Gouraudshading

Gouraud shading is a shading technique used in computer graphics to render smooth lighting on polygonal surfaces. It computes lighting at the vertices of each polygon using a local illumination model, typically including ambient, diffuse, and specular components. The resulting color at each vertex is then interpolated across the polygon's interior, producing a smoothly shaded appearance without computing lighting for every pixel.

Named after Henri Gouraud, who introduced the method in 1971, Gouraud shading relies on per-vertex normals, usually derived by averaging the normals of adjacent faces (smoothing) or from explicitly defined vertex normals. The interpolation of vertex colors across the polygon is linear in screen space, which makes the technique efficient for hardware that can interpolate vertex attributes across fragments.

Limitations of Gouraud shading include the potential for specular highlights to appear only at vertices, which can cause highlights to be missed or smeared within large polygons. The technique can also exhibit visible color bands (Mach bands) when vertex sampling is sparse. For higher fidelity, especially for accurate specular highlights, per-pixel lighting is used by Phong shading, which interpolates normals across the surface and computes lighting for each fragment.

In practice, Gouraud shading played a foundational role in early real-time rendering and remains a useful concept for understanding vertex-based lighting and interpolation. Modern graphics pipelines favor per-fragment shading or physically based shading for greater realism, but Gouraud shading is still encountered in legacy content and education.