Returning to the gold question.

We must understand the behaviour of metals and the behaviour of metals can be very different depending on the roughness of its surface.

1-

Metals with very low roughness behaves as a mirror. In this case if we use Phong, Blinn or Torrance it will not make any difference. Just set the reflective color and use raytracing. With gold you set the reflective color to some yellow, with copper to some orange and with silver to white.

A metal with very low roughness it will act as a mirror and you will not see the metallic object, you will see distorsed reflections from the environment where the distorsion will depend on the geometric shape of the object. With planar surfaces there is not disrtion at all and you see a perfect mirror image on the object's surface.

2-

The other case are very rough metals. Very rough metals don't produce mirror-like reflections. Phong and Blinn are inadequated for this case. Torrance works pretty well. The main characteristics of very rough metals is that have a big specular area and illumination falling quickly beyond this area. You can approximate using Phong with very low exponent values (1 to 4), but is not the same as Torrance and don't produce off-specular peaks. This is what I used in my picture posted aomewhere above.

3-

For intermediate roughness metals there is no simple solution, there do not exist a simple model for this.

4-

In all the cases the diffuse compont of metals is very small or zero.

5-

For making images of metallic objects things get very complicated, unless we use simple geometric shapes as the famous metallic spheres.

The behaviour of a metallic surface changes dramatically with the roughness and roughness can be changed polishing the metallic object. If the object is not a simple geometrical shape and many times has a complicated shape the polishing is never perfect and so, we will have areas that have been polished very well, for eample the borders, and other areas where we were not able to polish.

The result is that we have a metallic object with areas that act as an almost perfect mirror and other areas that have a very rough surface.

 

Stupidity also evolves!