feat: implement premultiplied and add alpha modes in OIT

Signed-off-by: Schmarni <marnistromer@gmail.com>

Author: Schmarni-Dev

crates/bevy_core_pipeline/src/oit/oit_draw.wgsl

@@ -14,7 +14,11 @@ fn oit_draw(position: vec4f, color: vec4f) {
 #endif
     // Don't add fully transparent fragments to the list
     // because we don't want to have to sort them in the resolve pass
-    if color.a < oit_settings.alpha_threshold {
+    if color.r < oit_settings.alpha_threshold
+        && color.g < oit_settings.alpha_threshold
+        && color.b < oit_settings.alpha_threshold
+        && color.a < oit_settings.alpha_threshold
+    {
         return;
     }
     // get the index of the current fragment relative to the screen size

crates/bevy_core_pipeline/src/oit/resolve/oit_resolve.wgsl

@@ -116,7 +116,7 @@ fn resolve(head: u32, opaque_depth: f32) -> vec4<f32> {
     for (var i = 0u; i < sorted_frag_count; i += 1) {
         let color = bevy_pbr::rgb9e5::rgb9e5_to_vec3_(fragment_list[i].color);
         let alpha = packed_depth_alpha_get_alpha(fragment_list[i].depth_alpha);
-        var base_color = vec4(color.rgb * alpha, alpha);
+        var base_color = vec4(color.rgb, alpha);
         final_color = blend(final_color, base_color);
         if final_color.a == 1.0 {
             break;

crates/bevy_pbr/src/render/mesh.rs

@@ -3416,7 +3416,12 @@ impl SpecializedMeshPipeline for MeshPipeline {
         let (label, blend, depth_write_enabled);
         let pass = key.intersection(MeshPipelineKey::BLEND_RESERVED_BITS);
         let (mut is_opaque, mut alpha_to_coverage_enabled) = (false, false);
-        if key.contains(MeshPipelineKey::OIT_ENABLED) && pass == MeshPipelineKey::BLEND_ALPHA {
+        if key.contains(MeshPipelineKey::OIT_ENABLED)
+            && matches!(
+                pass,
+                MeshPipelineKey::BLEND_ALPHA | MeshPipelineKey::BLEND_PREMULTIPLIED_ALPHA
+            )
+        {
             label = "oit_mesh_pipeline".into();
             // TODO tail blending would need alpha blending
             blend = None;

crates/bevy_pbr/src/render/pbr.wgsl

@@ -91,7 +91,17 @@ fn fragment(
 
 #ifdef OIT_ENABLED
     let alpha_mode = pbr_input.material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_RESERVED_BITS;
-    if alpha_mode != pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_OPAQUE {
+    if alpha_mode == pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_BLEND {
+        // The fragments will only be drawn during the oit resolve pass.
+        oit_draw(in.position, vec4(out.color.rgb * out.color.a, out.color.a));
+        discard;
+    }
+    if alpha_mode == pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_PREMULTIPLIED {
+        // The fragments will only be drawn during the oit resolve pass.
+        oit_draw(in.position, out.color);
+        discard;
+    }
+    if alpha_mode == pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_ADD {
         // The fragments will only be drawn during the oit resolve pass.
         oit_draw(in.position, out.color);
         discard;

examples/3d/order_independent_transparency.rs

@@ -162,7 +162,7 @@ fn spawn_spheres(
         Mesh3d(sphere_handle.clone()),
         MeshMaterial3d(materials.add(StandardMaterial {
             base_color: GREEN.with_alpha(alpha).into(),
-            alpha_mode: AlphaMode::Blend,
+            alpha_mode: AlphaMode::Premultiplied,
             ..default()
         })),
         Transform::from_translation(pos_b + offset),
@@ -172,7 +172,7 @@ fn spawn_spheres(
         Mesh3d(sphere_handle.clone()),
         MeshMaterial3d(materials.add(StandardMaterial {
             base_color: BLUE.with_alpha(alpha).into(),
-            alpha_mode: AlphaMode::Blend,
+            alpha_mode: AlphaMode::Add,
             ..default()
         })),
         Transform::from_translation(pos_c + offset),