From 925ade1ef789fd4dc804507c462fa8e42c509119 Mon Sep 17 00:00:00 2001
From: James Gayfer <10660608+jgayfer@users.noreply.github.com>
Date: Mon, 27 May 2024 12:54:59 -0700
Subject: [PATCH] Use dynamic uniform buffer for post process settings

While learning about shaders and pipelines, I found this example
to be misleading; it wasn't clear to me how the node knew what the correct
"instance" of `PostProcessSettings` we should send to the GPU (as
the combination of `ExtractComponentPlugin` and `UniformComponentPlugin`
extracts + sends _all_ of our `PostProcessSetting` components).

While the example in its current state is _correct_, I believe that fact
that it's intended to showcase a per camera post processing effect
warrants a dynamic uniform buffer (even though in the context of this
example we have only one camera, and therefore no adverse behaviour).
---
 examples/shader/post_processing.rs | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/examples/shader/post_processing.rs b/examples/shader/post_processing.rs
index b3b7330bca8fe..f42e8075ce7e4 100644
--- a/examples/shader/post_processing.rs
+++ b/examples/shader/post_processing.rs
@@ -15,7 +15,8 @@ use bevy::{
     prelude::*,
     render::{
         extract_component::{
-            ComponentUniforms, ExtractComponent, ExtractComponentPlugin, UniformComponentPlugin,
+            ComponentUniforms, DynamicUniformIndex, ExtractComponent, ExtractComponentPlugin,
+            UniformComponentPlugin,
         },
         render_graph::{
             NodeRunError, RenderGraphApp, RenderGraphContext, RenderLabel, ViewNode, ViewNodeRunner,
@@ -124,6 +125,9 @@ impl ViewNode for PostProcessNode {
         &'static ViewTarget,
         // This makes sure the node only runs on cameras with the PostProcessSettings component
         &'static PostProcessSettings,
+        // As there could be multiple post processing components sent to the GPU (one per camera),
+        // we need to get the index of the one that is associated with the current view.
+        &'static DynamicUniformIndex<PostProcessSettings>,
     );
 
     // Runs the node logic
@@ -137,7 +141,7 @@ impl ViewNode for PostProcessNode {
         &self,
         _graph: &mut RenderGraphContext,
         render_context: &mut RenderContext,
-        (view_target, _post_process_settings): QueryItem<Self::ViewQuery>,
+        (view_target, _post_process_settings, settings_index): QueryItem<Self::ViewQuery>,
         world: &World,
     ) -> Result<(), NodeRunError> {
         // Get the pipeline resource that contains the global data we need
@@ -209,7 +213,10 @@ impl ViewNode for PostProcessNode {
         // This is mostly just wgpu boilerplate for drawing a fullscreen triangle,
         // using the pipeline/bind_group created above
         render_pass.set_render_pipeline(pipeline);
-        render_pass.set_bind_group(0, &bind_group, &[]);
+        // By passing in the index of the post process settings on this view, we ensure
+        // that in the event that multiple settings were sent to the GPU (as would be the
+        // case with multiple cameras), we use the correct one.
+        render_pass.set_bind_group(0, &bind_group, &[settings_index.index()]);
         render_pass.draw(0..3, 0..1);
 
         Ok(())
@@ -240,7 +247,7 @@ impl FromWorld for PostProcessPipeline {
                     // The sampler that will be used to sample the screen texture
                     sampler(SamplerBindingType::Filtering),
                     // The settings uniform that will control the effect
-                    uniform_buffer::<PostProcessSettings>(false),
+                    uniform_buffer::<PostProcessSettings>(true),
                 ),
             ),
         );