makes marble look like marble rather than grey paint.
Thin fruit skin glows in backlit conditions. Mushroom caps, flower petals, alien tissue — any organic material benefits from SSS.
Thin fabric like silk or linen scatters light differently than thick fabric. SSS adds realism to close-up cloth rendering where the difference is visible.
Baking Thickness Maps
Thickness maps are baked by casting rays from each surface point inward through the mesh — in the direction opposite to the surface normal — and measuring how far the ray travels before it exits through the other side of the mesh. Short travel distance means thin geometry (light value). Long travel distance means thick geometry (dark value).
Setting up the engine — per engine
| Engine | Where to Connect the Thickness Map | Notes |
|---|---|---|
| Unreal Engine 5 | Subsurface Profile material → Opacity input | Set Shading Model to Subsurface Profile. Configure an SSS profile asset with the correct scattering radius and color for your material type (skin, plant, wax). |
| Unity HDRP | Lit Shader → Thickness Map slot | Enable Subsurface Scattering on the material. Assign an SSS Profile with appropriate scattering settings. Thickness map plugs directly into the dedicated Thickness slot. |
| Godot 4 | BaseMaterial3D → Backlight section | Godot uses a Backlight texture for a simplified SSS approximation. Connect the thickness map to the Backlight texture slot and set a backlight color to taste. |
Thickness Baking in Trumble
Trumble includes a thickness bake pass in its bake tool. Import your watertight mesh, run the bake, and the thickness map is generated alongside your normal, AO, curvature, and height maps — all in one pass. No additional configuration is needed beyond ensuring the mesh is closed before import.
The thickness map exports as part of your engine texture pack. For skin and organic assets, Trumble's character workflow is: bake all maps including thickness, paint PBR channels in the texture tool, export the full pack including the thickness map, and connect it to the SSS slot in your engine's material.
Thickness bakes as part of the standard map pass. Requires a watertight mesh. Outputs alongside normal, AO, and curvature automatically.
Check that thin areas (ears, fingertips, leaf edges) are bright and thick areas (torso, trunk) are dark. Inverted output indicates an issue with mesh normals.
Included in the full texture export. Connect it to the SSS or Thickness slot in your engine material alongside the rest of the Trumble texture pack.