glTF 2.0 Reference Viewer

This is the offical Khronos glTF 2.0 reference viewer using WebGL.

Table of Contents

Viewer
- Usage
- Setup
- Debugging
Physically-Based Materials in glTF 2.0
Appendix
- Specular Term
- Diffuse Term
Features

Viewer

Usage

If you would like to see this in action, view the live demo.

Controls

click + drag : Rotate model

scroll : Zoom camera

GUI : Use to change models and settings

Change glTF model

Choose one of the glTF models in the selction list

Drag and drop glTF files into viewer

Offline / Headless Rendering

NOTE: The dimensions of the rendered image are limited by the (virtual) desktop size.

Requirements

NodeJS
Electron (Installed automatically)

Configure environment

npm install (also installs Electron)
npm run build (“compile” the code)

Run

npm run start-offscreen -- -- -h for a list of available options

Example

npm run start-offscreen -- -- assets\models\2.0\FlightHelmet\glTF\FlightHelmet.gltf

After execution, the screenshot is stored as output.png on the file system.

Setup

For local usage and debugging, please follow these instructions:

(1) Checkout the reference-viewer branch

(2) Install dependencies with npm install

(3) Pull the submodules for the required glTF sample models and environments git submodule update --init --recursive

(4a) Start a demo in the browser with npm run dev, and open http://localhost:8000.

(4b) Start a demo in Electron with npm run dev:electron.

When making changes, the project is automatically rebuilt and the dist/ folder is updated. Files in the dist/ folder should not be included in pull requests — they will be updated by project maintainers with each new release.

Debugging

Requirements
- Visual Studio Code
- Mozilla Firefox
Install the Debugger for Firefox extension for Visual Studio Code
Open the project folder in Visual Studio Code and select Debug->Add Configuration->Firefox so the .vscode/launch.json file is created.
Debug->Start Debugging should now launch a Firefox window with the reference viewer and VS Code breakpoints should be hit.

VS Code

Modified VSCode gltf-vscode plugin:

gltf-vscode

Physically-Based Materials in glTF 2.0

With the change from glTF 1.0 to glTF 2.0, one of the largest changes included core support for materials that could be used for physically-based shading. Part of this process involved chosing technically accurate, yet user-friendly, parameters for which developers and artists could use intuitively. This resulted in the introduction of the Metallic-Roughness Material to glTF. If you would like to read more about glTF, you can find the content at its GitHub page.

A good reference about Physically-Based Materials and its workflow can be found on the THE PBR GUIDE - PART 1 and THE PBR GUIDE - PART 2 from allegorithmic.

For implementation details and further theory, please find more information in the Real Shading in Unreal Engine 4 presentation from the SIGGRAPH 2013 course.

Appendix

For further reference, please read the glTF 2.0: Appendix B: BRDF Implementation
The following sections do summarize the important shader code.

vec3 specularContribution = D * Vis * G;
vec3 diffuseContribution = (1.0 - F) * diffuse;

Please note: Vis = G / (4 * NdotL * NdotV)

Specular Term

Microfaced Distribution (D)

Trowbridge-Reitz GGX

float microfacetDistribution(MaterialInfo materialInfo, AngularInfo angularInfo)
{
    float alphaRoughnessSq = materialInfo.alphaRoughness * materialInfo.alphaRoughness;
    float f = (angularInfo.NdotH * alphaRoughnessSq - angularInfo.NdotH) * angularInfo.NdotH + 1.0;
    return alphaRoughnessSq / (M_PI * f * f);
}

Surface Reflection Ratio (F)

Fresnel Schlick

vec3 specularReflection(MaterialInfo materialInfo, AngularInfo angularInfo)
{
    return materialInfo.reflectance0 + (materialInfo.reflectance90 - materialInfo.reflectance0) * pow(clamp(1.0 - angularInfo.VdotH, 0.0, 1.0), 5.0);
}

Please note, that the above shader code includes the optimization for "turning off" the Fresnel edge brightening (see "Real-Time Rendering" Fourth Edition on page 325).

Geometric Occlusion (G)

Smith Joint GGX

float visibilityOcclusion(MaterialInfo materialInfo, AngularInfo angularInfo)
{
    float NdotL = angularInfo.NdotL;
    float NdotV = angularInfo.NdotV;
    float alphaRoughnessSq = materialInfo.alphaRoughness * materialInfo.alphaRoughness;

    float GGXV = NdotL * sqrt(NdotV * NdotV * (1.0 - alphaRoughnessSq) + alphaRoughnessSq);
    float GGXL = NdotV * sqrt(NdotL * NdotL * (1.0 - alphaRoughnessSq) + alphaRoughnessSq);

    return 0.5 / (GGXV + GGXL);
}

Diffuse Term

Lambert

vec3 diffuse(MaterialInfo materialInfo)
{
    return materialInfo.diffuseColor / M_PI;
}

emackey / glTF-WebGL-PBR