# Objective
- Fixes https://github.com/bevyengine/bevy/issues/13552
## Solution
- Thanks for the guidance from @DGriffin91, the current solution is to
transmit the light_map through the emissive channel to avoid increasing
the bandwidth of deferred shading.
- <del>Store lightmap sample result into G-Buffer and pass them into the
`Deferred Lighting Pipeline`, therefore we can get the correct indirect
lighting via the `apply_pbr_lighting` function.</del>
- <del>The original G-Buffer lacks storage for lightmap data, therefore
a new buffer is added. We can only use Rgba16Uint here due to the
32-byte limit on the render targets.</del>
## Testing
- Need to test all the examples that contains a prepass, with both the
forward and deferred rendering mode.
- I have tested the ones below.
- `lightmaps` (adjust the code based on the issue and check the
rendering result)
- `transmission` (it contains a prepass)
- `ssr` (it also uses the G-Bufffer)
- `meshlet` (forward and deferred)
- `pbr`
## Showcase
By updating the `lightmaps` example to use deferred rendering, this pull
request enables correct rendering result of the Cornell Box.
```
diff --git a/examples/3d/lightmaps.rs b/examples/3d/lightmaps.rs
index 564a3162b..11a748fba 100644
--- a/examples/3d/lightmaps.rs
+++ b/examples/3d/lightmaps.rs
@@ -1,12 +1,14 @@
//! Rendering a scene with baked lightmaps.
-use bevy::pbr::Lightmap;
+use bevy::core_pipeline::prepass::DeferredPrepass;
+use bevy::pbr::{DefaultOpaqueRendererMethod, Lightmap};
use bevy::prelude::*;
fn main() {
App::new()
.add_plugins(DefaultPlugins)
.insert_resource(AmbientLight::NONE)
+ .insert_resource(DefaultOpaqueRendererMethod::deferred())
.add_systems(Startup, setup)
.add_systems(Update, add_lightmaps_to_meshes)
.run();
@@ -19,10 +21,12 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
..default()
});
- commands.spawn(Camera3dBundle {
- transform: Transform::from_xyz(-278.0, 273.0, 800.0),
- ..default()
- });
+ commands
+ .spawn(Camera3dBundle {
+ transform: Transform::from_xyz(-278.0, 273.0, 800.0),
+ ..default()
+ })
+ .insert(DeferredPrepass);
}
fn add_lightmaps_to_meshes(
```
<img width="1280" alt="image"
src="https://github.com/user-attachments/assets/17fd3367-61cc-4c23-b956-e7cfc751af3c">
## Emissive Issue
**The emissive light object appears incorrectly rendered because the
alpha channel of emission is set to 1 in deferred rendering and 0 in
forward rendering, leading to different emissive light result. Could
this be a bug?**
```wgsl
// pbr_deferred_functions.wgsl - pbr_input_from_deferred_gbuffer
let emissive = rgb9e5::rgb9e5_to_vec3_(gbuffer.g);
if ((pbr.material.flags & STANDARD_MATERIAL_FLAGS_UNLIT_BIT) != 0u) {
pbr.material.base_color = vec4(emissive, 1.0);
pbr.material.emissive = vec4(vec3(0.0), 1.0);
} else {
pbr.material.base_color = vec4(pow(base_rough.rgb, vec3(2.2)), 1.0);
pbr.material.emissive = vec4(emissive, 1.0);
}
// pbr_functions.wgsl - apply_pbr_lighting
emissive_light = emissive_light * mix(1.0, view_bindings::view.exposure, emissive.a);
```
---------
Co-authored-by: JMS55 <47158642+JMS55@users.noreply.github.com>
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