Geodecals

A geodecal (also written geo decal, short for geometry decal) is a decal made of real geometry: a thin strip or small patch of polygons that floats just above a surface and shows part of a decal or trim texture. It is not an engine feature with its own documentation. It is artists' shorthand, and in most places it means the same thing as a mesh decal.

On Polycount, for example, an artist describes his spaceship as "Lots of floating geo decals and tiling textures" (Polycount: Star Swarm space fighter craft). Another team calls the same idea "decal mapping planes" and says it is "a similar method used in Star Citizen and Alien Isolation" (Polycount: Starfighter Inc. - Creating Next-gen…). Epic's documentation uses the term mesh decal (Unreal Engine documentation: Using Mesh Decals in Unreal Engine).

#How they differ from projected decals

A projected decal is a box that throws a material onto whatever is inside it. A geodecal is a mesh with its own UVs, so it can wrap around edges and follow curves where a projection box would "shear and distort" (Unreal Engine documentation: Using Mesh Decals in Unreal Engine).

#How they are made

The workflow on Polycount and in later breakdowns is consistent:

1. Build the strip. Duplicate faces from the base mesh or draw new strips that hug it. Hussain Dabhiya used Maya's retopology tools so the strips would "wrap closely with the actual model" (Medium: Sentinel Drone: Workflow).
2. UV it onto an atlas. He UV'd them "the same way we UV trim meshes", onto a normal trim sheet (Medium: Sentinel Drone: Workflow).
3. Offset it slightly. Obscura's rule: "a little offset, yes, but not too much", or the decal will "swim" (Polycount: Decal technique from Star Citizen). In Unreal, a small depth bias helps and World Position Offset can push a decal outwards (Unreal Engine documentation: Using Mesh Decals in Unreal Engine).
4. Blend it. Either alpha-test it (cheap, hard edges) or render it as a decal that changes only some channels of the surface below. Dabhiya set Unreal's material domain to Deferred Decal with a translucent or modulate blend (Medium: Sentinel Drone: Workflow); Epic notes that mesh decals blend softly but "you don't get shadows or proper occlusion" (Unreal Engine documentation: Using Mesh Decals in Unreal Engine).

#Three uses of the term

#Pros and cons

• Pros: crisp detail at any size from a small atlas; no unique bake; works on curves; fewer draw calls than large projected decals, and flat back-facing decals cover no pixels (Unreal Engine documentation: Using Mesh Decals in Unreal Engine).
• Cons: extra vertices; offsets to manage; Unreal gives mesh decals no artist-set sort order (Unreal Engine documentation: Using Mesh Decals in Unreal Engine); and in Blender's forward renderer they are opaque, with "no mixing" (machin3.io: DECALmachine FAQ).

#Further reading and watching

• See also mesh decals, decals, trim sheets, normal map, weighted (custom) normals and floaters.