A decal is an image projected onto surfaces — a stain, a poster, a bullet hole, a warning label. It sits on top of whatever is underneath.
Why they matter
Tiling and modular construction produce repetition. Decals are the cheapest way to make repeated geometry feel specific: the same wall module reads as three different walls with different grime, cracks and signage on each.
Kinds
- Deferred decals, projected in screen space onto whatever geometry is behind. Flexible, cheap, and can be placed anywhere at runtime.
- Mesh decals, small pieces of geometry with transparent materials hovering just above a surface. More control, more draw calls, and z-fighting risk if placed too close.
Where they earn their cost
- Contact grime where surfaces meet the floor
- Water streaks under ledges
- Wear on edges and around handles
- Signage, numbers, labels
- Damage — impacts, cracks, scorch marks
- Breaking up any surface that tiles visibly
Costs to watch
Decals are transparent surfaces, so they contribute overdraw. A wall covered in overlapping decals can be more expensive to render than the wall. On mobile and VR this matters considerably.
Practical discipline
- Build a decal library and reuse it. Bespoke decals per location defeat the purpose.
- Keep them small; large decals cost more overdraw and reveal their projection.
- Vary rotation and scale so the same decal does not read as a repeated stamp.
- Watch the projection angle — a decal projected onto a surface at a steep angle stretches badly.
With generated assets
Generated props arrive with their detail already in their texture, so decals are less about them and more about the environment they sit in. The relationship is the other way round: decals are what stop a scene of generated props from reading as a collection of separate objects dropped onto a clean surface.