Drainage-aligned coordinates as a river-rendering parameterization

A coordinate transform that approximately straightens major rivers could double as a flow-aligned shader/UV parameterization. Treat one coordinate as downstream arc length and the other as signed cross-channel distance. This would simplify streaming normal/foam textures, procedural displacement, bank masks, width profiles, anisotropic highlights, and river-specific LOD. Existing river rendering supports the idea: arc length along a Bézier river is used for streaming texture coordinates #M78ELM distance to the curve defines river boundaries #QGGCCR and junctions blend coordinates from overlapping branches #KNWKGF Texture advection is otherwise difficult because a texture must move like a carpet around a curved track with spatially varying direction and speed #W32P5Z The transform would be a rendering parameterization, not a replacement for velocity simulation; junction singularities, distortion, and inverse-map/Jacobian handling remain important. Coarse 3D displacement is compatible with a 2D river animation model #YTDJGS