Allow volumetric fog to be localized to specific, optionally voxelized, regions. (#14099)

Currently, volumetric fog is global and affects the entire scene
uniformly. This is inadequate for many use cases, such as local smoke
effects. To address this problem, this commit introduces *fog volumes*,
which are axis-aligned bounding boxes (AABBs) that specify fog
parameters inside their boundaries. Such volumes can also specify a
*density texture*, a 3D texture of voxels that specifies the density of
the fog at each point.

To create a fog volume, add a `FogVolume` component to an entity (which
is included in the new `FogVolumeBundle` convenience bundle). Like light
probes, a fog volume is conceptually a 1×1×1 cube centered on the
origin; a transform can be used to position and resize this region. Many
of the fields on the existing `VolumetricFogSettings` have migrated to
the new `FogVolume` component. `VolumetricFogSettings` on a camera is
still needed to enable volumetric fog. However, by itself
`VolumetricFogSettings` is no longer sufficient to enable volumetric
fog; a `FogVolume` must be present. Applications that wish to retain the
old global fog behavior can simply surround the scene with a large fog
volume.

By way of implementation, this commit converts the volumetric fog shader
from a full-screen shader to one applied to a mesh. The strategy is
different depending on whether the camera is inside or outside the fog
volume. If the camera is inside the fog volume, the mesh is simply a
plane scaled to the viewport, effectively falling back to a full-screen
pass. If the camera is outside the fog volume, the mesh is a cube
transformed to coincide with the boundaries of the fog volume's AABB.
Importantly, in the latter case, only the front faces of the cuboid are
rendered. Instead of treating the boundaries of the fog as a sphere
centered on the camera position, as we did prior to this patch, we
raytrace the far planes of the AABB to determine the portion of each ray
contained within the fog volume. We then raymarch in shadow map space as
usual. If a density texture is present, we modulate the fixed density
value with the trilinearly-interpolated value from that texture.

Furthermore, this patch introduces optional jitter to fog volumes,
intended for use with TAA. This modifies the position of the ray from
frame to frame using interleaved gradient noise, in order to reduce
aliasing artifacts. Many implementations of volumetric fog in games use
this technique. Note that this patch makes no attempt to write a motion
vector; this is because when a view ray intersects multiple voxels
there's no single direction of motion. Consequently, fog volumes can
have ghosting artifacts, but because fog is "ghostly" by its nature,
these artifacts are less objectionable than they would be for opaque
objects.

A new example, `fog_volumes`, has been added. It demonstrates a single
fog volume containing a voxelized representation of the Stanford bunny.
The existing `volumetric_fog` example has been updated to use the new
local volumetrics API.

## Changelog

### Added

* Local `FogVolume`s are now supported, to localize fog to specific
regions. They can optionally have 3D density voxel textures for precise
control over the distribution of the fog.

### Changed

* `VolumetricFogSettings` on a camera no longer enables volumetric fog;
instead, it simply enables the processing of `FogVolume`s within the
scene.

## Migration Guide

* A `FogVolume` is now necessary in order to enable volumetric fog, in
addition to `VolumetricFogSettings` on the camera. Existing uses of
volumetric fog can be migrated by placing a large `FogVolume`
surrounding the scene.

---------

Co-authored-by: Alice Cecile <[email protected]>
Co-authored-by: François Mockers <[email protected]>

Author: 3 people

Cargo.toml

@@ -3262,6 +3262,17 @@ description = "Demonstrates how to enqueue custom draw commands in a render phas
 category = "Shaders"
 wasm = true
 
+[[example]]
+name = "fog_volumes"
+path = "examples/3d/fog_volumes.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.fog_volumes]
+name = "Fog volumes"
+description = "Demonstrates fog volumes"
+category = "3D Rendering"
+wasm = false
+
 [[example]]
 name = "physics_in_fixed_timestep"
 path = "examples/movement/physics_in_fixed_timestep.rs"

assets/volumes/bunny.ktx2

@@ -57,7 +57,9 @@ pub use prepass::*;
 pub use render::*;
 pub use ssao::*;
 pub use ssr::*;
-pub use volumetric_fog::*;
+pub use volumetric_fog::{
+    FogVolume, FogVolumeBundle, VolumetricFogPlugin, VolumetricFogSettings, VolumetricLight,
+};
 
 pub mod prelude {
     #[doc(hidden)]