PBR (physically based rendering)

PBR describes a surface with a handful of physical properties instead of hand-tuned fudge factors: base color (albedo), metalness (is this metal or not), roughness (how blurry the reflection is), plus usually a normal map and ambient occlusion. The shading math conserves energy, so a surface never reflects more light than it receives. The payoff: the same material looks correct in a sunset, a studio, and a dark room without you touching it. Almost every tool speaks a variant of the same microfacet dialect. glTF's core metallic-roughness model is the compact version: base color, metalness, roughness, normal, occlusion, emissive, and little else. The Disney principled BRDF your modeling app exposes is a bigger animal, with extra lobes and controls (sheen, clearcoat, subsurface, anisotropy) that glTF either leaves out or ships as separate extensions, which is why a principled material does not always survive the export intact.

Reach for it for anything meant to read as a real object: product viewers, configurators, packshots. Rough dielectrics (0.6 roughness, 0 metalness) give plastic and rubber, low roughness with metalness 1 gives chrome, mid roughness with metalness 1 gives brushed aluminum.

Two gotchas that account for most bad-looking PBR. First, metalness is effectively binary: 0 or 1, with in-between values only where paint chips off metal. And for metals, base color is the tint of the reflection, not a diffuse color. Metal needs something to reflect: direct lights do illuminate it, but with no environment map and nothing useful landing on it, a metal renders nearly black. Second, color textures must be tagged sRGB while roughness, metalness, and normal maps are raw linear data. Mix those up and everything goes flat or chalky.