glTF IBL Sampler, enhanced "lite" version

Introduction

This project pre-filters an environment High Dynamic Range (HDR) panorama image or cube map and stores diffuse and specular parts in cube maps respectively. Considering different material characteristics, the specular part consists of several mip-map levels corresponding to different roughness values of the modeled material. The result is written to a KTX2 texture.

Algorithm references:

• Real Shading in Unreal Engine 4
• GPU-Based Importance Sampling
• Runtime Environment Map Filtering for Image Based Lighting
• Filtering shader

Enhanced "lite" version

In order to make the library easier to integrate into external projects and other build systems, this "lite" version drops all external dependencies on KTX2-Software, glslang, and the Vulkan SDK. It also eliminates Git submodules in favor of Git subtrees, so that the repository is fully self-contained.

This enhanced version of the glTF IBL Sampler allows a cubemap in KTX2 format to be supplied as input. Note that at this point compressed input textures aren't supported. It also features a "None" filter mode to convert a panorama into a KTX2 cubemap without doing any further processing.

Workflow

The individual transformations can be executed separately in the library, enabling the use of a cube map as input.

• Environment HDR image → Cube Map
• Cube Map → Filtered Cube Map
• Filtered Cube Map → KTX2 Output File

This separation of shader passes ensures both, flexibility and extensibility. The pre-filtering will use the algorithms noted in the glTF Sample Environments repository.

Building

The project provides a cmake file to generate respective build files.

There are no third-party requirements, other than a C++ compiler and CMake; everything in this repository is self-contained.

The glTF-IBL-Sampler consists of two projects: lib (shared library) and cli (executable).

Usage

The CLI takes an environment HDR image or a cubemap in uncompressed KTX2 format as input. The filtered specular and diffuse cube maps can be stored as uncompressed KTX2.

• -inputPath: path to panorama image or cube map
• -outCubeMap: output path for filtered cube map (default=outputCubeMap.ktx2)
• -outLUT: output path for BRDF LUT (default=outputLUT.png)
• -distribution: NDF to sample (None, Lambertian, GGX, Charlie)
• -sampleCount: number of samples used for filtering (default = 1024)
• -mipLevelCount: number of mip levels of specular cube map. If omitted, an optimal mipmap level is chosen, based on the input panorama's resolution.
• -cubeMapResolution: resolution of output cube map. If omitted, an optimal resolution is chosen based on the input panorama's resolution.
• -targetFormat: specify output texture format (R8G8B8A8_UNORM, R16G16B16A16_SFLOAT, R32G32B32A32_SFLOAT)
• -lodBias: level of detail bias applied to filtering (default = 0)

Example

.\cli.exe -inputPath ..\cubemap_in.hdr -outCubeMap ..\..\specular_out.ktx2 -distribution GGX -sampleCount 1024 -targetFormat R16G16B16A16_SFLOAT
.\cli.exe -inputPath ..\cubemap_in.hdr -outCubeMap ..\diffuse_out.ktx2 -distribution Lambertian -sampleCount 1024 -targetFormat R16G16B16A16_SFLOAT