Phongshading

Phong shading is a computer graphics technique used to produce smooth-looking surfaces by computing lighting at the pixel level while interpolating surface normals across polygonal faces. Named after Bui Tuong Phong, the method contrasts with per-vertex shading approaches by evaluating the illumination model at every pixel rather than at polygon vertices.

In Phong shading, normals are defined at the vertices of a polygon and then interpolated across the interior of the polygon. At each pixel, the interpolated normal is normalized and used with a lighting model to determine color. The standard lighting model combines ambient, diffuse, and specular components. A common formulation is:

I = k_a * I_a + k_d * max(0, N · L) * I_l + k_s * max(0, R · V)^n * I_l

where N is the interpolated normal, L is the light direction, R is the reflection of L around N, V is the view direction, I_l is the light intensity, I_a is ambient light, and k_a, k_d, k_s are coefficients for ambient, diffuse, and specular reflection, with n controlling the shininess.

Compared with Gouraud shading, which interpolates color across vertices and computes lighting only at those vertices, Phong shading yields more accurate specular highlights and reduces color banding, especially on curved surfaces. However, it requires more per-pixel computations, which historically increased rendering cost, though modern hardware handles per-fragment shading efficiently.

Applications of Phong shading include real-time graphics, educational demonstrations of lighting, and any scenario where smooth appearance and specular highlights are desired. It remains a foundational concept in shading pipelines and a stepping stone to more physically based rendering techniques.