A metal surface lit only by punctual lights is black everywhere except its highlight

Fact

In a physically based shading model a metal has no diffuse term: its colour is entirely the specular reflection of whatever radiance arrives at it. Point, spot and directional lights are punctual — they deliver energy from a single direction — so they can only produce a narrow highlight where the mirror direction happens to line up with the viewer. Everywhere else a metal has nothing to reflect, and nothing is what it shows.

Measured on one chrome object at metalness: 1.0, roughness: 0.3, lit by a single directional light and no environment, three sample points read (63,33,13), (12,8,4) and (0,0,0) — mean 15/255. Adding an image-based environment and changing nothing else took the same points to (102,53,23), (72,48,28) and (26,6,1), mean 40. The literal black pixel is the ordinary case, not an edge one.

Why it matters

The failure looks like a lighting problem, so the response is to add lights or turn them up — which cannot work, because more punctual light only brightens the highlight that is already the one lit part. Time then goes into the light rig, the tone mapper and the exposure, none of which is the missing term.

It is most common when porting art from a renderer that had no notion of metalness, where "shiny metal" was a colour and a specular power, and the same material description now means something entirely different.

How to apply

• Give the scene an environment before tuning any light. A 64×32 canvas gradient run through a prefilter is enough to make metal read as metal; it does not need to be a captured HDRI.
• Sample the darkest metal away from its highlight to check the fix. The highlight looks correct under every wrong setting.
• If metal must stay black for a reason, lower metalness instead — a partly dielectric material keeps a diffuse term that punctual lights can reach.
• Roughness does not rescue it: a rougher metal spreads the same missing energy over a wider lobe, so it gets dimmer, not fuller.

Related: [[palette-is-calibrated-to-its-shading-model]], [[lamp-power-cannot-widen-the-pool]].