This is a high-performance renderer designed among other things to render glTF models that use the KHR_materials_transmission and KHR_materials_volume extensions.

It can:

• Handle rough transmissive materials such as frosted glass.
• Approximate refracted light through transmissive models that specify a thickness.
• Attenuate light via Beer's law.

It wasn't easy working out how to render these models. While the glTF extension READMEs as fairly extensive, I had to look through the code of the glTF-Sample-Viewer to work out implementation details.

Transmissive materials that are totally smooth and do not have a volume can be rendered without any super special tricks via pre-multiplied alpha and additive blending. Transmissive materials with any kind of roughness or volume are quite a bit trickier.

The main trick I realised from reading over the code from the glTF-Sample-Viewer is that you need to render or blit all the rendered diffuse objects to a second framebuffer texture, which you then sample in a seperate pass for all the transmissive objects. If you want to handle rough transmissive objects then you need to generate a mip-chain for this sampled texture, as rougher objects need to fetch the transmitted light of the pixels behind them in a 'blurrier' way.

In my shader code, this is done via two functions (these I did mostly copy from the gltf-Sample-Viewer). The first, ibl_volume_refraction is only ran once per pixel. It handles fetching the transmitted light from the sampled framebuffer and attenuating it.

The second, the transmission_btdf is ran once per light and handles light that is scattered through the model and out onto the other side. This only effects materials that are a bit rough.

Running

Clone https://github.com/KhronosGroup/glTF-Sample-Models into this directory as run with:

cargo run -- <gltf-sample-model-name>

transmission-renderer 0.1.0

USAGE:
    transmission-renderer [FLAGS] [OPTIONS] <gltf-sample-model-name>

FLAGS:
        --external-model    Render a model external to the glTF-Sample-Models directory, in which case the full path
                            needs to be specified
    -h, --help              Prints help information
        --log-leaks         Log allocator leaks on shutdown. Off by default because it makes panics hard to debug
    -V, --version           Prints version information

OPTIONS:
        --roughness-override <roughness-override>
            Override the default roughness factor of the model. Doesn't effect models that use a
texture for roughness

    -s, --scale <scale>                              A scale factor to be applied to the model [default: 1.0]

ARGS:
    <gltf-sample-model-name>    The name of the model inside the glTF-Sample-Models directory to
render

For some cool examples:

cargo run -- DragonAttenuation --roughness-override 0.25

cargo run -- IridescentDishWithOlives --scale 5

cargo run -- MosquitoInAmber --scale 20

cargo run -- AttenuationTest --scale 0.1

Shader Compiling

I'm using my own project, rust-gpu-cli-builder to build the shader module as I prefer this to using a build script. The arguments that I use to build it are:

--target spirv-unknown-spv1.3 --capabilities RuntimeDescriptorArray --extensions SPV_EXT_descriptor_indexing

Other Fun Stuff

Oh, it also features

• Shader written entirely in https://github.com/EmbarkStudios/rust-gpu.
• Basic frustum culling via a compute shader and vkCmdDrawIndexedIndirectCount.
• A depth pre-pass setup where at max 2 full screens of fragments are processed (1 screen of opaque and 1 of transmissive objects on top).