From 38658f0a2ad911cc17656fa84c7722cc664886c4 Mon Sep 17 00:00:00 2001
From: robtfm <50659922+robtfm@users.noreply.github.com>
Date: Wed, 16 Aug 2023 13:54:20 +0100
Subject: [PATCH] add transparent depth bias for stability

---
 .../scene_runner/src/update_world/material.rs | 26 +++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/crates/scene_runner/src/update_world/material.rs b/crates/scene_runner/src/update_world/material.rs
index ba64b02a..934937e9 100644
--- a/crates/scene_runner/src/update_world/material.rs
+++ b/crates/scene_runner/src/update_world/material.rs
@@ -1,4 +1,4 @@
-use bevy::{ecs::system::SystemParam, pbr::NotShadowCaster, prelude::*};
+use bevy::{ecs::system::SystemParam, pbr::NotShadowCaster, prelude::*, render::primitives::Aabb};
 
 use crate::{renderer_context::RendererSceneContext, ContainerEntity, SceneSets};
 use common::util::TryInsertEx;
@@ -128,7 +128,10 @@ impl Plugin for MaterialDefinitionPlugin {
             ComponentPosition::EntityOnly,
         );
 
-        app.add_systems(Update, update_materials.in_set(SceneSets::PostLoop));
+        app.add_systems(
+            Update,
+            (update_materials, update_bias).in_set(SceneSets::PostLoop),
+        );
     }
 }
 
@@ -272,3 +275,22 @@ fn update_materials(
         materials.get_mut(touch);
     }
 }
+
+#[allow(clippy::type_complexity)]
+fn update_bias(
+    mut materials: ResMut<Assets<StandardMaterial>>,
+    query: Query<
+        (&Aabb, &Handle<StandardMaterial>),
+        Or<(Changed<Handle<StandardMaterial>>, Changed<Aabb>)>,
+    >,
+) {
+    for (aabb, h_material) in query.iter() {
+        if let Some(material) = materials.get_mut(h_material) {
+            if material.alpha_mode == AlphaMode::Blend {
+                // add a bias based on the aabb size, to force an explicit transparent order which is
+                // hopefully correct, but should be better than nothing even if not always perfect
+                material.depth_bias = aabb.half_extents.length() * 1e-5;
+            }
+        }
+    }
+}