Add support for opaque, alpha mask, and alpha blend modes (#3072)

# Objective

Add depth prepass and support for opaque, alpha mask, and alpha blend modes for the 3D PBR target.

## Solution

NOTE: This is based on top of #2861 frustum culling. Just lining it up to keep @cart loaded with the review train. 🚂 

There are a lot of important details here. Big thanks to @cwfitzgerald of wgpu, naga, and rend3 fame for explaining how to do it properly!

* An `AlphaMode` component is added that defines whether a material should be considered opaque, an alpha mask (with a cutoff value that defaults to 0.5, the same as glTF), or transparent and should be alpha blended
* Two depth prepasses are added:
  * Opaque does a plain vertex stage
  * Alpha mask does the vertex stage but also a fragment stage that samples the colour for the fragment and discards if its alpha value is below the cutoff value
  * Both are sorted front to back, not that it matters for these passes. (Maybe there should be a way to skip sorting?)
* Three main passes are added:
  * Opaque and alpha mask passes use a depth comparison function of Equal such that only the geometry that was closest is processed further, due to early-z testing
  * The transparent pass uses the Greater depth comparison function so that only transparent objects that are closer than anything opaque are rendered
  * The opaque fragment shading is as before except that alpha is explicitly set to 1.0
  * Alpha mask fragment shading sets the alpha value to 1.0 if it is equal to or above the cutoff, as defined by glTF
  * Opaque and alpha mask are sorted front to back (again not that it matters as we will skip anything that is not equal... maybe sorting is no longer needed here?)
  * Transparent is sorted back to front. Transparent fragment shading uses the alpha blending over operator

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

pipelined/bevy_core_pipeline/src/lib.rs

@@ -75,11 +75,15 @@ impl Plugin for CorePipelinePlugin {
         let render_app = app.sub_app(RenderApp);
         render_app
             .init_resource::<DrawFunctions<Transparent2d>>()
+            .init_resource::<DrawFunctions<Opaque3d>>()
+            .init_resource::<DrawFunctions<AlphaMask3d>>()
             .init_resource::<DrawFunctions<Transparent3d>>()
             .add_system_to_stage(RenderStage::Extract, extract_clear_color)
             .add_system_to_stage(RenderStage::Extract, extract_core_pipeline_camera_phases)
             .add_system_to_stage(RenderStage::Prepare, prepare_core_views_system)
             .add_system_to_stage(RenderStage::PhaseSort, sort_phase_system::<Transparent2d>)
+            .add_system_to_stage(RenderStage::PhaseSort, sort_phase_system::<Opaque3d>)
+            .add_system_to_stage(RenderStage::PhaseSort, sort_phase_system::<AlphaMask3d>)
             .add_system_to_stage(RenderStage::PhaseSort, sort_phase_system::<Transparent3d>);
 
         let pass_node_2d = MainPass2dNode::new(&mut render_app.world);
@@ -147,6 +151,76 @@ impl PhaseItem for Transparent2d {
     }
 }
 
+pub struct Opaque3d {
+    pub distance: f32,
+    pub pipeline: CachedPipelineId,
+    pub entity: Entity,
+    pub draw_function: DrawFunctionId,
+}
+
+impl PhaseItem for Opaque3d {
+    type SortKey = FloatOrd;
+
+    #[inline]
+    fn sort_key(&self) -> Self::SortKey {
+        FloatOrd(self.distance)
+    }
+
+    #[inline]
+    fn draw_function(&self) -> DrawFunctionId {
+        self.draw_function
+    }
+}
+
+impl EntityPhaseItem for Opaque3d {
+    #[inline]
+    fn entity(&self) -> Entity {
+        self.entity
+    }
+}
+
+impl CachedPipelinePhaseItem for Opaque3d {
+    #[inline]
+    fn cached_pipeline(&self) -> CachedPipelineId {
+        self.pipeline
+    }
+}
+
+pub struct AlphaMask3d {
+    pub distance: f32,
+    pub pipeline: CachedPipelineId,
+    pub entity: Entity,
+    pub draw_function: DrawFunctionId,
+}
+
+impl PhaseItem for AlphaMask3d {
+    type SortKey = FloatOrd;
+
+    #[inline]
+    fn sort_key(&self) -> Self::SortKey {
+        FloatOrd(self.distance)
+    }
+
+    #[inline]
+    fn draw_function(&self) -> DrawFunctionId {
+        self.draw_function
+    }
+}
+
+impl EntityPhaseItem for AlphaMask3d {
+    #[inline]
+    fn entity(&self) -> Entity {
+        self.entity
+    }
+}
+
+impl CachedPipelinePhaseItem for AlphaMask3d {
+    #[inline]
+    fn cached_pipeline(&self) -> CachedPipelineId {
+        self.pipeline
+    }
+}
+
 pub struct Transparent3d {
     pub distance: f32,
     pub pipeline: CachedPipelineId,
@@ -203,9 +277,11 @@ pub fn extract_core_pipeline_camera_phases(
     }
     if let Some(camera_3d) = active_cameras.get(CameraPlugin::CAMERA_3D) {
         if let Some(entity) = camera_3d.entity {
-            commands
-                .get_or_spawn(entity)
-                .insert(RenderPhase::<Transparent3d>::default());
+            commands.get_or_spawn(entity).insert_bundle((
+                RenderPhase::<Opaque3d>::default(),
+                RenderPhase::<AlphaMask3d>::default(),
+                RenderPhase::<Transparent3d>::default(),
+            ));
         }
     }
 }
@@ -215,7 +291,14 @@ pub fn prepare_core_views_system(
     mut texture_cache: ResMut<TextureCache>,
     msaa: Res<Msaa>,
     render_device: Res<RenderDevice>,
-    views_3d: Query<(Entity, &ExtractedView), With<RenderPhase<Transparent3d>>>,
+    views_3d: Query<
+        (Entity, &ExtractedView),
+        (
+            With<RenderPhase<Opaque3d>>,
+            With<RenderPhase<AlphaMask3d>>,
+            With<RenderPhase<Transparent3d>>,
+        ),
+    >,
 ) {
     for (entity, view) in views_3d.iter() {
         let cached_texture = texture_cache.get(

pipelined/bevy_core_pipeline/src/main_pass_3d.rs

@@ -1,12 +1,9 @@
-use crate::{ClearColor, Transparent3d};
+use crate::{AlphaMask3d, ClearColor, Opaque3d, Transparent3d};
 use bevy_ecs::prelude::*;
 use bevy_render2::{
     render_graph::{Node, NodeRunError, RenderGraphContext, SlotInfo, SlotType},
     render_phase::{DrawFunctions, RenderPhase, TrackedRenderPass},
-    render_resource::{
-        LoadOp, Operations, RenderPassColorAttachment, RenderPassDepthStencilAttachment,
-        RenderPassDescriptor,
-    },
+    render_resource::{LoadOp, Operations, RenderPassDepthStencilAttachment, RenderPassDescriptor},
     renderer::RenderContext,
     view::{ExtractedView, ViewDepthTexture, ViewTarget},
 };
@@ -14,6 +11,8 @@ use bevy_render2::{
 pub struct MainPass3dNode {
     query: QueryState<
         (
+            &'static RenderPhase<Opaque3d>,
+            &'static RenderPhase<AlphaMask3d>,
             &'static RenderPhase<Transparent3d>,
             &'static ViewTarget,
             &'static ViewDepthTexture,
@@ -48,52 +47,119 @@ impl Node for MainPass3dNode {
         world: &World,
     ) -> Result<(), NodeRunError> {
         let view_entity = graph.get_input_entity(Self::IN_VIEW)?;
-        let (transparent_phase, target, depth) = self
+        let (opaque_phase, alpha_mask_phase, transparent_phase, target, depth) = self
             .query
             .get_manual(world, view_entity)
             .expect("view entity should exist");
         let clear_color = world.get_resource::<ClearColor>().unwrap();
-        let pass_descriptor = RenderPassDescriptor {
-            label: Some("main_pass_3d"),
-            color_attachments: &[RenderPassColorAttachment {
-                view: if let Some(sampled_target) = &target.sampled_target {
-                    sampled_target
-                } else {
-                    &target.view
-                },
-                resolve_target: if target.sampled_target.is_some() {
-                    Some(&target.view)
-                } else {
-                    None
-                },
-                ops: Operations {
+
+        {
+            // Run the opaque pass, sorted front-to-back
+            // NOTE: Scoped to drop the mutable borrow of render_context
+            let pass_descriptor = RenderPassDescriptor {
+                label: Some("main_opaque_pass_3d"),
+                // NOTE: The opaque pass clears and initializes the color
+                //       buffer as well as writing to it.
+                color_attachments: &[target.get_color_attachment(Operations {
                     load: LoadOp::Clear(clear_color.0.into()),
                     store: true,
-                },
-            }],
-            depth_stencil_attachment: Some(RenderPassDepthStencilAttachment {
-                view: &depth.view,
-                depth_ops: Some(Operations {
-                    load: LoadOp::Clear(0.0),
-                    store: true,
+                })],
+                depth_stencil_attachment: Some(RenderPassDepthStencilAttachment {
+                    view: &depth.view,
+                    // NOTE: The opaque main pass clears and writes to the depth buffer.
+                    depth_ops: Some(Operations {
+                        load: LoadOp::Clear(0.0),
+                        store: true,
+                    }),
+                    stencil_ops: None,
                 }),
-                stencil_ops: None,
-            }),
-        };
+            };
 
-        let draw_functions = world
-            .get_resource::<DrawFunctions<Transparent3d>>()
-            .unwrap();
+            let draw_functions = world.get_resource::<DrawFunctions<Opaque3d>>().unwrap();
 
-        let render_pass = render_context
-            .command_encoder
-            .begin_render_pass(&pass_descriptor);
-        let mut draw_functions = draw_functions.write();
-        let mut tracked_pass = TrackedRenderPass::new(render_pass);
-        for item in transparent_phase.items.iter() {
-            let draw_function = draw_functions.get_mut(item.draw_function).unwrap();
-            draw_function.draw(world, &mut tracked_pass, view_entity, item);
+            let render_pass = render_context
+                .command_encoder
+                .begin_render_pass(&pass_descriptor);
+            let mut draw_functions = draw_functions.write();
+            let mut tracked_pass = TrackedRenderPass::new(render_pass);
+            for item in opaque_phase.items.iter() {
+                let draw_function = draw_functions.get_mut(item.draw_function).unwrap();
+                draw_function.draw(world, &mut tracked_pass, view_entity, item);
+            }
         }
+
+        {
+            // Run the alpha mask pass, sorted front-to-back
+            // NOTE: Scoped to drop the mutable borrow of render_context
+            let pass_descriptor = RenderPassDescriptor {
+                label: Some("main_alpha_mask_pass_3d"),
+                // NOTE: The alpha_mask pass loads the color buffer as well as overwriting it where appropriate.
+                color_attachments: &[target.get_color_attachment(Operations {
+                    load: LoadOp::Load,
+                    store: true,
+                })],
+                depth_stencil_attachment: Some(RenderPassDepthStencilAttachment {
+                    view: &depth.view,
+                    // NOTE: The alpha mask pass loads the depth buffer and possibly overwrites it
+                    depth_ops: Some(Operations {
+                        load: LoadOp::Load,
+                        store: true,
+                    }),
+                    stencil_ops: None,
+                }),
+            };
+
+            let draw_functions = world.get_resource::<DrawFunctions<AlphaMask3d>>().unwrap();
+
+            let render_pass = render_context
+                .command_encoder
+                .begin_render_pass(&pass_descriptor);
+            let mut draw_functions = draw_functions.write();
+            let mut tracked_pass = TrackedRenderPass::new(render_pass);
+            for item in alpha_mask_phase.items.iter() {
+                let draw_function = draw_functions.get_mut(item.draw_function).unwrap();
+                draw_function.draw(world, &mut tracked_pass, view_entity, item);
+            }
+        }
+
+        {
+            // Run the transparent pass, sorted back-to-front
+            // NOTE: Scoped to drop the mutable borrow of render_context
+            let pass_descriptor = RenderPassDescriptor {
+                label: Some("main_transparent_pass_3d"),
+                // NOTE: The transparent pass loads the color buffer as well as overwriting it where appropriate.
+                color_attachments: &[target.get_color_attachment(Operations {
+                    load: LoadOp::Load,
+                    store: true,
+                })],
+                depth_stencil_attachment: Some(RenderPassDepthStencilAttachment {
+                    view: &depth.view,
+                    // NOTE: For the transparent pass we load the depth buffer but do not write to it.
+                    //       As the opaque and alpha mask passes run first, opaque meshes can occlude
+                    //       transparent ones.
+                    depth_ops: Some(Operations {
+                        load: LoadOp::Load,
+                        store: false,
+                    }),
+                    stencil_ops: None,
+                }),
+            };
+
+            let draw_functions = world
+                .get_resource::<DrawFunctions<Transparent3d>>()
+                .unwrap();
+
+            let render_pass = render_context
+                .command_encoder
+                .begin_render_pass(&pass_descriptor);
+            let mut draw_functions = draw_functions.write();
+            let mut tracked_pass = TrackedRenderPass::new(render_pass);
+            for item in transparent_phase.items.iter() {
+                let draw_function = draw_functions.get_mut(item.draw_function).unwrap();
+                draw_function.draw(world, &mut tracked_pass, view_entity, item);
+            }
+        }
+
         Ok(())
     }
 }

pipelined/bevy_gltf2/src/loader.rs

@@ -6,7 +6,7 @@ use bevy_core::Name;
 use bevy_ecs::world::World;
 use bevy_log::warn;
 use bevy_math::{Mat4, Vec3};
-use bevy_pbr2::{PbrBundle, StandardMaterial};
+use bevy_pbr2::{AlphaMode, PbrBundle, StandardMaterial};
 use bevy_render2::{
     camera::{
         Camera, CameraPlugin, CameraProjection, OrthographicProjection, PerspectiveProjection,
@@ -438,6 +438,7 @@ fn load_material(material: &Material, load_context: &mut LoadContext) -> Handle<
             emissive: Color::rgba(emissive[0], emissive[1], emissive[2], 1.0),
             emissive_texture,
             unlit: material.unlit(),
+            alpha_mode: alpha_mode(material),
             ..Default::default()
         }),
     )
@@ -649,6 +650,14 @@ fn get_primitive_topology(mode: Mode) -> Result<PrimitiveTopology, GltfError> {
     }
 }
 
+fn alpha_mode(material: &Material) -> AlphaMode {
+    match material.alpha_mode() {
+        gltf::material::AlphaMode::Opaque => AlphaMode::Opaque,
+        gltf::material::AlphaMode::Mask => AlphaMode::Mask(material.alpha_cutoff().unwrap_or(0.5)),
+        gltf::material::AlphaMode::Blend => AlphaMode::Blend,
+    }
+}
+
 async fn load_buffers(
     gltf: &gltf::Gltf,
     load_context: &LoadContext<'_>,

pipelined/bevy_pbr2/src/alpha.rs

@@ -0,0 +1,22 @@
+use bevy_ecs::reflect::ReflectComponent;
+use bevy_reflect::Reflect;
+
+// FIXME: This should probably be part of bevy_render2!
+/// Alpha mode
+#[derive(Debug, Reflect, Clone, PartialEq)]
+#[reflect(Component)]
+pub enum AlphaMode {
+    Opaque,
+    /// An alpha cutoff must be supplied where alpha values >= the cutoff
+    /// will be fully opaque and < will be fully transparent
+    Mask(f32),
+    Blend,
+}
+
+impl Eq for AlphaMode {}
+
+impl Default for AlphaMode {
+    fn default() -> Self {
+        AlphaMode::Opaque
+    }
+}

pipelined/bevy_pbr2/src/lib.rs

@@ -1,8 +1,10 @@
+mod alpha;
 mod bundle;
 mod light;
 mod material;
 mod render;
 
+pub use alpha::*;
 pub use bundle::*;
 pub use light::*;
 pub use material::*;
@@ -10,7 +12,7 @@ pub use render::*;
 
 use bevy_app::prelude::*;
 use bevy_asset::{Assets, Handle, HandleUntyped};
-use bevy_core_pipeline::Transparent3d;
+use bevy_core_pipeline::{AlphaMask3d, Opaque3d, Transparent3d};
 use bevy_ecs::prelude::*;
 use bevy_reflect::TypeUuid;
 use bevy_render2::{
@@ -109,6 +111,8 @@ impl Plugin for PbrPlugin {
             .init_resource::<SpecializedPipelines<ShadowPipeline>>();
 
         let shadow_pass_node = ShadowPassNode::new(&mut render_app.world);
+        render_app.add_render_command::<Opaque3d, DrawPbr>();
+        render_app.add_render_command::<AlphaMask3d, DrawPbr>();
         render_app.add_render_command::<Transparent3d, DrawPbr>();
         render_app.add_render_command::<Shadow, DrawShadowMesh>();
         let mut graph = render_app.world.get_resource_mut::<RenderGraph>().unwrap();

pipelined/bevy_pbr2/src/material.rs

@@ -1,4 +1,4 @@
-use crate::{PbrPipeline, StandardMaterialFlags};
+use crate::{AlphaMode, PbrPipeline, StandardMaterialFlags};
 use bevy_app::{App, Plugin};
 use bevy_asset::{AddAsset, Handle};
 use bevy_ecs::system::{lifetimeless::SRes, SystemParamItem};
@@ -7,9 +7,7 @@ use bevy_reflect::TypeUuid;
 use bevy_render2::{
     color::Color,
     render_asset::{PrepareAssetError, RenderAsset, RenderAssetPlugin, RenderAssets},
-    render_resource::{
-        BindGroup, Buffer, BufferInitDescriptor, BufferUsages, Sampler, TextureView,
-    },
+    render_resource::{BindGroup, Buffer, BufferInitDescriptor, BufferUsages},
     renderer::RenderDevice,
     texture::Image,
 };
@@ -47,6 +45,7 @@ pub struct StandardMaterial {
     pub occlusion_texture: Option<Handle<Image>>,
     pub double_sided: bool,
     pub unlit: bool,
+    pub alpha_mode: AlphaMode,
 }
 
 impl Default for StandardMaterial {
@@ -73,6 +72,7 @@ impl Default for StandardMaterial {
             normal_map_texture: None,
             double_sided: false,
             unlit: false,
+            alpha_mode: AlphaMode::Opaque,
         }
     }
 }
@@ -95,7 +95,7 @@ impl From<Handle<Image>> for StandardMaterial {
     }
 }
 
-#[derive(Clone, AsStd140)]
+#[derive(Clone, Default, AsStd140)]
 pub struct StandardMaterialUniformData {
     /// Doubles as diffuse albedo for non-metallic, specular for metallic and a mix for everything
     /// in between.
@@ -112,6 +112,9 @@ pub struct StandardMaterialUniformData {
     /// defaults to 0.5 which is mapped to 4% reflectance in the shader
     pub reflectance: f32,
     pub flags: u32,
+    /// When the alpha mode mask flag is set, any base color alpha above this cutoff means fully opaque,
+    /// and any below means fully transparent.
+    pub alpha_cutoff: f32,
 }
 
 pub struct StandardMaterialPlugin;
@@ -127,7 +130,10 @@ impl Plugin for StandardMaterialPlugin {
 pub struct GpuStandardMaterial {
     pub buffer: Buffer,
     pub bind_group: BindGroup,
+    pub has_normal_map: bool,
     pub flags: StandardMaterialFlags,
+    pub base_color_texture: Option<Handle<Image>>,
+    pub alpha_mode: AlphaMode,
 }
 
 impl RenderAsset for StandardMaterial {
@@ -148,7 +154,7 @@ impl RenderAsset for StandardMaterial {
         (render_device, pbr_pipeline, gpu_images): &mut SystemParamItem<Self::Param>,
     ) -> Result<Self::PreparedAsset, PrepareAssetError<Self::ExtractedAsset>> {
         let (base_color_texture_view, base_color_sampler) = if let Some(result) =
-            image_handle_to_view_sampler(pbr_pipeline, gpu_images, &material.base_color_texture)
+            pbr_pipeline.image_handle_to_texture(gpu_images, &material.base_color_texture)
         {
             result
         } else {
@@ -156,7 +162,7 @@ impl RenderAsset for StandardMaterial {
         };
 
         let (emissive_texture_view, emissive_sampler) = if let Some(result) =
-            image_handle_to_view_sampler(pbr_pipeline, gpu_images, &material.emissive_texture)
+            pbr_pipeline.image_handle_to_texture(gpu_images, &material.emissive_texture)
         {
             result
         } else {
@@ -164,24 +170,21 @@ impl RenderAsset for StandardMaterial {
         };
 
         let (metallic_roughness_texture_view, metallic_roughness_sampler) = if let Some(result) =
-            image_handle_to_view_sampler(
-                pbr_pipeline,
-                gpu_images,
-                &material.metallic_roughness_texture,
-            ) {
+            pbr_pipeline.image_handle_to_texture(gpu_images, &material.metallic_roughness_texture)
+        {
             result
         } else {
             return Err(PrepareAssetError::RetryNextUpdate(material));
         };
         let (normal_map_texture_view, normal_map_sampler) = if let Some(result) =
-            image_handle_to_view_sampler(pbr_pipeline, gpu_images, &material.normal_map_texture)
+            pbr_pipeline.image_handle_to_texture(gpu_images, &material.normal_map_texture)
         {
             result
         } else {
             return Err(PrepareAssetError::RetryNextUpdate(material));
         };
         let (occlusion_texture_view, occlusion_sampler) = if let Some(result) =
-            image_handle_to_view_sampler(pbr_pipeline, gpu_images, &material.occlusion_texture)
+            pbr_pipeline.image_handle_to_texture(gpu_images, &material.occlusion_texture)
         {
             result
         } else {
@@ -206,9 +209,18 @@ impl RenderAsset for StandardMaterial {
         if material.unlit {
             flags |= StandardMaterialFlags::UNLIT;
         }
-        if material.normal_map_texture.is_some() {
-            flags |= StandardMaterialFlags::NORMAL_MAP_TEXTURE;
-        }
+        let has_normal_map = material.normal_map_texture.is_some();
+        // NOTE: 0.5 is from the glTF default - do we want this?
+        let mut alpha_cutoff = 0.5;
+        match material.alpha_mode {
+            AlphaMode::Opaque => flags |= StandardMaterialFlags::ALPHA_MODE_OPAQUE,
+            AlphaMode::Mask(c) => {
+                alpha_cutoff = c;
+                flags |= StandardMaterialFlags::ALPHA_MODE_MASK
+            }
+            AlphaMode::Blend => flags |= StandardMaterialFlags::ALPHA_MODE_BLEND,
+        };
+
         let value = StandardMaterialUniformData {
             base_color: material.base_color.as_rgba_linear().into(),
             emissive: material.emissive.into(),
@@ -216,6 +228,7 @@ impl RenderAsset for StandardMaterial {
             metallic: material.metallic,
             reflectance: material.reflectance,
             flags: flags.bits(),
+            alpha_cutoff,
         };
         let value_std140 = value.as_std140();
 
@@ -279,22 +292,9 @@ impl RenderAsset for StandardMaterial {
             buffer,
             bind_group,
             flags,
+            has_normal_map,
+            base_color_texture: material.base_color_texture,
+            alpha_mode: material.alpha_mode,
         })
     }
 }
-
-fn image_handle_to_view_sampler<'a>(
-    pbr_pipeline: &'a PbrPipeline,
-    gpu_images: &'a RenderAssets<Image>,
-    handle_option: &Option<Handle<Image>>,
-) -> Option<(&'a TextureView, &'a Sampler)> {
-    if let Some(handle) = handle_option {
-        let gpu_image = gpu_images.get(handle)?;
-        Some((&gpu_image.texture_view, &gpu_image.sampler))
-    } else {
-        Some((
-            &pbr_pipeline.dummy_white_gpu_image.texture_view,
-            &pbr_pipeline.dummy_white_gpu_image.sampler,
-        ))
-    }
-}

pipelined/bevy_pbr2/src/render/mod.rs

@@ -3,11 +3,11 @@ mod light;
 pub use light::*;
 
 use crate::{
-    NotShadowCaster, NotShadowReceiver, StandardMaterial, StandardMaterialUniformData,
+    AlphaMode, NotShadowCaster, NotShadowReceiver, StandardMaterial, StandardMaterialUniformData,
     PBR_SHADER_HANDLE,
 };
 use bevy_asset::Handle;
-use bevy_core_pipeline::Transparent3d;
+use bevy_core_pipeline::{AlphaMask3d, Opaque3d, Transparent3d};
 use bevy_ecs::{
     prelude::*,
     system::{lifetimeless::*, SystemParamItem},
@@ -61,7 +61,9 @@ bitflags::bitflags! {
         const OCCLUSION_TEXTURE          = (1 << 3);
         const DOUBLE_SIDED               = (1 << 4);
         const UNLIT                      = (1 << 5);
-        const NORMAL_MAP_TEXTURE         = (1 << 6);
+        const ALPHA_MODE_OPAQUE          = (1 << 6);
+        const ALPHA_MODE_MASK            = (1 << 7);
+        const ALPHA_MODE_BLEND           = (1 << 8);
         const NONE                       = 0;
         const UNINITIALIZED              = 0xFFFF;
     }
@@ -153,6 +155,24 @@ pub struct PbrPipeline {
     pub dummy_white_gpu_image: GpuImage,
 }
 
+impl PbrPipeline {
+    pub fn image_handle_to_texture<'a>(
+        &'a self,
+        gpu_images: &'a RenderAssets<Image>,
+        handle_option: &Option<Handle<Image>>,
+    ) -> Option<(&'a TextureView, &'a Sampler)> {
+        if let Some(handle) = handle_option {
+            let gpu_image = gpu_images.get(handle)?;
+            Some((&gpu_image.texture_view, &gpu_image.sampler))
+        } else {
+            Some((
+                &self.dummy_white_gpu_image.texture_view,
+                &self.dummy_white_gpu_image.sampler,
+            ))
+        }
+    }
+}
+
 impl FromWorld for PbrPipeline {
     fn from_world(world: &mut World) -> Self {
         let render_device = world.get_resource::<RenderDevice>().unwrap();
@@ -426,6 +446,9 @@ bitflags::bitflags! {
         const NONE                        = 0;
         const VERTEX_TANGENTS             = (1 << 0);
         const STANDARDMATERIAL_NORMAL_MAP = (1 << 1);
+        const OPAQUE_MAIN_PASS            = (1 << 2);
+        const ALPHA_MASK_MAIN_PASS        = (1 << 3);
+        const TRANSPARENT_MAIN_PASS       = (1 << 4);
         const MSAA_RESERVED_BITS          = PbrPipelineKey::MSAA_MASK_BITS << PbrPipelineKey::MSAA_SHIFT_BITS;
     }
 }
@@ -511,6 +534,21 @@ impl SpecializedPipeline for PbrPipeline {
         if key.contains(PbrPipelineKey::STANDARDMATERIAL_NORMAL_MAP) {
             shader_defs.push(String::from("STANDARDMATERIAL_NORMAL_MAP"));
         }
+        let (label, blend, depth_write_enabled);
+        if key.contains(PbrPipelineKey::TRANSPARENT_MAIN_PASS) {
+            label = Some("transparent_pbr_pipeline".into());
+            blend = Some(BlendState::ALPHA_BLENDING);
+            // For the transparent pass, fragments that are closer will be alpha blended
+            // but their depth is not written to the depth buffer
+            depth_write_enabled = false;
+        } else {
+            label = Some("opaque_pbr_pipeline".into());
+            blend = Some(BlendState::REPLACE);
+            // For the opaque and alpha mask passes, fragments that are closer will replace
+            // the current fragment value in the output and the depth is written to the
+            // depth buffer
+            depth_write_enabled = true;
+        }
         RenderPipelineDescriptor {
             vertex: VertexState {
                 shader: PBR_SHADER_HANDLE.typed::<Shader>(),
@@ -528,18 +566,7 @@ impl SpecializedPipeline for PbrPipeline {
                 entry_point: "fragment".into(),
                 targets: vec![ColorTargetState {
                     format: TextureFormat::bevy_default(),
-                    blend: Some(BlendState {
-                        color: BlendComponent {
-                            src_factor: BlendFactor::SrcAlpha,
-                            dst_factor: BlendFactor::OneMinusSrcAlpha,
-                            operation: BlendOperation::Add,
-                        },
-                        alpha: BlendComponent {
-                            src_factor: BlendFactor::One,
-                            dst_factor: BlendFactor::One,
-                            operation: BlendOperation::Add,
-                        },
-                    }),
+                    blend,
                     write_mask: ColorWrites::ALL,
                 }],
             }),
@@ -559,7 +586,7 @@ impl SpecializedPipeline for PbrPipeline {
             },
             depth_stencil: Some(DepthStencilState {
                 format: TextureFormat::Depth32Float,
-                depth_write_enabled: true,
+                depth_write_enabled,
                 depth_compare: CompareFunction::Greater,
                 stencil: StencilState {
                     front: StencilFaceState::IGNORE,
@@ -578,7 +605,7 @@ impl SpecializedPipeline for PbrPipeline {
                 mask: !0,
                 alpha_to_coverage_enabled: false,
             },
-            label: Some("pbr_pipeline".into()),
+            label,
         }
     }
 }
@@ -614,7 +641,9 @@ pub struct PbrViewBindGroup {
 #[allow(clippy::too_many_arguments)]
 pub fn queue_meshes(
     mut commands: Commands,
-    transparent_3d_draw_functions: Res<DrawFunctions<Transparent3d>>,
+    opaque_draw_functions: Res<DrawFunctions<Opaque3d>>,
+    alpha_mask_draw_functions: Res<DrawFunctions<AlphaMask3d>>,
+    transparent_draw_functions: Res<DrawFunctions<Transparent3d>>,
     render_device: Res<RenderDevice>,
     pbr_pipeline: Res<PbrPipeline>,
     shadow_pipeline: Res<ShadowPipeline>,
@@ -631,6 +660,8 @@ pub fn queue_meshes(
         &ExtractedView,
         &ViewLights,
         &VisibleEntities,
+        &mut RenderPhase<Opaque3d>,
+        &mut RenderPhase<AlphaMask3d>,
         &mut RenderPhase<Transparent3d>,
     )>,
 ) {
@@ -638,7 +669,15 @@ pub fn queue_meshes(
         view_uniforms.uniforms.binding(),
         light_meta.view_gpu_lights.binding(),
     ) {
-        for (entity, view, view_lights, visible_entities, mut transparent_phase) in views.iter_mut()
+        for (
+            entity,
+            view,
+            view_lights,
+            visible_entities,
+            mut opaque_phase,
+            mut alpha_mask_phase,
+            mut transparent_phase,
+        ) in views.iter_mut()
         {
             let view_bind_group = render_device.create_bind_group(&BindGroupDescriptor {
                 entries: &[
@@ -681,46 +720,85 @@ pub fn queue_meshes(
                 value: view_bind_group,
             });
 
-            let draw_pbr = transparent_3d_draw_functions
+            let draw_opaque_pbr = opaque_draw_functions.read().get_id::<DrawPbr>().unwrap();
+            let draw_alpha_mask_pbr = alpha_mask_draw_functions
+                .read()
+                .get_id::<DrawPbr>()
+                .unwrap();
+            let draw_transparent_pbr = transparent_draw_functions
                 .read()
                 .get_id::<DrawPbr>()
                 .unwrap();
 
-            let view_matrix = view.transform.compute_matrix();
-            let view_row_2 = view_matrix.row(2);
+            let inverse_view_matrix = view.transform.compute_matrix().inverse();
+            let inverse_view_row_2 = inverse_view_matrix.row(2);
 
             for visible_entity in &visible_entities.entities {
                 if let Ok((material_handle, mesh_handle, mesh_uniform)) =
                     standard_material_meshes.get(visible_entity.entity)
                 {
                     let mut key = PbrPipelineKey::from_msaa_samples(msaa.samples);
-                    if let Some(material) = render_materials.get(material_handle) {
-                        if material
-                            .flags
-                            .contains(StandardMaterialFlags::NORMAL_MAP_TEXTURE)
-                        {
+                    let alpha_mode = if let Some(material) = render_materials.get(material_handle) {
+                        if material.has_normal_map {
                             key |= PbrPipelineKey::STANDARDMATERIAL_NORMAL_MAP;
                         }
+                        material.alpha_mode.clone()
                     } else {
                         continue;
-                    }
+                    };
                     if let Some(mesh) = render_meshes.get(mesh_handle) {
                         if mesh.has_tangents {
                             key |= PbrPipelineKey::VERTEX_TANGENTS;
                         }
                     }
+                    key |= match alpha_mode {
+                        AlphaMode::Opaque => PbrPipelineKey::OPAQUE_MAIN_PASS,
+                        AlphaMode::Mask(_) => PbrPipelineKey::ALPHA_MASK_MAIN_PASS,
+                        AlphaMode::Blend => PbrPipelineKey::TRANSPARENT_MAIN_PASS,
+                    };
                     let pipeline_id = pipelines.specialize(&mut pipeline_cache, &pbr_pipeline, key);
 
-                    // NOTE: row 2 of the view matrix dotted with column 3 of the model matrix
+                    // NOTE: row 2 of the inverse view matrix dotted with column 3 of the model matrix
                     // gives the z component of translation of the mesh in view space
-                    let mesh_z = view_row_2.dot(mesh_uniform.transform.col(3));
-                    // TODO: currently there is only "transparent phase". this should pick transparent vs opaque according to the mesh material
-                    transparent_phase.add(Transparent3d {
-                        entity: visible_entity.entity,
-                        draw_function: draw_pbr,
-                        pipeline: pipeline_id,
-                        distance: mesh_z,
-                    });
+                    let mesh_z = inverse_view_row_2.dot(mesh_uniform.transform.col(3));
+                    match alpha_mode {
+                        AlphaMode::Opaque => {
+                            opaque_phase.add(Opaque3d {
+                                entity: visible_entity.entity,
+                                draw_function: draw_opaque_pbr,
+                                pipeline: pipeline_id,
+                                // NOTE: Front-to-back ordering for opaque with ascending sort means near should have the
+                                //       lowest sort key and getting further away should increase. As we have
+                                //       -z in front of the camera, values in view space decrease away from the
+                                //       camera. Flipping the sign of mesh_z results in the correct front-to-back ordering
+                                distance: -mesh_z,
+                            });
+                        }
+                        AlphaMode::Mask(_) => {
+                            alpha_mask_phase.add(AlphaMask3d {
+                                entity: visible_entity.entity,
+                                draw_function: draw_alpha_mask_pbr,
+                                pipeline: pipeline_id,
+                                // NOTE: Front-to-back ordering for alpha mask with ascending sort means near should have the
+                                //       lowest sort key and getting further away should increase. As we have
+                                //       -z in front of the camera, values in view space decrease away from the
+                                //       camera. Flipping the sign of mesh_z results in the correct front-to-back ordering
+                                distance: -mesh_z,
+                            });
+                        }
+                        AlphaMode::Blend => {
+                            transparent_phase.add(Transparent3d {
+                                entity: visible_entity.entity,
+                                draw_function: draw_transparent_pbr,
+                                pipeline: pipeline_id,
+                                // NOTE: Back-to-front ordering for transparent with ascending sort means far should have the
+                                //       lowest sort key and getting closer should increase. As we have
+                                //       -z in front of the camera, the largest distance is -far with values increasing toward the
+                                //       camera. As such we can just use mesh_z as the distance
+                                distance: mesh_z,
+                            });
+                        }
+                    }
                 }
             }
         }

pipelined/bevy_pbr2/src/render/pbr.wgsl

@@ -110,6 +110,7 @@ struct StandardMaterial {
     reflectance: f32;
     // 'flags' is a bit field indicating various options. u32 is 32 bits so we have up to 32 options.
     flags: u32;
+    alpha_cutoff: f32;
 };
 
 let STANDARD_MATERIAL_FLAGS_BASE_COLOR_TEXTURE_BIT: u32         = 1u;
@@ -118,6 +119,9 @@ let STANDARD_MATERIAL_FLAGS_METALLIC_ROUGHNESS_TEXTURE_BIT: u32 = 4u;
 let STANDARD_MATERIAL_FLAGS_OCCLUSION_TEXTURE_BIT: u32          = 8u;
 let STANDARD_MATERIAL_FLAGS_DOUBLE_SIDED_BIT: u32               = 16u;
 let STANDARD_MATERIAL_FLAGS_UNLIT_BIT: u32                      = 32u;
+let STANDARD_MATERIAL_FLAGS_ALPHA_MODE_OPAQUE: u32              = 64u;
+let STANDARD_MATERIAL_FLAGS_ALPHA_MODE_MASK: u32                = 128u;
+let STANDARD_MATERIAL_FLAGS_ALPHA_MODE_BLEND: u32               = 256u;
 
 struct PointLight {
     projection: mat4x4<f32>;
@@ -559,6 +563,20 @@ fn fragment(in: FragmentInput) -> [[location(0)]] vec4<f32> {
 #endif
 #endif
 
+        if ((material.flags & STANDARD_MATERIAL_FLAGS_ALPHA_MODE_OPAQUE) != 0u) {
+            // NOTE: If rendering as opaque, alpha should be ignored so set to 1.0
+            output_color.a = 1.0;
+        } elseif ((material.flags & STANDARD_MATERIAL_FLAGS_ALPHA_MODE_MASK) != 0u) {
+            if (output_color.a >= material.alpha_cutoff) {
+                // NOTE: If rendering as masked alpha and >= the cutoff, render as fully opaque
+                output_color.a = 1.0;
+            } else {
+                // NOTE: output_color.a < material.alpha_cutoff should not is not rendered
+                // NOTE: This and any other discards mean that early-z testing cannot be done!
+                discard;
+            }
+        }
+
         var V: vec3<f32>;
         if (view.projection[3].w != 1.0) { // If the projection is not orthographic
             // Only valid for a perpective projection

pipelined/bevy_render2/src/view/mod.rs

@@ -2,7 +2,10 @@ pub mod visibility;
 pub mod window;
 
 pub use visibility::*;
-use wgpu::{Extent3d, TextureDescriptor, TextureDimension, TextureFormat, TextureUsages};
+use wgpu::{
+    Color, Extent3d, Operations, RenderPassColorAttachment, TextureDescriptor, TextureDimension,
+    TextureFormat, TextureUsages,
+};
 pub use window::*;
 
 use crate::{
@@ -83,6 +86,24 @@ pub struct ViewTarget {
     pub sampled_target: Option<TextureView>,
 }
 
+impl ViewTarget {
+    pub fn get_color_attachment(&self, ops: Operations<Color>) -> RenderPassColorAttachment {
+        RenderPassColorAttachment {
+            view: if let Some(sampled_target) = &self.sampled_target {
+                sampled_target
+            } else {
+                &self.view
+            },
+            resolve_target: if self.sampled_target.is_some() {
+                Some(&self.view)
+            } else {
+                None
+            },
+            ops,
+        }
+    }
+}
+
 pub struct ViewDepthTexture {
     pub texture: Texture,
     pub view: TextureView,

pipelined/bevy_sprite2/src/render/mod.rs

@@ -133,18 +133,7 @@ impl SpecializedPipeline for SpritePipeline {
                 entry_point: "fragment".into(),
                 targets: vec![ColorTargetState {
                     format: TextureFormat::bevy_default(),
-                    blend: Some(BlendState {
-                        color: BlendComponent {
-                            src_factor: BlendFactor::SrcAlpha,
-                            dst_factor: BlendFactor::OneMinusSrcAlpha,
-                            operation: BlendOperation::Add,
-                        },
-                        alpha: BlendComponent {
-                            src_factor: BlendFactor::One,
-                            dst_factor: BlendFactor::One,
-                            operation: BlendOperation::Add,
-                        },
-                    }),
+                    blend: Some(BlendState::ALPHA_BLENDING),
                     write_mask: ColorWrites::ALL,
                 }],
             }),