Support dual source blending (#4022)

Co-authored-by: Teodor Tanasoaia <28601907+teoxoy@users.noreply.github.com>

CHANGELOG.md

@@ -100,6 +100,7 @@ By @wumpf in [#4147](https://github.com/gfx-rs/wgpu/pull/4147)
 - Add validation in accordance with WebGPU `setViewport` valid usage for `x`, `y` and `this.[[attachment_size]]`. By @James2022-rgb in [#4058](https://github.com/gfx-rs/wgpu/pull/4058)
 - `wgpu::CreateSurfaceError` and `wgpu::RequestDeviceError` now give details of the failure, but no longer implement `PartialEq` and cannot be constructed. By @kpreid in [#4066](https://github.com/gfx-rs/wgpu/pull/4066) and [#4145](https://github.com/gfx-rs/wgpu/pull/4145)
 - Make `WGPU_POWER_PREF=none` a valid value. By @fornwall in [4076](https://github.com/gfx-rs/wgpu/pull/4076)
+- Support dual source blending in OpenGL ES, Metal, Vulkan & DX12. By @freqmod in [4022](https://github.com/gfx-rs/wgpu/pull/4022)
 
 #### Vulkan
 

wgpu-core/src/device/resource.rs

@@ -1276,6 +1276,10 @@ impl<A: HalApi> Device<A> {
                 .flags
                 .contains(wgt::DownlevelFlags::MULTISAMPLED_SHADING),
         );
+        caps.set(
+            Caps::DUAL_SOURCE_BLENDING,
+            self.features.contains(wgt::Features::DUAL_SOURCE_BLENDING),
+        );
 
         let info = naga::valid::Validator::new(naga::valid::ValidationFlags::all(), caps)
             .validate(&module)
@@ -2560,6 +2564,8 @@ impl<A: HalApi> Device<A> {
         let mut vertex_steps = Vec::with_capacity(desc.vertex.buffers.len());
         let mut vertex_buffers = Vec::with_capacity(desc.vertex.buffers.len());
         let mut total_attributes = 0;
+        let mut shader_expects_dual_source_blending = false;
+        let mut pipeline_expects_dual_source_blending = false;
         for (i, vb_state) in desc.vertex.buffers.iter().enumerate() {
             vertex_steps.push(pipeline::VertexStep {
                 stride: vb_state.array_stride,
@@ -2700,7 +2706,25 @@ impl<A: HalApi> Device<A> {
                     {
                         break Some(pipeline::ColorStateError::FormatNotMultisampled(cs.format));
                     }
-
+                    if let Some(blend_mode) = cs.blend {
+                        for factor in [
+                            blend_mode.color.src_factor,
+                            blend_mode.color.dst_factor,
+                            blend_mode.alpha.src_factor,
+                            blend_mode.alpha.dst_factor,
+                        ] {
+                            if factor.ref_second_blend_source() {
+                                self.require_features(wgt::Features::DUAL_SOURCE_BLENDING)?;
+                                if i == 0 {
+                                    pipeline_expects_dual_source_blending = true;
+                                    break;
+                                } else {
+                                    return Err(crate::pipeline::CreateRenderPipelineError
+                            ::BlendFactorOnUnsupportedTarget { factor, target: i as u32 });
+                                }
+                            }
+                        }
+                    }
                     break None;
                 };
                 if let Some(e) = error {
@@ -2857,6 +2881,15 @@ impl<A: HalApi> Device<A> {
                     }
                 }
 
+                if let Some(ref interface) = shader_module.interface {
+                    shader_expects_dual_source_blending = interface
+                        .fragment_uses_dual_source_blending(&fragment.stage.entry_point)
+                        .map_err(|error| pipeline::CreateRenderPipelineError::Stage {
+                            stage: flag,
+                            error,
+                        })?;
+                }
+
                 Some(hal::ProgrammableStage {
                     module: &shader_module.raw,
                     entry_point: fragment.stage.entry_point.as_ref(),
@@ -2865,6 +2898,17 @@ impl<A: HalApi> Device<A> {
             None => None,
         };
 
+        if !pipeline_expects_dual_source_blending && shader_expects_dual_source_blending {
+            return Err(
+                pipeline::CreateRenderPipelineError::ShaderExpectsPipelineToUseDualSourceBlending,
+            );
+        }
+        if pipeline_expects_dual_source_blending && !shader_expects_dual_source_blending {
+            return Err(
+                pipeline::CreateRenderPipelineError::PipelineExpectsShaderToUseDualSourceBlending,
+            );
+        }
+
         if validated_stages.contains(wgt::ShaderStages::FRAGMENT) {
             for (i, output) in io.iter() {
                 match color_targets.get(*i as usize) {

wgpu-core/src/pipeline.rs

@@ -384,6 +384,15 @@ pub enum CreateRenderPipelineError {
     },
     #[error("In the provided shader, the type given for group {group} binding {binding} has a size of {size}. As the device does not support `DownlevelFlags::BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED`, the type must have a size that is a multiple of 16 bytes.")]
     UnalignedShader { group: u32, binding: u32, size: u64 },
+    #[error("Using the blend factor {factor:?} for render target {target} is not possible. Only the first render target may be used when dual-source blending.")]
+    BlendFactorOnUnsupportedTarget {
+        factor: wgt::BlendFactor,
+        target: u32,
+    },
+    #[error("Pipeline expects the shader entry point to make use of dual-source blending.")]
+    PipelineExpectsShaderToUseDualSourceBlending,
+    #[error("Shader entry point expects the pipeline to make use of dual-source blending.")]
+    ShaderExpectsPipelineToUseDualSourceBlending,
 }
 
 bitflags::bitflags! {

wgpu-core/src/validation.rs

@@ -116,6 +116,7 @@ struct EntryPoint {
     spec_constants: Vec<SpecializationConstant>,
     sampling_pairs: FastHashSet<(naga::Handle<Resource>, naga::Handle<Resource>)>,
     workgroup_size: [u32; 3],
+    dual_source_blending: bool,
 }
 
 #[derive(Debug)]
@@ -903,7 +904,7 @@ impl Interface {
                 ep.sampling_pairs
                     .insert((resource_mapping[&key.image], resource_mapping[&key.sampler]));
             }
-
+            ep.dual_source_blending = info.dual_source_blending;
             ep.workgroup_size = entry_point.workgroup_size;
 
             entry_points.insert((entry_point.stage, entry_point.name.clone()), ep);
@@ -1177,4 +1178,15 @@ impl Interface {
             .collect();
         Ok(outputs)
     }
+
+    pub fn fragment_uses_dual_source_blending(
+        &self,
+        entry_point_name: &str,
+    ) -> Result<bool, StageError> {
+        let pair = (naga::ShaderStage::Fragment, entry_point_name.to_string());
+        self.entry_points
+            .get(&pair)
+            .ok_or(StageError::MissingEntryPoint(pair.1))
+            .map(|ep| ep.dual_source_blending)
+    }
 }

wgpu-hal/src/dx12/adapter.rs

@@ -250,7 +250,9 @@ impl super::Adapter {
             | wgt::Features::TEXTURE_FORMAT_16BIT_NORM
             | wgt::Features::PUSH_CONSTANTS
             | wgt::Features::SHADER_PRIMITIVE_INDEX
-            | wgt::Features::RG11B10UFLOAT_RENDERABLE;
+            | wgt::Features::RG11B10UFLOAT_RENDERABLE
+            | wgt::Features::DUAL_SOURCE_BLENDING;
+
         //TODO: in order to expose this, we need to run a compute shader
         // that extract the necessary statistics out of the D3D12 result.
         // Alternatively, we could allocate a buffer for the query set,

wgpu-hal/src/dx12/conv.rs

@@ -246,12 +246,12 @@ fn map_blend_factor(factor: wgt::BlendFactor, is_alpha: bool) -> d3d12_ty::D3D12
         Bf::Constant => d3d12_ty::D3D12_BLEND_BLEND_FACTOR,
         Bf::OneMinusConstant => d3d12_ty::D3D12_BLEND_INV_BLEND_FACTOR,
         Bf::SrcAlphaSaturated => d3d12_ty::D3D12_BLEND_SRC_ALPHA_SAT,
-        //Bf::Src1Color if is_alpha => d3d12_ty::D3D12_BLEND_SRC1_ALPHA,
+        Bf::Src1 if is_alpha => d3d12_ty::D3D12_BLEND_SRC1_ALPHA,
-        //Bf::Src1Color => d3d12_ty::D3D12_BLEND_SRC1_COLOR,
+        Bf::Src1 => d3d12_ty::D3D12_BLEND_SRC1_COLOR,
-        //Bf::OneMinusSrc1Color if is_alpha => d3d12_ty::D3D12_BLEND_INV_SRC1_ALPHA,
+        Bf::OneMinusSrc1 if is_alpha => d3d12_ty::D3D12_BLEND_INV_SRC1_ALPHA,
-        //Bf::OneMinusSrc1Color => d3d12_ty::D3D12_BLEND_INV_SRC1_COLOR,
+        Bf::OneMinusSrc1 => d3d12_ty::D3D12_BLEND_INV_SRC1_COLOR,
-        //Bf::Src1Alpha => d3d12_ty::D3D12_BLEND_SRC1_ALPHA,
+        Bf::Src1Alpha => d3d12_ty::D3D12_BLEND_SRC1_ALPHA,
-        //Bf::OneMinusSrc1Alpha => d3d12_ty::D3D12_BLEND_INV_SRC1_ALPHA,
+        Bf::OneMinusSrc1Alpha => d3d12_ty::D3D12_BLEND_INV_SRC1_ALPHA,
     }
 }
 

wgpu-hal/src/gles/adapter.rs

@@ -363,6 +363,10 @@ impl super::Adapter {
             wgt::Features::MULTIVIEW,
             extensions.contains("OVR_multiview2"),
         );
+        features.set(
+            wgt::Features::DUAL_SOURCE_BLENDING,
+            extensions.contains("GL_EXT_blend_func_extended"),
+        );
         features.set(
             wgt::Features::SHADER_PRIMITIVE_INDEX,
             ver >= (3, 2) || extensions.contains("OES_geometry_shader"),

wgpu-hal/src/gles/conv.rs

@@ -376,6 +376,10 @@ fn map_blend_factor(factor: wgt::BlendFactor) -> u32 {
         Bf::Constant => glow::CONSTANT_COLOR,
         Bf::OneMinusConstant => glow::ONE_MINUS_CONSTANT_COLOR,
         Bf::SrcAlphaSaturated => glow::SRC_ALPHA_SATURATE,
+        Bf::Src1 => glow::SRC1_COLOR,
+        Bf::OneMinusSrc1 => glow::ONE_MINUS_SRC1_COLOR,
+        Bf::Src1Alpha => glow::SRC1_ALPHA,
+        Bf::OneMinusSrc1Alpha => glow::ONE_MINUS_SRC1_ALPHA,
     }
 }
 

wgpu-hal/src/metal/adapter.rs

@@ -833,6 +833,10 @@ impl super::PrivateCapabilities {
             self.timestamp_query_support
                 .contains(TimestampQuerySupport::INSIDE_WGPU_PASSES),
         );
+        features.set(
+            F::DUAL_SOURCE_BLENDING,
+            self.msl_version >= MTLLanguageVersion::V1_2 && self.dual_source_blending,
+        );
         features.set(F::TEXTURE_COMPRESSION_ASTC, self.format_astc);
         features.set(F::TEXTURE_COMPRESSION_ASTC_HDR, self.format_astc_hdr);
         features.set(F::TEXTURE_COMPRESSION_BC, self.format_bc);

wgpu-hal/src/metal/conv.rs

@@ -152,13 +152,11 @@ pub fn map_blend_factor(factor: wgt::BlendFactor) -> metal::MTLBlendFactor {
         Bf::OneMinusDstAlpha => OneMinusDestinationAlpha,
         Bf::Constant => BlendColor,
         Bf::OneMinusConstant => OneMinusBlendColor,
-        //Bf::ConstantAlpha => BlendAlpha,
-        //Bf::OneMinusConstantAlpha => OneMinusBlendAlpha,
         Bf::SrcAlphaSaturated => SourceAlphaSaturated,
-        //Bf::Src1 => Source1Color,
+        Bf::Src1 => Source1Color,
-        //Bf::OneMinusSrc1 => OneMinusSource1Color,
+        Bf::OneMinusSrc1 => OneMinusSource1Color,
-        //Bf::Src1Alpha => Source1Alpha,
+        Bf::Src1Alpha => Source1Alpha,
-        //Bf::OneMinusSrc1Alpha => OneMinusSource1Alpha,
+        Bf::OneMinusSrc1Alpha => OneMinusSource1Alpha,
     }
 }
 

wgpu-hal/src/vulkan/adapter.rs

@@ -177,6 +177,7 @@ impl PhysicalDeviceFeatures {
                 //.shader_resource_residency(requested_features.contains(wgt::Features::SHADER_RESOURCE_RESIDENCY))
                 .geometry_shader(requested_features.contains(wgt::Features::SHADER_PRIMITIVE_INDEX))
                 .depth_clamp(requested_features.contains(wgt::Features::DEPTH_CLIP_CONTROL))
+                .dual_src_blend(requested_features.contains(wgt::Features::DUAL_SOURCE_BLENDING))
                 .build(),
             descriptor_indexing: if requested_features.intersects(indexing_features()) {
                 Some(
@@ -460,6 +461,7 @@ impl PhysicalDeviceFeatures {
         }
 
         features.set(F::DEPTH_CLIP_CONTROL, self.core.depth_clamp != 0);
+        features.set(F::DUAL_SOURCE_BLENDING, self.core.dual_src_blend != 0);
 
         if let Some(ref multiview) = self.multiview {
             features.set(F::MULTIVIEW, multiview.multiview != 0);

wgpu-hal/src/vulkan/conv.rs

@@ -792,6 +792,10 @@ fn map_blend_factor(factor: wgt::BlendFactor) -> vk::BlendFactor {
         Bf::SrcAlphaSaturated => vk::BlendFactor::SRC_ALPHA_SATURATE,
         Bf::Constant => vk::BlendFactor::CONSTANT_COLOR,
         Bf::OneMinusConstant => vk::BlendFactor::ONE_MINUS_CONSTANT_COLOR,
+        Bf::Src1 => vk::BlendFactor::SRC1_COLOR,
+        Bf::OneMinusSrc1 => vk::BlendFactor::ONE_MINUS_SRC1_COLOR,
+        Bf::Src1Alpha => vk::BlendFactor::SRC1_ALPHA,
+        Bf::OneMinusSrc1Alpha => vk::BlendFactor::ONE_MINUS_SRC1_ALPHA,
     }
 }
 

wgpu-types/src/lib.rs

@@ -781,7 +781,17 @@ bitflags::bitflags! {
         /// This is a native only feature.
         const SHADER_EARLY_DEPTH_TEST = 1 << 62;
 
-        // 62..64 available
+        /// Allows two outputs from a shader to be used for blending.
+        /// Note that dual-source blending doesn't support multiple render targets.
+        ///
+        /// For more info see the OpenGL ES extension GL_EXT_blend_func_extended.
+        ///
+        /// Supported platforms:
+        /// - OpenGL ES (with GL_EXT_blend_func_extended)
+        /// - Metal (with MSL 1.2+)
+        /// - Vulkan (with dualSrcBlend)
+        /// - DX12
+        const DUAL_SOURCE_BLENDING = 1 << 63;
     }
 }
 
@@ -1549,6 +1559,8 @@ impl TextureViewDimension {
 ///
 /// Corresponds to [WebGPU `GPUBlendFactor`](
 /// https://gpuweb.github.io/gpuweb/#enumdef-gpublendfactor).
+/// Values using S1 requires [`Features::DUAL_SOURCE_BLENDING`] and can only be
+/// used with the first render target.
 #[repr(C)]
 #[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
 #[cfg_attr(feature = "trace", derive(Serialize))]
@@ -1581,6 +1593,29 @@ pub enum BlendFactor {
     Constant = 11,
     /// 1.0 - Constant
     OneMinusConstant = 12,
+    /// S1.component
+    Src1 = 13,
+    /// 1.0 - S1.component
+    OneMinusSrc1 = 14,
+    /// S1.alpha
+    Src1Alpha = 15,
+    /// 1.0 - S1.alpha
+    OneMinusSrc1Alpha = 16,
+}
+
+impl BlendFactor {
+    /// Returns `true` if the blend factor references the second blend source.
+    ///
+    /// Note that the usage of those blend factors require [`Features::DUAL_SOURCE_BLENDING`].
+    pub fn ref_second_blend_source(&self) -> bool {
+        match self {
+            BlendFactor::Src1
+            | BlendFactor::OneMinusSrc1
+            | BlendFactor::Src1Alpha
+            | BlendFactor::OneMinusSrc1Alpha => true,
+            _ => false,
+        }
+    }
 }
 
 /// Alpha blend operation.

wgpu/src/backend/web.rs

@@ -421,6 +421,15 @@ fn map_blend_factor(factor: wgt::BlendFactor) -> web_sys::GpuBlendFactor {
         BlendFactor::SrcAlphaSaturated => bf::SrcAlphaSaturated,
         BlendFactor::Constant => bf::Constant,
         BlendFactor::OneMinusConstant => bf::OneMinusConstant,
+        BlendFactor::Src1
+        | BlendFactor::OneMinusSrc1
+        | BlendFactor::Src1Alpha
+        | BlendFactor::OneMinusSrc1Alpha => {
+            panic!(
+                "{:?} is not enabled for this backend",
+                wgt::Features::DUAL_SOURCE_BLENDING
+            )
+        }
     }
 }