Anisotropy
Direction-dependent reflection
Also known as: Anisotropic shading
Definition
Anisotropic shading produces reflections that stretch along a specific surface direction — the long horizontal highlight on brushed metal, the radial shimmer on a vinyl record, the streaked specular on hair. Standard PBR is isotropic (reflections are circular); anisotropic shading adds a tangent direction the highlight stretches along. Essential for hair, brushed metals, fabric with directional weave.
In production
In UE5: switch material shading model to Default Lit and enable Anisotropy + Tangent inputs. The tangent direction is usually derived from a flow-map texture aligned with hair strand or brushed-metal direction, since the default mesh tangent basis alone can't represent the fine per-pixel stroke direction needed for convincing anisotropic highlights. Unity's HDRP Lit shader exposes the same feature under "Anisotropy" with a tangent map input, commonly used for hair cards, guitar strings, and CD/vinyl-style circular highlights where the tangent flow is procedurally generated as a radial gradient. A common mistake is leaving the tangent direction at its default mesh-derived value on brushed metal, which produces a highlight that stretches in the wrong direction relative to the visible brush strokes baked into the albedo texture. Strand-based hair systems, like UE5's Groom system, compute anisotropic highlights natively per-strand using the Marschner hair shading model, a more physically accurate but far more expensive approach than a flat anisotropic shading model applied to hair card meshes.
