[dxvk] Fix blending with A8 render targets

src/dxvk/dxvk_graphics.cpp

@@ -250,6 +250,16 @@ namespace dxvk {
           if (writeMask) {
             cbAttachments[i] = state.omBlend[i].state();
             cbAttachments[i].colorWriteMask = writeMask;
+
+            // If we're rendering to an emulated alpha-only render target, fix up blending
+            if (cbAttachments[i].blendEnable && formatInfo->componentMask == VK_COLOR_COMPONENT_R_BIT && state.omSwizzle[i].rIndex() == 3) {
+              cbAttachments[i].srcColorBlendFactor = util::remapAlphaToColorBlendFactor(
+                std::exchange(cbAttachments[i].srcAlphaBlendFactor, VK_BLEND_FACTOR_ONE));
+              cbAttachments[i].dstColorBlendFactor = util::remapAlphaToColorBlendFactor(
+                std::exchange(cbAttachments[i].dstAlphaBlendFactor, VK_BLEND_FACTOR_ZERO));
+              cbAttachments[i].colorBlendOp =
+                std::exchange(cbAttachments[i].alphaBlendOp, VK_BLEND_OP_ADD);
+            }
           }
         }
       }

src/dxvk/dxvk_util.cpp

@@ -272,6 +272,48 @@ namespace dxvk::util {
   }
 
 
+  VkBlendFactor remapAlphaToColorBlendFactor(VkBlendFactor factor) {
+    switch (factor) {
+      // Make sure we use the red component from the
+      // fragment shader since alpha may be undefined
+      case VK_BLEND_FACTOR_SRC_ALPHA:
+        return VK_BLEND_FACTOR_SRC_COLOR;
+
+      case VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA:
+        return VK_BLEND_FACTOR_ONE_MINUS_SRC_COLOR;
+
+      case VK_BLEND_FACTOR_SRC1_ALPHA:
+        return VK_BLEND_FACTOR_SRC1_COLOR;
+
+      case VK_BLEND_FACTOR_ONE_MINUS_SRC1_ALPHA:
+        return VK_BLEND_FACTOR_ONE_MINUS_SRC1_COLOR;
+
+      // This is defined to always be 1 for alpha
+      case VK_BLEND_FACTOR_SRC_ALPHA_SATURATE:
+        return VK_BLEND_FACTOR_ONE;
+
+      // Make sure we use the red component from the
+      // attachment since there is no alpha component
+      case VK_BLEND_FACTOR_DST_ALPHA:
+        return VK_BLEND_FACTOR_DST_COLOR;
+
+      case VK_BLEND_FACTOR_ONE_MINUS_DST_ALPHA:
+        return VK_BLEND_FACTOR_ONE_MINUS_DST_COLOR;
+
+      // For blend constants we actually need to do the
+      // opposite and make sure we always use alpha
+      case VK_BLEND_FACTOR_CONSTANT_COLOR:
+        return VK_BLEND_FACTOR_CONSTANT_ALPHA;
+
+      case VK_BLEND_FACTOR_ONE_MINUS_CONSTANT_COLOR:
+        return VK_BLEND_FACTOR_ONE_MINUS_CONSTANT_ALPHA;
+
+      default:
+        return factor;
+    }
+  }
+
+
   bool isIdentityMapping(
           VkComponentMapping          mapping) {
     return (mapping.r == VK_COMPONENT_SWIZZLE_R || mapping.r == VK_COMPONENT_SWIZZLE_IDENTITY)

src/dxvk/dxvk_util.h

@@ -339,7 +339,17 @@ namespace dxvk::util {
   VkComponentMapping resolveSrcComponentMapping(
           VkComponentMapping          dstMapping,
           VkComponentMapping          srcMapping);
-  
+
+  /**
+   * \brief Remaps alpha blend factor to a color one
+   *
+   * Needed when rendering to alpha-only render targets
+   * which we only support through single-channel formats.
+   * \param [in] factor Alpha blend factor
+   * \returns Corresponding color blend factor
+   */
+  VkBlendFactor remapAlphaToColorBlendFactor(VkBlendFactor factor);
+
   bool isIdentityMapping(
           VkComponentMapping          mapping);