Add support for alpha to coverage dithering (#3069)

* Add support for alpha to coverage dithering

* Shader cache version bump

* Fix wrong alpha register

* Ensure support buffer is cleared

* New shader specialization based approach

3 files changed, 41 insertions, 1 deletions

diff --git a/Ryujinx.Graphics.Shader/CodeGen/Glsl/Declarations.cs b/Ryujinx.Graphics.Shader/CodeGen/Glsl/Declarations.cs
index 59a7ccdc..54578b79 100644
--- a/Ryujinx.Graphics.Shader/CodeGen/Glsl/Declarations.cs
+++ b/Ryujinx.Graphics.Shader/CodeGen/Glsl/Declarations.cs
@@ -615,7 +615,7 @@ namespace Ryujinx.Graphics.Shader.CodeGen.Glsl
 
         private static void DeclareSupportUniformBlock(CodeGenContext context, ShaderStage stage, int scaleElements)
         {
-            bool needsSupportBlock = stage == ShaderStage.Fragment || 
+            bool needsSupportBlock = stage == ShaderStage.Fragment ||
                 (context.Config.LastInVertexPipeline && context.Config.GpuAccessor.QueryViewportTransformDisable());
 
             if (!needsSupportBlock && scaleElements == 0)
diff --git a/Ryujinx.Graphics.Shader/IGpuAccessor.cs b/Ryujinx.Graphics.Shader/IGpuAccessor.cs
index 42f210a5..878c7180 100644
--- a/Ryujinx.Graphics.Shader/IGpuAccessor.cs
+++ b/Ryujinx.Graphics.Shader/IGpuAccessor.cs
@@ -35,6 +35,15 @@ namespace Ryujinx.Graphics.Shader
         ReadOnlySpan<ulong> GetCode(ulong address, int minimumSize);
 
         /// <summary>
+        /// Queries whenever the alpha-to-coverage dithering feature is enabled.
+        /// </summary>
+        /// <returns>True if the feature is enabled, false otherwise</returns>
+        bool QueryAlphaToCoverageDitherEnable()
+        {
+            return false;
+        }
+
+        /// <summary>
         /// Queries the binding number of a constant buffer.
         /// </summary>
         /// <param name="index">Constant buffer index</param>
diff --git a/Ryujinx.Graphics.Shader/Translation/EmitterContext.cs b/Ryujinx.Graphics.Shader/Translation/EmitterContext.cs
index ba3b551d..332b3e02 100644
--- a/Ryujinx.Graphics.Shader/Translation/EmitterContext.cs
+++ b/Ryujinx.Graphics.Shader/Translation/EmitterContext.cs
@@ -205,6 +205,8 @@ namespace Ryujinx.Graphics.Shader.Translation
             }
             else if (Config.Stage == ShaderStage.Fragment)
             {
+                GenerateAlphaToCoverageDitherDiscard();
+
                 if (Config.OmapDepth)
                 {
                     Operand dest = Attribute(AttributeConsts.FragmentOutputDepth);
@@ -266,6 +268,35 @@ namespace Ryujinx.Graphics.Shader.Translation
             }
         }
 
+        private void GenerateAlphaToCoverageDitherDiscard()
+        {
+            // If the feature is disabled, or alpha is not written, then we're done.
+            if (!Config.GpuAccessor.QueryAlphaToCoverageDitherEnable() || (Config.OmapTargets & 8) == 0)
+            {
+                return;
+            }
+
+            // 11 11 11 10 10 10 10 00
+            // 11 01 01 01 01 00 00 00
+            Operand ditherMask = Const(unchecked((int)0xfbb99110u));
+
+            Operand x = this.BitwiseAnd(this.FP32ConvertToU32(Attribute(AttributeConsts.PositionX)), Const(1));
+            Operand y = this.BitwiseAnd(this.FP32ConvertToU32(Attribute(AttributeConsts.PositionY)), Const(1));
+            Operand xy = this.BitwiseOr(x, this.ShiftLeft(y, Const(1)));
+
+            Operand alpha = Register(3, RegisterType.Gpr);
+            Operand scaledAlpha = this.FPMultiply(this.FPSaturate(alpha), ConstF(8));
+            Operand quantizedAlpha = this.IMinimumU32(this.FP32ConvertToU32(scaledAlpha), Const(7));
+            Operand shift = this.BitwiseOr(this.ShiftLeft(quantizedAlpha, Const(2)), xy);
+            Operand opaque = this.BitwiseAnd(this.ShiftRightU32(ditherMask, shift), Const(1));
+
+            Operand a2cDitherEndLabel = Label();
+
+            this.BranchIfTrue(a2cDitherEndLabel, opaque);
+            this.Discard();
+            this.MarkLabel(a2cDitherEndLabel);
+        }
+
         public Operation[] GetOperations()
         {
             return _operations.ToArray();