gpuweb · kainino0x · Jun 18, 2024 · May 6, 2024 · May 6, 2024 · May 7, 2024
diff --git a/spec/index.bs b/spec/index.bs
@@ -88,6 +88,7 @@ spec: WGSL; urlPrefix: https://gpuweb.github.io/gpuweb/wgsl/#
     type: dfn
         text: f16; url: f16
         text: location; url: input-output-locations
+        text: blend_src; url: input-output-locations
         text: interpolation; url: interpolation
         text: pipeline-overridable; url: pipeline-overridable
         text: pipeline constant ID; url: pipeline-constant-id
@@ -124,6 +125,7 @@ spec: WGSL; urlPrefix: https://gpuweb.github.io/gpuweb/wgsl/#
             text: frag_depth; url: built-in-values-frag_depth
         for: extension
             text: f16; url: extension-f16
+            text: dual_source_blending; url: extension-dual_source_blending
     type: abstract-op
         text: SizeOf; url: sizeof
 spec: Internationalization Glossary; urlPrefix: https://www.w3.org/TR/i18n-glossary/#
@@ -8101,20 +8103,30 @@ dictionary GPUFragmentState
             - [$validating GPUProgrammableStage$]({{GPUShaderStage/FRAGMENT}}, |descriptor|, |layout|) succeeds.
             - |descriptor|.{{GPUFragmentState/targets}}.length must be &le;
                 |device|.{{device/[[limits]]}}.{{supported limits/maxColorAttachments}}.
+            - Let |entryPoint| be [$get the entry point$]({{GPUShaderStage/FRAGMENT}}, |descriptor|).
+            - Let |usesDualSourceBlending| be `false`.
             - [=list/For each=] |index| of the [=list/indices=] of |descriptor|.{{GPUFragmentState/targets}}
                 containing a non-`null` value |colorState|:
                 - |colorState|.{{GPUColorTargetState/format}} must be listed in [[#plain-color-formats]]
                     with {{GPUTextureUsage/RENDER_ATTACHMENT}} capability.
+                - |colorState|.{{GPUColorTargetState/writeMask}} must be &lt; 16.
                 - If |colorState|.{{GPUColorTargetState/blend}} is [=map/exist|provided=]:
                     - The |colorState|.{{GPUColorTargetState/format}} must be [=blendable=].
                     - |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/color}}
                         must be a [=valid GPUBlendComponent=].
                     - |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/alpha}}
                         must be a [=valid GPUBlendComponent=].
-                - |colorState|.{{GPUColorTargetState/writeMask}} must be &lt; 16.
-                - If [$get the entry point$]({{GPUShaderStage/FRAGMENT}}, |descriptor|) has a
-                    [=shader stage output=] value |output| with [=location=] attribute equal to |index|:
-
+                - If |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/color}}.{{GPUBlendComponent/srcFactor}} or
+                    |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/color}}.{{GPUBlendComponent/dstFactor}} or
+                    |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/alpha}}.{{GPUBlendComponent/srcFactor}} or
+                    |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/alpha}}.{{GPUBlendComponent/dstFactor}}
+                    uses the second input of the corresponding blending unit (is any of
+                        {{GPUBlendFactor/"src1"}}, {{GPUBlendFactor/"one-minus-src1"}},
+                        {{GPUBlendFactor/"src1-alpha"}}, {{GPUBlendFactor/"one-minus-src1-alpha"}}), then:
+
+                    - Set |usesDualSourceBlending| to `true`.
+                - For each [=shader stage output=] value |output| with [=location=] attribute equal to |index|
+                    in |entryPoint|:
                     - For each component in |colorState|.{{GPUColorTargetState/format}}, there must be a
                         corresponding component in |output|.
                         (That is, RGBA requires vec4, RGB requires vec3 or vec4, RG requires vec2 or vec3 or vec4.)
@@ -8129,16 +8141,33 @@ dictionary GPUFragmentState
                             : {{GPUTextureSampleType/"uint"}}
                             :: |output| must have an unsigned integer scalar type.
                         </dl>
-                    - If |colorState|.{{GPUColorTargetState/blend}} is [=map/exist|provided=] and
-                        |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/color}}.{{GPUBlendComponent/srcFactor}}
-                        or .{{GPUBlendComponent/dstFactor}} uses the source alpha
-                        (is any of {{GPUBlendFactor/"src-alpha"}}, {{GPUBlendFactor/"one-minus-src-alpha"}},
-                        or {{GPUBlendFactor/"src-alpha-saturated"}}), then:
-                        - |output| must have an alpha channel (that is, it must be a vec4).
+                    - If |colorState|.{{GPUColorTargetState/blend}} is [=map/exist|provided=] and **either**:
+                        - The [=blend_src=] attribute of |output| is omitted or 0 and
+                            |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/color}}.{{GPUBlendComponent/srcFactor}} or
+                            |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/color}}.{{GPUBlendComponent/dstFactor}}
+                            uses the alpha channel of the first input of the corresponding blend unit (is any of
+                            {{GPUBlendFactor/"src-alpha"}}, {{GPUBlendFactor/"one-minus-src-alpha"}} or
+                            {{GPUBlendFactor/"src-alpha-saturated"}})
+                        - The [=blend_src=] attribute of |output| is 1 and
+                            |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/color}}.{{GPUBlendComponent/srcFactor}} or
+                            |colorState|.{{GPUColorTargetState/blend}}.{{GPUBlendState/color}}.{{GPUBlendComponent/dstFactor}}
+                            uses the alpha channel of the second input of the corresponding blending unit (is any of
+                            {{GPUBlendFactor/"src1-alpha"}} or {{GPUBlendFactor/"one-minus-src1-alpha"}})
+
+                        Then:
 
-                    Otherwise, since there is no shader output for the attachment:
+                        - |output| must have an alpha channel (that is, it must be a vec4).
 
-                    - |colorState|.{{GPUColorTargetState/writeMask}} must be 0.
+                - If |colorState|.{{GPUColorTargetState/writeMask}} is not 0:
+                    - |entryPoint| must have a [=shader stage output=] with [=location=] equal to |index|
+                         and [=blend_src=] omitted or equal to 0.
+            - If |usesDualSourceBlending| is `true`:
+                - All the [=shader stage output=] values of |entryPoint| must have a [=blend_src=] attribute.
+                - |descriptor|.{{GPUFragmentState/targets}}.length must be `1`.
+                - Let |colorState| be |descriptor|.{{GPUFragmentState/targets}}[0].
+                - If |colorState|.{{GPUColorTargetState/writeMask}} is not 0:
+                - |entryPoint| must have a [=shader stage output=] with [=location=] equal to 0
+                    and [=blend_src=] equal to 1.    
             - [$Validating GPUFragmentState's color attachment bytes per sample$](|device|, |descriptor|.{{GPUFragmentState/targets}}) succeeds.
         </div>
 </div>
@@ -8269,6 +8298,12 @@ location:
             <td><b><code>RGBA<sub>src</src></code></b>
             <td>Color output by the fragment shader for the color attachment.
                 If the shader doesn't return an alpha channel, src-alpha blend factors cannot be used.
+        <tr>
+            <td><b><code>RGBA<sub>src1</src></code></b>
+            <td>Color output by the fragment shader for the color attachment with
+                <a href="https://gpuweb.github.io/gpuweb/wgsl/#input-output-locations">"@blend_src" attribute</a>
+                equal to `1`.
+                If the shader doesn't return an alpha channel, src1-alpha blend factors cannot be used.
         <tr>
             <td><b><code>RGBA<sub>dst</src></code></b>
             <td>Color currently in the color attachment.
@@ -8300,6 +8335,10 @@ enum GPUBlendFactor {
     "src-alpha-saturated",
     "constant",
     "one-minus-constant",
+    "src1",
+    "one-minus-src1",
+    "src1-alpha",
+    "one-minus-src1-alpha",
 };
 </script>
 
@@ -8352,6 +8391,18 @@ enum GPUBlendFactor {
         <tr>
             <td><dfn>"one-minus-constant"</dfn>
             <td><code>(1 - R<sub>const</sub>, 1 - G<sub>const</sub>, 1 - B<sub>const</sub>, 1 - A<sub>const</sub>)</code>
+        <tr>
+            <td><dfn>"src1"</dfn>
+            <td><code>(R<sub>src1</sub>, G<sub>src1</sub>, B<sub>src1</sub>, A<sub>src1</sub>)</code>
+        <tr>
+            <td><dfn>"one-minus-src1"</dfn>
+            <td><code>(1 - R<sub>src1</sub>, 1 - G<sub>src1</sub>, 1 - B<sub>src1</sub>, 1 - A<sub>src1</sub>)</code>
+        <tr>
+            <td><dfn>"src1-alpha"</dfn>
+            <td><code>(A<sub>src1</sub>, A<sub>src1</sub>, A<sub>src1</sub>, A<sub>src1</sub>)</code>
+        <tr>
+            <td><dfn>"one-minus-src1-alpha"</dfn>
+            <td><code>(1 - A<sub>src1</sub>, 1 - A<sub>src1</sub>, 1 - A<sub>src1</sub>, 1 - A<sub>src1</sub>)</code>
     </tbody>
 </table>
 
@@ -15257,6 +15308,23 @@ This feature adds no [=optional API surfaces=].
 Makes textures with formats {{GPUTextureFormat/"r32float"}}, {{GPUTextureFormat/"rg32float"}}, and
 {{GPUTextureFormat/"rgba32float"}} [=filterable=].
 
+<h3 id=dom-gpufeaturename-dual-source-blending data-dfn-type=enum-value data-dfn-for=GPUFeatureName>`"dual-source-blending"`
+</h3>
+
+Allows the use of [=blend_src=] in WGSL and simultaneously using both pixel shader outputs
+(`@blend_src(0)` and `@blend_src(1)`) as inputs to a blending operation with the single color
+attachment at [=location=] `0`.
+
+This feature adds the following [=optional API surfaces=]:
+- Allows the use of the below {{GPUBlendFactor}}s:
+  - {{GPUBlendFactor/"src1"}}
+  - {{GPUBlendFactor/"one-minus-src1"}}
+  - {{GPUBlendFactor/"src1-alpha"}}
+  - {{GPUBlendFactor/"one-minus-src1-alpha"}}
+
+- New WGSL extensions:
+    - [=extension/dual_source_blending=]
+
 # Appendices # {#appendices}
 
 ## Texture Format Capabilities ## {#texture-format-caps}

diff --git a/wgsl/index.bs b/wgsl/index.bs
@@ -1335,6 +1335,7 @@ The [=syntax/attribute=] names are:
 * <a for=attribute lt=interpolate>`'interpolate'`</a>
 * <a for=attribute lt=invariant>`'invariant'`</a>
 * <a for=attribute lt=location>`'location'`</a>
+* <a for=attribute lt=blend_src>`'blend_src'`</a>
 * <a for=attribute lt=must_use>`'must_use'`</a>
 * <a for=attribute lt=size>`'size'`</a>
 * <a for=attribute lt=vertex>`'vertex'`</a>
@@ -1751,6 +1752,10 @@ The valid [=enable-extensions=] are listed in the following table.
   <tr><td><dfn noexport dfn-for="extension">`f16`</dfn>
       <td>`"shader-f16"`
       <td>The [=f16=] type is valid to use in the WGSL module. Otherwise, using [=f16=] (directly or indirectly) will result in a [=shader-creation error=].
+  <tr><td><dfn noexport dfn-for="extension">`dual_source_blending`</dfn>
+      <td>`"dual_source_blending"`
+      <td>The attribute [=attribute/blend_src=] is valid to use in the WGSL module. Otherwise, using
+          [=attribute/blend_src=] will result in a [=shader-creation error=].
 </table>
 
 <div class='example wgsl using extensions expect-error' heading="Using hypothetical enable-extensions">
@@ -3099,12 +3104,13 @@ The following attributes can be applied to structure members:
  * [=attribute/align=]
  * [=attribute/builtin=]
  * [=attribute/location=]
+ * [=attribute/blend_src=]
  * [=attribute/interpolate=]
  * [=attribute/invariant=]
  * [=attribute/size=]
 
-Attributes [=attribute/builtin=], [=attribute/location=], [=attribute/interpolate=], and [=attribute/invariant=]
-are [=IO attributes=].
+Attributes [=attribute/builtin=], [=attribute/location=], [=attribute/blend_src=],
+[=attribute/interpolate=], and [=attribute/invariant=] are [=IO attributes=].
 An [=IO attribute=] on a member of a structure *S* has effect only when
 *S* is used as the type of a [=formal parameter=] or [=return type=] of an [=entry point=].
 See [[#stage-inputs-outputs]].
@@ -8013,6 +8019,7 @@ WGSL defines the following attributes that can be applied to function
 parameters and return types:
  * [=attribute/builtin=]
  * [=attribute/location=]
+ * [=attribute/blend_src=]
  * [=attribute/interpolate=]
  * [=attribute/invariant=]
 
@@ -8608,6 +8615,30 @@ path: syntax/location_attr.syntax.bs.include
 
 </table>
 
+<pre class=include>
+path: syntax/blend_src_attr.syntax.bs.include
+</pre>
+
+<table class='data builtin'>
+  <caption><dfn noexport dfn-for="attribute">`blend_src`</dfn> Attribute</caption>
+  <tr>
+    <td style="width:10%">Description
+    <td>
+    Specifies a part of the [=fragment=] output when the feature [=extension/dual_source_blending=]
+    is enabled.
+    See [[#input-output-locations]].
+
+    [=shader-creation error|Must=] only be applied to a member of a [=structure=] type.
+    [=shader-creation error|Must=] only be applied to declarations of objects with [=numeric scalar=]
+    or [=numeric vector=] type.
+    [=shader-creation error|Must=] only be used as an output of the [=fragment=] shader stage.
+
+  <tr>
+    <td>Parameters
+    <td>[=shader-creation error|Must=] be a [=const-expression=] that [=resolves=] to an i32 or u32 with value of `0` or `1`.
+
+</table>
+
 ## `must_use` ## {#must-use-attr}
 
 <pre class=include>
@@ -8915,6 +8946,7 @@ or to further describe the properties of an input or output.
 The <dfn noexport>IO attributes</dfn> are:
 * [=attribute/builtin=]
 * [=attribute/location=]
+* [=attribute/blend_src=]
 * [=attribute/interpolate=]
 * [=attribute/invariant=]
 
@@ -9390,18 +9422,32 @@ Each input-output location can store a value up to 16 bytes in size.
 The byte size of a type is defined using the *SizeOf* column in [[#alignment-and-size]].
 For example, a four-component vector of floating-point values occupies a single location.
 
-IO locations are specified via the [=attribute/location=] attribute.
-
 Each user-defined [=user-defined input datum|input=] and [=user-defined output datum|output=] [=shader-creation error|must=] have an explicitly specified IO location.
 Each structure member in the entry point IO [=shader-creation error|must=] be one of either a built-in value
 (see [[#builtin-inputs-outputs]]), or assigned a location.
 
+A location can be specified via either the [=attribute/location=] attribute only or both the
+[=attribute/location=] attribute and the [=attribute/blend_src=] attribute. The optional
+[=attribute/blend_src=] attribute can only be specified on a [=fragment=] shader
+[=user-defined output datum|output=] and used to specify up to two [=fragment=] shader
+[=user-defined output datum|outputs=] for the only color attachment as the inputs to the blend
+equation.
+
 Locations [=shader-creation error|must not=] overlap within each of the following sets:
 * Members within a structure type.
     This applies to any structure, not just those used in shader stage inputs or outputs.
 * An entry point's shader stage inputs,
     i.e. locations for its formal parameters, or for the members of its formal parameters of structure type.
 
+When the [=attribute/blend_src=] attribute is specified:
+* There [=shader-creation error|must=] be exactly `2` [=fragment=] shader
+    [=user-defined output datum|user-defined outputs=].
+* The [=attribute/location=] attribute of all [=fragment=] shader
+    [=user-defined output datum|outputs=] [=shader-creation error|must=] be `0`.
+* The value of [=attribute/blend_src=] attribute [=shader-creation error|must=] be `0` for one
+    [=fragment=] shader [=user-defined output datum|output=], and `1` for the other [=fragment=]
+    shader [=user-defined output datum|output=].
+
 Note: Location numbering is distinct between inputs and outputs:
 Location numbers for an entry point's shader stage inputs do not conflict with location numbers for the entry point's shader stage outputs.
 

diff --git a/wgsl/syntax.bnf b/wgsl/syntax.bnf
@@ -109,6 +109,10 @@ binding_attr :
   '@' 'binding' '(' expression ',' ? ')'
 ;
 
+blend_src_attr :
+  '@' 'blend_src' '(' expression ',' ? ')'
+;
+
 builtin_attr :
   '@' 'builtin' '(' builtin_value_name ',' ? ')'
 ;
@@ -184,6 +188,7 @@ attribute :
   '@' ident_pattern_token argument_expression_list ?
 | align_attr
 | binding_attr
+| blend_src_attr
 | builtin_attr
 | const_attr
 | diagnostic_attr

diff --git a/wgsl/syntax/attribute.syntax.bs.include b/wgsl/syntax/attribute.syntax.bs.include
@@ -7,6 +7,8 @@
 
     <span class="choice">|</span> [=syntax/binding_attr=]
 
+    <span class="choice">|</span> [=syntax/blend_src_attr=]
+
     <span class="choice">|</span> [=syntax/builtin_attr=]
 
     <span class="choice">|</span> [=syntax/const_attr=]

diff --git a/wgsl/syntax/blend_src_attr.syntax.bs.include b/wgsl/syntax/blend_src_attr.syntax.bs.include
@@ -0,0 +1,5 @@
+<div class='syntax' noexport='true'>
+  <dfn for=syntax>blend_src_attr</dfn> :
+
+    <span class="choice"></span> <a for=syntax_sym lt=attr>`'@'`</a> `'blend_src'` <a for=syntax_sym lt=paren_left>`'('`</a> [=syntax/expression=] <a for=syntax_sym lt=comma>`','`</a> ? <a for=syntax_sym lt=paren_right>`')'`</a>
+</div>
diff --git a/wgsl/wgsl.recursive.bs.include b/wgsl/wgsl.recursive.bs.include
@@ -45,6 +45,8 @@
 
  | `'@'` `'binding'` `'('` [=recursive descent syntax/expression=] `','` ? `')'`
 
+ | `'@'` `'blend_src'` `'('` [=recursive descent syntax/expression=] `','` ? `')'`
+
  | `'@'` `'builtin'` `'('` [=syntax/ident_pattern_token=] `','` ? `')'`
 
  | `'@'` `'diagnostic'` [=recursive descent syntax/diagnostic_control=]