Added the platform independent interface and the Direct2D1 implementation of opacity masks.

src/Avalonia.SceneGraph/Media/DrawingContext.cs

@@ -128,7 +128,8 @@ namespace Avalonia.Media
                 Opacity,
                 Clip,
                 MatrixContainer,
-                GeometryClip
+                GeometryClip,
+                OpacityMask
             }
 
             public PushedState(DrawingContext context, PushedStateType type, Matrix matrix = default(Matrix))
@@ -156,6 +157,8 @@ namespace Avalonia.Media
                     _context._impl.PopOpacity();
                 else if (_type == PushedStateType.GeometryClip)
                     _context._impl.PopGeometryClip();
+                else if (_type == PushedStateType.OpacityMask)
+                    _context._impl.PopOpacityMask();
                 else if (_type == PushedStateType.MatrixContainer)
                 {
                     var cont = _context._transformContainers.Pop();
@@ -201,6 +204,17 @@ namespace Avalonia.Media
             return new PushedState(this, PushedState.PushedStateType.Opacity);
         }
 
+        /// <summary>
+        /// Pushes an opacity mask.
+        /// </summary>
+        /// <param name="mask">The opacity mask.</param>
+        /// <returns>A disposable to undo the opacity mask.</returns>
+        public PushedState PushOpacityMask(IBrush mask, Rect bounds)
+        {
+            _impl.PushOpacityMask(mask, bounds);
+            return new PushedState(this, PushedState.PushedStateType.OpacityMask);
+        }
+
         /// <summary>
         /// Pushes a matrix transformation.
         /// </summary>

src/Avalonia.SceneGraph/Media/IDrawingContext.cs

@@ -81,6 +81,10 @@ namespace Avalonia.Media
 
         void PopOpacity();
 
+        void PushOpacityMask(IBrush mask, Rect bounds);
+
+        void PopOpacityMask();
+
         /// <summary>
         /// Pushes a clip geometry.
         /// </summary>

src/Avalonia.SceneGraph/Rendering/RendererMixin.cs

@@ -120,6 +120,7 @@ namespace Avalonia.Rendering
                 using (context.PushOpacity(opacity))
                 using (clipToBounds ? context.PushClip(bounds) : default(DrawingContext.PushedState))
                 using (visual.Clip != null ? context.PushGeometryClip(visual.Clip) : default(DrawingContext.PushedState))
+                using (visual.OpacityMask != null ? context.PushOpacityMask(visual.OpacityMask, clipToBounds ? bounds : clipRect) : default(DrawingContext.PushedState))
                 using (context.PushTransformContainer())
                 {
                     visual.Render(context);

src/Avalonia.SceneGraph/Visual.cs

@@ -57,6 +57,12 @@ namespace Avalonia
         public static readonly StyledProperty<double> OpacityProperty =
             AvaloniaProperty.Register<Visual, double>(nameof(Opacity), 1);
 
+        /// <summary>
+        /// Defines the <see cref="OpacityMask"/> property.
+        /// </summary>
+        public static readonly StyledProperty<IBrush> OpacityMaskProperty =
+            AvaloniaProperty.Register<Visual, IBrush>(nameof(OpacityMask));
+
         /// <summary>
         /// Defines the <see cref="RenderTransform"/> property.
         /// </summary>
@@ -168,6 +174,16 @@ namespace Avalonia
             set { SetValue(OpacityProperty, value); }
         }
 
+
+        /// <summary>
+        /// Gets the opacity mask of the scene graph node.
+        /// </summary>
+        public IBrush OpacityMask
+        {
+            get { return GetValue(OpacityMaskProperty); }
+            set { SetValue(OpacityMaskProperty, value); }
+        }
+
         /// <summary>
         /// Gets the render transform of the scene graph node.
         /// </summary>

src/Avalonia.SceneGraph/VisualTree/IVisual.cs

@@ -66,6 +66,11 @@ namespace Avalonia.VisualTree
         /// </summary>
         double Opacity { get; set; }
 
+        /// <summary>
+        /// Gets or sets the opacity mask of the scene graph node.
+        /// </summary>
+        IBrush OpacityMask { get; set; }
+
         /// <summary>
         /// Gets or sets the render transform of the scene graph node.
         /// </summary>

src/Windows/Avalonia.Direct2D1/Media/DrawingContext.cs

@@ -350,5 +350,25 @@ namespace Avalonia.Direct2D1.Media
         {
             PopLayer();
         }
+
+        public void PushOpacityMask(IBrush mask, Rect bounds)
+        {
+            var parameters = new LayerParameters
+            {
+                ContentBounds = PrimitiveExtensions.RectangleInfinite,
+                MaskTransform = PrimitiveExtensions.Matrix3x2Identity,
+                Opacity = 1,
+                OpacityBrush = CreateBrush(mask, bounds.Size).PlatformBrush
+            };
+            var layer = _layerPool.Count != 0 ? _layerPool.Pop() : new Layer(_renderTarget);
+            _renderTarget.PushLayer(ref parameters, layer);
+
+            _layers.Push(layer);
+        }
+
+        public void PopOpacityMask()
+        {
+            PopLayer();
+        }
     }
 }