Bake cage

A bake cage is an inflated copy of the low-poly mesh that defines where the baker's rays start and which way they travel. Marmoset: "A cage is essentially a copy of your low poly that expands outward to enclose the high poly" (Marmoset: The Toolbag Baking Tutorial). Some tools call it a projection mesh or envelope; the Warframe guide treats "a cage or bake envelope" as "essentially equivalent" (Warframe Steam Workshop guide: Normal Map Baking).

#Why it matters

During a bake, rays travel from the cage inward to the low poly and record the high-poly surface they hit. The cage therefore controls two things:

• Distance. It must enclose the high poly, but only "just far enough to cover the highest points" (Marmoset: The Toolbag Baking Tutorial). "If the cage extends too far, it can cause problems with complex meshes that have overlaps or tight corners" (Marmoset: The Toolbag Baking Tutorial): rays catch nearby parts. Too close, and rays start inside the high poly and miss it.
• Direction. Rays travel along the cage's vertex normals. This decides whether hard edges cause gaps and whether details skew.

Without a cage, bakers use ray distances instead. Substance's frontal and rear distances have "no effect when a Cage is used" (Substance 3D Bakers documentation: Common parameters); Blender's Max Ray Distance "is only available when not using Cage" (Blender Manual: Render Baking).

#Averaged versus split cages

If your low poly has hard edges, its vertices have split normals. A cage built from those normals splits too. Marmoset describes this unsmoothed cage as one that "results in gaps or seams at hard edge borders and is generally not recommended" (Marmoset: The Toolbag Baking Tutorial). On Polycount, mikhga explains that without averaging "the cage will physically split at that edge, and cause artifacts along those edges" (Polycount: Mobile Asset and Optimization. Tips Please).

An averaged (smoothed) cage "ignores the low poly mesh normals when determining the projection direction" (Marmoset: The Toolbag Baking Tutorial). EarthQuake: "This method will ignore your lowpoly mesh normals", and with it "you can use hard edges along uv borders with no negative side effects" (Polycount forum: Making sense of hard edges, uvs, normal maps…). Substance's Average Normals option does the same, computing "the average normal of a vertex to know in which direction to send rays" (Substance 3D Bakers documentation: Common parameters).

#Making and editing a cage

• Marmoset Toolbag generates a cage automatically; you set its offset and can paint it. "By default the offset map is set to an even gray to provide room to sculpt the cage in both directions" (Marmoset: The Toolbag Baking Tutorial). You can also paint skew to correct details "recorded poorly due to an off-axis projection direction" (Marmoset: The Toolbag Baking Tutorial).
• Substance 3D Painter offers three methods: distance based ("inflate the vertices away from the mesh a uniform distance"), automatic (experimental), or a custom cage file, which must have "the same number of vertices as the base mesh" (Substance 3D Painter documentation: How to bake mesh maps). Red spots in the viewport mark cage intersections (Substance 3D Painter documentation: How to bake mesh maps).
• Blender uses Selected to Active with an optional Cage object; Cage Extrusion sets the "distance to use for the inward ray cast" (Blender Manual: Render Baking).
• Houdini Labs Maps Baker can visualise "the cage or tracing distance" when tracing along surface normals (Houdini documentation: Labs Maps Baker 5.0 geometry node).
• Manual cage: duplicate the low poly and "push out the vertices along their normal", as mikhga describes (Polycount: Mobile Asset and Optimization. Tips Please). Keep the vertex count and order identical.

#Common mistakes

• Cage too large: rays hit neighbouring parts (Marmoset: The Toolbag Baking Tutorial; Substance 3D Bakers documentation: Mesh parts bleed between each other).
• Cage too small: parts of the high poly poke through it and are missed.
• Split cage on a mesh with hard edges: seams along every hard edge (Marmoset: The Toolbag Baking Tutorial).
• Custom cage with a different vertex count from the low poly: it will not load correctly (Substance 3D Painter documentation: How to bake mesh maps).
• Not having a cage at all on complex shapes. The 80 Level guide: "Make sure to have a proper cage that envelops your whole high poly" (80 Level: Best Practices for Hard Surface Normal Map…).

#Related terms

baking · bake groups and exploded bakes · fixing baking artefacts · smoothing, hard edges and smoothing groups · the high-to-low workflow · normal map

#Further reading and watching