> [!abstract] Summary > Reference guide for **PBR shading** using Arnold `standard_surface`. Covers every lobe of the uber-shader (diffuse, specular, metal, transmission, SSS, coat, sheen, emission), the difference between bump / normal / displacement, and a practical lookdev checklist to build a material the right way. --- # Part 1 — Theory ## Overview - We assign materials to objects. - To simulate realism we use **PBR** (Physically Based Rendering/Shading) — as opposed to toon shaders. - PBR relies on real-world physical principles: **energy conservation**, **Fresnel reflectance**, **microfacet distribution**, and a **linear color workflow**. > [!info] Shader of reference > In Arnold, assign a **`standard_surface`** shader (not the legacy `standard`). It is an uber-shader handling diffuse, specular, metal, transmission, SSS, coat, sheen and emission in a single layered node. --- # Part 2 — Shading Lobes ## Color and Spec ### Diffuse (Albedo) - **Diffuse = Albedo** — the base color of an object before it is affected by lighting. - Author albedo maps, not baked-light textures. ![[takavach-rafizadeh-renders-albedo-final-f.jpg]] ![[takavach-rafizadeh-albedo.jpg]] ### Specular | Parameter | Meaning | | --- | --- | | **Specular color** | Color of the light reflected off the surface | | **Specular roughness** | 0 = mirror-smooth (calm water), 1 = fully diffuse/blurred reflection | ![[Heart_of_the_City_water_feature_Sheffield_-_geograph.org.uk_-_618552.jpg]] ### Dielectric vs Conductor - Most materials are **dielectric** (non-conductive). - Metals are **conductors** — handled via the Metalness lobe. ### Metalness - Defines the areas where reflection is so intense that the surface behaves as a metal (tinted specular, no diffuse). ### IOR — Index of Refraction Determines how light bends when passing between media. Higher IOR → more bending and stronger Fresnel at grazing angles. | Medium | IOR | | --- | --- | | Air | 1.0003 | | Ice | 1.31 | | Water | 1.33 | | Alcohol | 1.329 | | Glass | 1.5 | | Sapphire | 1.77 | | Diamond | 2.42 | > [!tip] Dielectric IORs typically sit between **1.3 and 1.8**. Values above 2.0 are rare outside of gemstones. ![[fresnel-ior_reflection-glossiness-matrix-1024x598.jpg]] ### Anisotropy - **Isotropic** = light scattered uniformly in all directions. - **Anisotropic** = light scattered non-uniformly, driven by the micro-structure of the surface. - Examples: brushed metal, silk, velvet, hair, vinyl records. ![[file-20240902211919111.png]] - **Rotation** = rotation angle of the anisotropy direction, usually driven by a tangent map or UV direction. ![[file-20240902212303312.png]] --- ## Relief ### Bump - Fakes depth using a **grayscale map** — white = raised, black = recessed, 0.5 gray = neutral. ![[bump-map-normal-map-768x377.webp|400]] ### Normal - Fakes depth using an **RGB map** — encodes the perturbed normal vector in tangent space. Blue = Z (up), Red = X, Green = Y. ![[normal-map-of-a-ten-pence-coin-300x296.jpg|250]] ### Normal vs Bump | Criteria | Normal | Bump | | --- | --- | --- | | Precision | High | Approximation | | File weight | RGB 16-bit (heavier) | Single channel (lighter) | | Recommended for | Mid-frequency detail | High-frequency detail (micro-scratches, pores) | > [!important] If using only one, choose **normal**. You can stack them: normal for mid-frequency + bump for high-frequency. > For actual geometric elevation, use **displacement**. --- ## Displacement - Actually moves geometry at render time without adding polygons in the viewport. - Elevation map, evaluated at tessellation time. - Heavier to render (requires subdivision + tessellation). - Use for **low/base-frequency** silhouette changes — not for fine details. - **Mid value**: 0.5 for signed displacement, 0 for unsigned (with a height multiplier). > [!tip] Always enable **auto-bump** in Arnold to recover sub-tessellation detail as bump. ![[bump-mapping-comparison-to-real-geometry.webp]] ![[different-texture-maps-guide.jpg]] --- ## Subsurface (SSS) - Simulates how light penetrates the interior of a translucent material (light passes through but diffusely — you can't see clearly through it). - Typical examples: **skin, wax, marble, milk, leaves, jade**. - Very expensive to compute. ![[ac-standard-subsurface-image2014-3-7-8-37-44-7.png|300]] ![[ac-standard-subsurface-dragon-sss-randomwalk-v2-500-5.jpg|300]] | Radius | Result | | --- | --- | | 0 | ![[ac-standard-subsurface-1-emily-radius-0-32.jpg\|250]] | | 1 | ![[ac-standard-subsurface-4-emily-radius-1-34.jpg\|250]] | ### Radius - Approximate distance up to which light can scatter below the surface. - Practical take: treat the color as transparency per wavelength. For skin, `1.0, 0.35, 0.2` → red wavelengths scatter deepest, green/blue bounce back closer to the surface — this is why skin has a reddish shadow terminator. ### Scale - Multiplies `sss_radius_color`. Tune scattering depth without re-authoring the color. ### Type | Type | Behavior | | --- | --- | | **Diffusion** | Cheap, dipole/normalized diffusion approximation | | **Randomwalk** (default) | Physically accurate, random path tracing inside the volume | | **Randomwalk V2** | More expensive, more accurate, better with thin geometry and anisotropic scattering | --- ## Glass (Transmission) - Very expensive. - Directly tied to sampling and **transmission depth** in the render settings. ### Depth - Higher value → thinner apparent volume, less scatter of rays inside. - Useful to match the transmission color when the model is too thin. ![[file-20240903174125083.png]] ### Scatter - For **thick liquids** (honey, murky water, soapy substances). > [!warning] The **Opaque** attribute must be **disabled** on the geometry for transmission/scatter to work correctly. ![[file-20240903174345816.png|925]] ### Dispersion (Abbe) - How much the IOR varies across wavelengths. - Creates the rainbow/diamond effect (chromatic aberration at refraction). - Glass: ~10 minimum; Diamond: ~55. **Lower Abbe = stronger dispersion.** ![[file-20240903174736545.png]] ### Dielectric Priority - Which material dominates when two surfaces overlap. - Typical case: a glass of water. Water and glass are both dielectric — Arnold needs to know which dominates at the interface. **Higher priority wins.** ### Specular IOR with Transmission - Default 1.0 = vacuum (no refraction). `standard_surface` assumes outward-facing normals, geometry embedded in air (IOR 1.0), and no overlapping surfaces. ![[file-20240902211443937.png]] | IOR 1.0 | IOR 1.5 | | --- | --- | |  |  | ### Going Further — Beer-Lambert Law - Describes how light is absorbed by a medium as a function of absorber concentration and path length. - Practical use: thicker glass/liquid appears more saturated because light travels a longer distance through the medium before reaching the camera. --- ## Metal ![[image-20250301 1.png]] - Metals have **no diffuse component** — base color tints the specular reflection directly. - Use **measured F0 values** (base color at 0°) and **F82 / edge tint** when available (complex IOR workflow). | Metal | F0 (linear RGB) | | --- | --- | | Gold | `1.00, 0.71, 0.29` | | Silver | `0.95, 0.93, 0.88` | | Copper | `0.95, 0.64, 0.54` | | Aluminum | `0.91, 0.92, 0.92` | | Iron | `0.56, 0.57, 0.58` | > [!tip] Roughness drives the visual identity of a metal far more than its base color (polished vs brushed vs oxidized). --- ## Coat - Simulates a thin transparent layer on top of the base: **clear coat, car paint varnish, wet skin, lacquered wood**. - Has its own IOR, roughness, and normal (often a smoother normal than the base). - Absorbs energy from the layers below (energy conservation). ## Sheen - Simulates **retro-reflection at grazing angles** on microfiber surfaces. - Use for: fabric (velvet, cotton, satin), dust layers, peach fuzz on skin. - Acts as an additional lobe — does **not** replace diffuse. ## Emission - Makes the surface **self-illuminate**. - Driven by color × weight (unbounded — can exceed 1 for true light emission). - Emissive geometry contributes to lighting only if Arnold's **Light Sampling** is enabled on the mesh. Otherwise it is just a bright surface. ## Opacity vs Transmission | | Opacity | Transmission | | ------- | ---------------------------------- | ----------------------------------- | | Type | Binary / alpha cutout | Physically correct glass/refraction | | Cost | Cheap (depend on render engine) | Expensive | | Use for | Leaves, hair cards, fences, decals | Solid transparent objects | | | | | > [!warning] Never mix both on the same surface unless you know what you are doing. --- # Part 3 — Practical Lookdev Checklist 1. Start from a **gray clay** shader with correct roughness — validate silhouette and lighting first. 2. Plug **albedo** → validate base color under neutral light (gray HDRI or 18% gray backdrop). 3. Plug **roughness** → validate specular behavior. 4. Plug **normal/bump** → validate surface detail. 5. Plug **displacement** last → most expensive, requires tessellation tweaks. 6. Add **SSS / coat / sheen** only where physically justified. 7. Always lookdev under **at least two lighting conditions** (studio HDRI + production HDRI). 8. Check albedo values stay within the **PBR safe range** (sRGB 30–240 — no pure black, no pure white). --- # Part 4 — Going Further ## Metals: MTL vs SPEC workflow ```embed title: "Metallic Magic" image: "https://miro.medium.com/v2/resize:fit:1200/1*yvVQ8HRMUiWyqFZ3E_nSDQ.jpeg" description: "Metalness Maps and Workflow Explained" url: "https://medium.com/gametextures/metallic-magic-2dce9001fe15#:~:text=A%20Metallic%2C%20or%20Metalness%2C%20map,and%20do%20not%20(black)." ``` ## Useful References - Arnold `standard_surface` documentation — Autodesk Help - *Physically Based Rendering: From Theory to Implementation* — Pharr, Jakob, Humphreys - Disney BRDF / Principled BSDF paper — Burley, 2012 - *Crafting a Next-Gen Material Pipeline for The Order: 1886* — David Neubelt & Matt Pettineo