Added support for opacity mask

src/Avalonia.Base/Rendering/Composition/CompositingRenderer.cs

@@ -189,6 +189,7 @@ public class CompositingRenderer : RendererBase, IRendererWithCompositor
             comp.Opacity = (float)visual.Opacity;
             comp.ClipToBounds = visual.ClipToBounds;
             comp.Clip = visual.Clip?.PlatformImpl;
+            comp.OpacityMask = visual.OpacityMask;
             
             var renderTransform = Matrix.Identity;
 

src/Avalonia.Base/Rendering/Composition/Server/ServerVisual.cs

@@ -24,10 +24,13 @@ namespace Avalonia.Rendering.Composition.Server
             canvas.Transform = Matrix.Identity;
             if (Opacity != 1)
                 canvas.PushOpacity(Opacity);
+            var boundsRect = new Rect(new Size(Size.X, Size.Y));
             if(ClipToBounds)
-                canvas.PushClip(new Rect(new Size(Size.X, Size.Y)));
+                canvas.PushClip(boundsRect);
             if (Clip != null) 
                 canvas.PushGeometryClip(Clip);
+            if(OpacityMaskBrush != null)
+                canvas.PushOpacityMask(OpacityMaskBrush, boundsRect);
             
             //TODO: Check clip
             RenderCore(canvas);
@@ -35,7 +38,9 @@ namespace Avalonia.Rendering.Composition.Server
             // Hack to force invalidation of SKMatrix
             canvas.PreTransform = MatrixUtils.ToMatrix(transform);
             canvas.Transform = Matrix.Identity;
-            
+
+            if (OpacityMaskBrush != null)
+                canvas.PopOpacityMask();
             if (Clip != null)
                 canvas.PopGeometryClip();
             if (ClipToBounds)

src/Avalonia.Base/Rendering/Composition/Visual.cs

@@ -1,11 +1,14 @@
 using System;
 using System.Numerics;
+using Avalonia.Media;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Rendering.Composition
 {
     public abstract partial class CompositionVisual
     {
+        private IBrush? _opacityMask;
+
         private protected virtual void OnRootChangedCore()
         {
         }
@@ -14,6 +17,16 @@ namespace Avalonia.Rendering.Composition
 
         partial void OnParentChanged() => Root = Parent?.Root;
 
+        public IBrush? OpacityMask
+        {
+            get => _opacityMask;
+            set
+            {
+                if (_opacityMask == value)
+                    return;
+                OpacityMaskBrush = (_opacityMask = value)?.ToImmutable();
+            }
+        }
 
         internal Matrix4x4? TryGetServerTransform()
         {

src/Avalonia.Base/composition-schema.xml

@@ -6,13 +6,15 @@
     <Using>Avalonia.Rendering.Composition.Animations</Using>
     
     <Manual Name="Avalonia.Platform.IGeometryImpl" Passthrough="true"/>
+    <Manual Name="Avalonia.Media.IBrush" Passthrough="true"/>
     <Manual Name="ICompositionSurface" ServerName="ServerCompositionSurface" />
     <Object Name="CompositionVisual" Abstract="true">
         <Property Name="Root" Type="CompositionTarget?" InternalSet="true" />
         <Property Name="Parent" Type="CompositionVisual?" InternalSet="true"  />
         <Property Name="Visible" Type="bool" Animated="true" DefaultValue="true"/>
         <Property Name="Opacity" Type="float" Animated="true" DefaultValue="1"/>
-        <Property Name="Clip" Type="Avalonia.Platform.IGeometryImpl?" />
+        
+        <Property Name="Clip" Type="Avalonia.Platform.IGeometryImpl?" Internal="true" />
         <Property Name="ClipToBounds" Type="bool" Animated="true" DefaultValue="true"/>
         <Property Name="Offset" Type="Vector3" Animated="true"/>
         <Property Name="Size" Type="Vector2" Animated="true"/>
@@ -23,6 +25,7 @@
         <Property Name="Scale" Type="Vector3" DefaultValue="new Vector3(1, 1, 1)" Animated="true"/>
         <Property Name="TransformMatrix" Type="Matrix4x4" DefaultValue="Matrix4x4.Identity" Animated="true"/>
         <Property Name="AdornedVisual" Type="CompositionVisual?" Internal="true" />
+        <Property Name="OpacityMaskBrush" Type="Avalonia.Media.IBrush?" Internal="true" />
     </Object>
     <Object Name="CompositionContainerVisual" Inherits="CompositionVisual"/>
     <List Name="CompositionVisualCollection" ItemType="CompositionVisual" CustomCtor="true"/>