Ink outlines

The ink outline around a level’s meshes is not a post-process edge detector. It is an inverted hull: the level meshes are drawn a second time with the opposite face culling and an outline flag set in the shader. Observed · STUNTBOOST The shell is pushed outwards by the thickness in the vertex shader, so the back faces show around the silhouette. Inferred

How it is drawn

DrawLevel.DrawWithOutline redraws the level meshes with CullCounterClockwise and ShaderRender.InverseHullOutlines = true. The shader’s vertex constants include OutlineThicknessOn0Meters and OutlineThicknessOn3Meters, InverseHullOutlines and VertexShiftEnabled; the thickness comes from settings that give 0.05 at 0 m and 2 at 3 m from the camera (the game multiplies both by a run-time scale each frame), so the outline grows with distance. Observed · STUNTBOOST

The outline colour is the surface colour times (0.30, 0.18, 0.15); the constant is in the compiled pixel shader. Observed · STUNTBOOST

A separate effect, Effects/D3/Outlines.fx, is a depth-edge post pass. Observed · STUNTBOOST

The per-vertex shift

With VertexShiftEnabled the shader offsets each vertex of the hull by a per-vertex attribute, _COLOR_SHIFT, which the exporter writes into the glTF as an application-specific attribute (two bytes per vertex) to make room for thick outlines. Observed · STUNTBOOST The exporter encodes the shift vector in colour space as X × 0.1 + 0.5 per component. Sourced · StuntboostTools

The exporter’s collection options set it: outline_shift=X|Y|Z sets the attribute of every vertex; outline_shift_relative adds to it; outline_shrink adds a gradient from 0 at the bottom to the given vector at the top (in local space); an empty named outline_shift or outline_shrink supplies the direction from its local up vector; and outline_max_thickness=X limits the thickness for a collection (default 1; 0 means no outline). Sourced · StuntboostTools