Gouraud Shading - Comparison With Other Shading Techniques

Comparison With Other Shading Techniques

Gouraud shading is considered superior to flat shading, which requires significantly less processing than Phong shading, but usually results in a faceted look.

In comparison to Phong shading, Gouraud shading's strength and weakness lies in its interpolation. If a mesh covers more pixels in screen space than it has vertices, interpolating colour values from samples of expensive lighting calculations at vertices is less processor intensive than performing the lighting calculation for each pixel as in Phong shading. However, highly localized lighting effects (such as specular highlights, e.g. the glint of reflected light on the surface of an apple) will not be rendered correctly, and if a highlight lies in the middle of a polygon, but does not spread to the polygon's vertex, it will not be apparent in a Gouraud rendering; conversely, if a highlight occurs at the vertex of a polygon, it will be rendered correctly at this vertex (as this is where the lighting model is applied), but will be spread unnaturally across all neighboring polygons via the interpolation method.

The problem is easily spotted in a rendering which ought to have a specular highlight moving smoothly across the surface of a model as it rotates. Gouraud shading will instead produce a highlight continuously fading in and out across neighboring portions of the model, peaking in intensity when the intended specular highlight passes over a vertex of the model.

(For clarity, note that the problem just described can be improved by increasing the density of vertices in the object (or perhaps increasing them just near the problem area), but of course, this solution applies to any shading paradigm whatsoever - indeed, with an "incredibly large" number of vertices there would never be any need at all for shading concepts.)

• Gouraud-shaded sphere - note the poor behaviour of the specular highlight.

• The same sphere rendered with a very high polygon count.