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bevy/docs/cargo_features.md
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StandardMaterial Light Transmission (#8015) 
# Objective

<img width="1920" alt="Screenshot 2023-04-26 at 01 07 34"
src="https://user-images.githubusercontent.com/418473/234467578-0f34187b-5863-4ea1-88e9-7a6bb8ce8da3.png">

This PR adds both diffuse and specular light transmission capabilities
to the `StandardMaterial`, with support for screen space refractions.
This enables realistically representing a wide range of real-world
materials, such as:

  - Glass; (Including frosted glass)
  - Transparent and translucent plastics;
  - Various liquids and gels;
  - Gemstones;
  - Marble;
  - Wax;
  - Paper;
  - Leaves;
  - Porcelain.

Unlike existing support for transparency, light transmission does not
rely on fixed function alpha blending, and therefore works with both
`AlphaMode::Opaque` and `AlphaMode::Mask` materials.

## Solution

- Introduces a number of transmission related fields in the
`StandardMaterial`;
- For specular transmission:
- Adds logic to take a view main texture snapshot after the opaque
phase; (in order to perform screen space refractions)
- Introduces a new `Transmissive3d` phase to the renderer, to which all
meshes with `transmission > 0.0` materials are sent.
- Calculates a light exit point (of the approximate mesh volume) using
`ior` and `thickness` properties
- Samples the snapshot texture with an adaptive number of taps across a
`roughness`-controlled radius enabling “blurry” refractions
- For diffuse transmission:
- Approximates transmitted diffuse light by using a second, flipped +
displaced, diffuse-only Lambertian lobe for each light source.

## To Do

- [x] Figure out where `fresnel_mix()` is taking place, if at all, and
where `dielectric_specular` is being calculated, if at all, and update
them to use the `ior` value (Not a blocker, just a nice-to-have for more
correct BSDF)
- To the _best of my knowledge, this is now taking place, after
964340cdd. The fresnel mix is actually "split" into two parts in our
implementation, one `(1 - fresnel(...))` in the transmission, and
`fresnel()` in the light implementations. A surface with more
reflectance now will produce slightly dimmer transmission towards the
grazing angle, as more of the light gets reflected.
- [x] Add `transmission_texture`
- [x] Add `diffuse_transmission_texture`
- [x] Add `thickness_texture`
- [x] Add `attenuation_distance` and `attenuation_color`
- [x] Connect values to glTF loader
  - [x] `transmission` and `transmission_texture`
  - [x] `thickness` and `thickness_texture`
  - [x] `ior`
- [ ] `diffuse_transmission` and `diffuse_transmission_texture` (needs
upstream support in `gltf` crate, not a blocker)
- [x] Add support for multiple screen space refraction “steps”
- [x] Conditionally create no transmission snapshot texture at all if
`steps == 0`
- [x] Conditionally enable/disable screen space refraction transmission
snapshots
- [x] Read from depth pre-pass to prevent refracting pixels in front of
the light exit point
- [x] Use `interleaved_gradient_noise()` function for sampling blur in a
way that benefits from TAA
- [x] Drill down a TAA `#define`, tweak some aspects of the effect
conditionally based on it
- [x] Remove const array that's crashing under HLSL (unless a new `naga`
release with https://github.com/gfx-rs/naga/pull/2496 comes out before
we merge this)
- [ ] Look into alternatives to the `switch` hack for dynamically
indexing the const array (might not be needed, compilers seem to be
decent at expanding it)
- [ ] Add pipeline keys for gating transmission (do we really want/need
this?)
- [x] Tweak some material field/function names?

## A Note on Texture Packing

_This was originally added as a comment to the
`specular_transmission_texture`, `thickness_texture` and
`diffuse_transmission_texture` documentation, I removed it since it was
more confusing than helpful, and will likely be made redundant/will need
to be updated once we have a better infrastructure for preprocessing
assets_

Due to how channels are mapped, you can more efficiently use a single
shared texture image
for configuring the following:

- R - `specular_transmission_texture`
- G - `thickness_texture`
- B - _unused_
- A - `diffuse_transmission_texture`

The `KHR_materials_diffuse_transmission` glTF extension also defines a
`diffuseTransmissionColorTexture`,
that _we don't currently support_. One might choose to pack the
intensity and color textures together,
using RGB for the color and A for the intensity, in which case this
packing advice doesn't really apply.

---

## Changelog

- Added a new `Transmissive3d` render phase for rendering specular
transmissive materials with screen space refractions
- Added rendering support for transmitted environment map light on the
`StandardMaterial` as a fallback for screen space refractions
- Added `diffuse_transmission`, `specular_transmission`, `thickness`,
`ior`, `attenuation_distance` and `attenuation_color` to the
`StandardMaterial`
- Added `diffuse_transmission_texture`, `specular_transmission_texture`,
`thickness_texture` to the `StandardMaterial`, gated behind a new
`pbr_transmission_textures` cargo feature (off by default, for maximum
hardware compatibility)
- Added `Camera3d::screen_space_specular_transmission_steps` for
controlling the number of “layers of transparency” rendered for
transmissive objects
- Added a `TransmittedShadowReceiver` component for enabling shadows in
(diffusely) transmitted light. (disabled by default, as it requires
carefully setting up the `thickness` to avoid self-shadow artifacts)
- Added support for the `KHR_materials_transmission`,
`KHR_materials_ior` and `KHR_materials_volume` glTF extensions
- Renamed items related to temporal jitter for greater consistency

## Migration Guide

- `SsaoPipelineKey::temporal_noise` has been renamed to
`SsaoPipelineKey::temporal_jitter`
- The `TAA` shader def (controlled by the presence of the
`TemporalAntiAliasSettings` component in the camera) has been replaced
with the `TEMPORAL_JITTER` shader def (controlled by the presence of the
`TemporalJitter` component in the camera)
- `MeshPipelineKey::TAA` has been replaced by
`MeshPipelineKey::TEMPORAL_JITTER`
- The `TEMPORAL_NOISE` shader def has been consolidated with
`TEMPORAL_JITTER`
2023-10-31 20:59:02 +00:00

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<!-- MD041 - This file will be included in docs and should not start with a top header -->
<!-- markdownlint-disable-file MD041 -->
## Cargo Features
Bevy exposes many features to customise the engine. Enabling them add functionalities but often come at the cost of longer compilation times and extra dependencies.
### Default Features
The default feature set enables most of the expected features of a game engine, like rendering in both 2D and 3D, asset loading, audio and UI. To help reduce compilation time, consider disabling default features and enabling only those you need.
|feature name|description|
|-|-|
|android_shared_stdcxx|Enable using a shared stdlib for cxx on Android|
|animation|Enable animation support, and glTF animation loading|
|bevy_animation|Provides animation functionality|
|bevy_asset|Provides asset functionality|
|bevy_audio|Provides audio functionality|
|bevy_core_pipeline|Provides cameras and other basic render pipeline features|
|bevy_gilrs|Adds gamepad support|
|bevy_gizmos|Adds support for rendering gizmos|
|bevy_gltf|[glTF](https://www.khronos.org/gltf/) support|
|bevy_pbr|Adds PBR rendering|
|bevy_render|Provides rendering functionality|
|bevy_scene|Provides scene functionality|
|bevy_sprite|Provides sprite functionality|
|bevy_text|Provides text functionality|
|bevy_ui|A custom ECS-driven UI framework|
|bevy_winit|winit window and input backend|
|default_font|Include a default font, containing only ASCII characters, at the cost of a 20kB binary size increase|
|hdr|HDR image format support|
|ktx2|KTX2 compressed texture support|
|multi-threaded|Enables multithreaded parallelism in the engine. Disabling it forces all engine tasks to run on a single thread.|
|png|PNG image format support|
|tonemapping_luts|Include tonemapping Look Up Tables KTX2 files. If everything is pink, you need to enable this feature or change the `Tonemapping` method on your `Camera2dBundle` or `Camera3dBundle`.|
|vorbis|OGG/VORBIS audio format support|
|webgl2|Enable some limitations to be able to use WebGL2. If not enabled, it will default to WebGPU in Wasm. Please refer to the [WebGL2 and WebGPU](https://github.com/bevyengine/bevy/tree/latest/examples#webgl2-and-webgpu) section of the examples README for more information on how to run Wasm builds with WebGPU.|
|x11|X11 display server support|
|zstd|For KTX2 supercompression|
### Optional Features
|feature name|description|
|-|-|
|accesskit_unix|Enable AccessKit on Unix backends (currently only works with experimental screen readers and forks.)|
|asset_processor|Enables the built-in asset processor for processed assets.|
|async-io|Use async-io's implementation of block_on instead of futures-lite's implementation. This is preferred if your application uses async-io.|
|basis-universal|Basis Universal compressed texture support|
|bevy_ci_testing|Enable systems that allow for automated testing on CI|
|bevy_dynamic_plugin|Plugin for dynamic loading (using [libloading](https://crates.io/crates/libloading))|
|bmp|BMP image format support|
|dds|DDS compressed texture support|
|debug_glam_assert|Enable assertions in debug builds to check the validity of parameters passed to glam|
|detailed_trace|Enable detailed trace event logging. These trace events are expensive even when off, thus they require compile time opt-in|
|dynamic_linking|Force dynamic linking, which improves iterative compile times|
|embedded_watcher|Enables watching in memory asset providers for Bevy Asset hot-reloading|
|exr|EXR image format support|
|file_watcher|Enables watching the filesystem for Bevy Asset hot-reloading|
|flac|FLAC audio format support|
|glam_assert|Enable assertions to check the validity of parameters passed to glam|
|jpeg|JPEG image format support|
|minimp3|MP3 audio format support (through minimp3)|
|mp3|MP3 audio format support|
|pbr_transmission_textures|Enable support for transmission-related textures in the `StandardMaterial`, at the risk of blowing past the global, per-shader texture limit on older/lower-end GPUs|
|pnm|PNM image format support, includes pam, pbm, pgm and ppm|
|serialize|Enable serialization support through serde|
|shader_format_glsl|Enable support for shaders in GLSL|
|shader_format_spirv|Enable support for shaders in SPIR-V|
|subpixel_glyph_atlas|Enable rendering of font glyphs using subpixel accuracy|
|symphonia-aac|AAC audio format support (through symphonia)|
|symphonia-all|AAC, FLAC, MP3, MP4, OGG/VORBIS, and WAV audio formats support (through symphonia)|
|symphonia-flac|FLAC audio format support (through symphonia)|
|symphonia-isomp4|MP4 audio format support (through symphonia)|
|symphonia-vorbis|OGG/VORBIS audio format support (through symphonia)|
|symphonia-wav|WAV audio format support (through symphonia)|
|tga|TGA image format support|
|trace|Tracing support|
|trace_chrome|Tracing support, saving a file in Chrome Tracing format|
|trace_tracy|Tracing support, exposing a port for Tracy|
|trace_tracy_memory|Tracing support, with memory profiling, exposing a port for Tracy|
|wav|WAV audio format support|
|wayland|Wayland display server support|
|webp|WebP image format support|
|wgpu_trace|Save a trace of all wgpu calls|
|zlib|For KTX2 supercompression|
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