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Using a shader that works like Phong, what would be the difference between painted Metal and plastic?

I seem to remember reading in books that I no longer have access to, the following:

  • Specular color for painted metal is same color as the paint.
  • Specular color for plastic is always white.
  • Ambient for metal should be darker than for plastic.

This also seems to correlate with what I see when I observe objects (in real world)

But I tried to verify this also by checking the Internet, and found conflicting versions, some said the complete opposite.

So what is true?

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  • \$\begingroup\$ This should probably go to cgse \$\endgroup\$ – Bálint Mar 22 '17 at 9:54
  • \$\begingroup\$ Is cgse a tag, don't know that term.. Edit: Figured it out, will post it there. \$\endgroup\$ – Invariant Mar 22 '17 at 9:56
  • \$\begingroup\$ Same opinion like @Balint. There are people which can help you a lot more \$\endgroup\$ – Arthur P. R. Mar 22 '17 at 10:03
  • \$\begingroup\$ Pleas avoid cross-posting across SE networks. Please delete this question. \$\endgroup\$ – Vaillancourt Mar 22 '17 at 10:19
  • \$\begingroup\$ Since I got a good answer now, do I still have to delete it? \$\endgroup\$ – Invariant Mar 22 '17 at 10:36
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General rule of thumb: under white light, plastics and other non-metallic surfaces tend to have white specular highlights, whereas metals tend to have specular highlights which are a similar shade as the metal. (Or to be more pedantic, most of the color of a metal is actually its specular reflection; they tend to be quite dark, in terms of their diffuse color)

Once paint is on the metal, though, all bets are off.

For example, opaque paint might reflect light exactly like plastic does, even though there's metal underneath. If the light's not making it down to the metal, then it doesn't really matter at all that there's metal down there.

Similarly, it's very common to put a clear coat of paint on top of metal ("clearcoat"), which will add a white specular highlight, similar to the one you get on plastic, with the metallic light behaviour underneath, on the next layer down. (Disney is fairly well known for putting simulated clearcoat on virtually every surface in their recent 3D-rendered films)

Car paints are often translucent; some light will interact with the paint, some will pass through to the metal underneath, and the light which passes through to the metal underneath and is reflected will pass back through the paint again on the way back, being affected in both directions.

Phong was one of the first algorithms for estimating specular lighting, and it's far from "state of the art", these days; it can't model any of these more complicated surfaces to any degree of accuracy. But if you're not after accuracy, it can still do stylised lighting okay, as long as you control the size and color of the highlights to be appropriate for the materials you're simulating.

As a rough rule of thumb, in the absence of other factors, and under a white light, plastics tend to have a white specular highlight, while metals tend to have a colored one.

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