add GrayScaleEffect

src/Shared/HandyControl_Shared/HandyControl_Shared.projitems

@@ -59,6 +59,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Media\Animation\ResolvedKeyFrameEntry.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Media\Animation\SplineGeometryKeyFrame.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Media\Effects\ColorComplementEffect.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Media\Effects\GrayScaleEffect.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\BorderCircularClipConverter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\ColLayoutConverter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\CornerRadiusSplitConverter.cs" />

src/Shared/HandyControl_Shared/Media/Effects/ColorComplementEffect.cs

@@ -22,6 +22,7 @@ namespace HandyControl.Media.Effects
         public ColorComplementEffect()
         {
             PixelShader = Shader;
+
             UpdateShaderValue(InputProperty);
         }
 

src/Shared/HandyControl_Shared/Media/Effects/GrayScaleEffect.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Windows;
+using System.Windows.Media;
+using System.Windows.Media.Effects;
+using HandyControl.Data;
+
+namespace HandyControl.Media.Effects
+{
+    public class GrayScaleEffect : ShaderEffect
+    {
+        public static readonly DependencyProperty InputProperty = RegisterPixelShaderSamplerProperty("Input", typeof(GrayScaleEffect), 0);
+
+        private static readonly PixelShader Shader;
+
+        static GrayScaleEffect()
+        {
+            Shader = new PixelShader
+            {
+                UriSource = new Uri("pack://application:,,,/HandyControl;component/Resources/Effects/GrayScaleEffect.ps")
+            };
+        }
+
+        public GrayScaleEffect()
+        {
+            PixelShader = Shader;
+
+            UpdateShaderValue(InputProperty);
+            UpdateShaderValue(ScaleProperty);
+        }
+
+        public Brush Input
+        {
+            get => (Brush)GetValue(InputProperty);
+            set => SetValue(InputProperty, value);
+        }
+
+        public static readonly DependencyProperty ScaleProperty = DependencyProperty.Register(
+            "Scale", typeof(double), typeof(GrayScaleEffect), new PropertyMetadata(ValueBoxes.Double1Box, PixelShaderConstantCallback(0)));
+
+        public double Scale
+        {
+            get => (double) GetValue(ScaleProperty);
+            set => SetValue(ScaleProperty, value);
+        }
+    }
+}