Physically Based Materials Explained

The core idea of physically based rendering (PBR) is a shift in what you author. In the old ad-hoc workflow you painted the result you wanted - a texture that already looked lit, with highlights and shadows baked in. That is brittle: the moment the light changes, the painted-in highlight is in the wrong place. PBR instead asks you to describe the material itself - its intrinsic properties, independent of any particular light - and hands the lighting job to the renderer.

The payoff is consistency. When a surface is described physically, the same material reads correctly whether it is in direct sun, deep shade, or a warm interior at night. You author it once and trust it everywhere, which is exactly what a design workflow needs: you are constantly moving the sun, swapping the sky and stepping between rooms, and you cannot re-tune every material each time.

Underneath sits a principle called energy conservation: a surface cannot reflect back more light than it receives. Old workflows happily broke this - you could crank a highlight until a wall glowed like a light source. PBR shaders enforce the physics, so a rough surface spreads its reflection dim and wide while a smooth one concentrates it bright and tight, but the total light bouncing off stays honest. That single constraint is why PBR surfaces sit naturally into a scene instead of looking pasted on. Under Unreal's Lumen global illumination (Module 4), a physically correct material also bounces physically correct light onto its neighbours - a red wall throws faint red onto the ceiling - which is only believable because the material obeys the same rules the light does.

Almost all real-time engines, Unreal included, use the metallic/roughness workflow. It reduces a surface to a handful of channels, each a value or a texture map:

Base Color (also called albedo): the pure surface color with no lighting or shadow in it - just the raw hue of the material. A red brick's base color is red; that is all. Critically, it should contain no baked highlights or ambient occlusion; the engine adds those.

Metallic: essentially a yes/no switch - is this surface a raw metal (1) or a non-metal / dielectric (0)? Metals and non-metals reflect light in fundamentally different ways, so this one channel reroutes the whole shading model.

Roughness: how microscopically rough the surface is, from 0 (a perfect mirror) to 1 (fully matte). This is the single most expressive dial in archviz - it is the difference between polished marble and honed stone, gloss paint and eggshell, a wet road and a dry one.

Normal: a map that fakes fine surface detail - the mortar grooves in brick, the grain in wood, the brushing in steel - by tilting how light bounces, without adding real geometry. It makes a flat polygon catch light as if it were bumpy.

Specular rounds it out: for non-metals it controls the small baseline reflection every surface has, and you leave it at the default 0.5 almost always - it exists for the rare case. Author these few channels well and you have described nearly any architectural surface truthfully.

Two channels trip up beginners most, so they are worth dwelling on.

Metallic wants to be 0 or 1, not something in between. In the real world a surface is either a raw conductor (metal) or it is not; there is very little that is genuinely half-metal. Brushed aluminium, chrome, copper, gold: metallic = 1. Brick, concrete, wood, plaster, plastic, fabric, painted steel: metallic = 0 - yes, even painted metal, because the paint is a dielectric coating sitting on top. The main honest use of an intermediate value is the transition edge of a texture, like where metal meets rust. If you find yourself dialling metallic to 0.4 to 'get the look', you are almost always fixing the wrong channel - the answer is usually roughness or base color instead.

Roughness is a full analog range and it does most of the storytelling. A value near 0 gives a tight, bright, mirror-like highlight; near 1 gives a soft, spread, matte surface with no clear reflection. Almost every real surface lives somewhere in between, and small moves read strongly: polished stone might be 0.15, honed stone 0.5, raw concrete 0.8. Real materials are also rarely uniform - a floor has scuffs, fingerprints, wear paths - so a subtle roughness map rather than a flat value is what pushes a material from 'CG' to 'real'. The eye reads variation in reflection as truth. This is the channel to reach for first when a material looks off.

Put the pieces together and you can see why a physically based material is robust. Because it stores intrinsic properties rather than a lit result, and because the shader conserves energy, the surface responds correctly to any lighting you throw at it. Move the sun and the highlight moves with it, the right size for that roughness, the right color for that metal. Switch to an overcast HDRI sky and the same material goes soft and even, because that is what real light does to it. Nothing needs re-authoring.

This is what makes PBR and real-time such a good match for design. In Unreal you are lighting with Lumen, changing the sun, the sky, and interior lamps constantly as you explore a scheme. If your materials were painted for one light, every change would break them. Because they are physically described, they hold - so you can trust what you see when you compare a morning and an evening walkthrough, and so can your client.

There is an honest caveat: real-time PBR is still an approximation. Engines use efficient formulas (a BRDF - a bidirectional reflectance distribution function - is the maths that models how a surface reflects light) rather than simulating every photon, and effects like frosted-glass blur, complex layered coatings, or true multi-bounce interreflection inside a material are simplified to hit the frame budget. For the overwhelming majority of architectural surfaces the approximation is indistinguishable from correct. Knowing where it simplifies just tells you which few surfaces - deep glass, brushed anisotropic metal - will need extra care in lesson 3.4.

When a PBR material looks wrong, the fault is almost always one of four things, and learning to name them is half the skill.

Base color too dark or too bright. Real materials sit in a fairly narrow reflectance band - there is almost nothing in the world as dark as pure black or as bright as pure white. If your base color hits 0 or 255, the material will read as a hole or a glowing patch under GI. Fresh snow tops out around 90 percent reflectance; fresh asphalt sits near 5 percent; keep base color between those and it behaves.

Highlights or shadows baked into base color. If your albedo texture already has shading painted in, the engine adds its own lighting on top and the surface double-darkens. Base color must be flat, lit-free color.

Metallic used to fake reflectivity. A shiny non-metal (glossy tile, wet stone) is not metal - it is a dielectric with low roughness. Reach for roughness, not metallic.

Roughness too flat and too clean. A single uniform roughness value is the fastest giveaway of a fake material. Reality has variation - wear, dust, fingerprints, edge polish. A gentle roughness map fixes more 'it looks CG' complaints than any other single change. Run through these four before you touch anything exotic, and most tired materials come right.