Artifice, Inc.
Artifice, Inc.
One factor which contributes to this elusive difference in appearance is that metals very often have different roughness and specular reflectivity charateristics than organic materials. Loosely speaking, metals are more often "shinny" than organic materials. For a quick, dirty, technical discussion of specular highlights and how they are defined in QD3D, see the Artifice Tech Note Specular Reflections in QD3D.
Another factor which contributes to this elusive difference in appearance is that light reflected from metals and from organic materials differs categorically. Physcially, these differences occur because the different types of material reflect light by means of different underlying physical mechanisms.
The typical spectrum of the light reflected from an organic material is about the same as the spectrum of the light incident on the material. In other words, the color of the reflected light is not affected very much by the color of the organic material.
This means, for instance, that the specular highlight seen from a white light bouncing off a ball of orangy-colored wood would be white, and the specular highlight seen from a white light bouncing off a greenish-colored wood would also be white.
In contrast, the typical spectrum of light reflected from a metallic material is _not_ the same as the spectrum of the light incident on the material. In other words, the color of the reflected light appears to be directly affected by the color of the organic material.
This means, for instance, that the specular highlight seen from a white light bouncing off a ball of orangy-colored copper would be orangy, while the specular highlight seen from a white light bouncing off a grayish-colored steel would still mostly be white.
Taking this distinction into the world of QuickDraw 3D, we could say that to establish appropriate specular highlights for an organic material, the RGB components of the SpecularColor for the material should typically be white. Thus, the spectrum of the specular lights will not be effected by the oreganic material.
For a metallic material, to establish appropriate specular highlights the RGB components of the SpecularColor should typically match the RGB color of the metal itself. Thus, the spectrum of the specular lights will be shifted to match the basic color of the metallic material.
http://www.artifice.com/tech/metals.html