This overview assumes that you are familiar with Brush objects. For an introduction to using brushes, see Painting with Solid Colors and Gradients Overview. For information about ImageBrush and DrawingBrush, see Painting with Images, Drawings, and Visuals.

An opacity mask works by mapping its contents to the element or visual. The alpha channel of each of the brush's pixels are then used to determine the resulting opacity of the element or visual's corresponding pixels; the actual color of the brush is ignored. If a given portion of the brush is transparent, the corresponding portion of the element or visual becomes transparent. If a given portion of the brush is opaque, the opacity of the corresponding portion of the element or visual is unchanged. The opacity specified by the opacity mask is combined with any opacity settings present in the element or visual. For example, if an element is 25 percent opaque and an opacity mask is applied that transitions from fully opaque to fully transparent, the result is an element that transitions from 25 percent opacity to fully transparent.

Note:
Although the examples in this overview demonstrate the use of opacity masks on image elements, an opacity mask may be applied to any element or Visual, including panels and controls.

Opacity masks are used to create interesting visual effects, such as to create images or buttons that fade from view, to add textures to elements, or to combine gradients to produce glass-like surfaces. The following illustration demonstrates the use of an opacity mask. A checkered background is used to show the transparent portions of the mask.

To create an opacity mask, you create a Brush and apply it to the OpacityMask property of an element or visual. You can use any type of Brush as an opacity mask.

• LinearGradientBrush, RadialGradientBrush: Used to make an element or visual fade from view.

  The following image shows a LinearGradientBrush used as an opacity mask.

  LinearGradientBrush Opacity Masking Example
  
  

• ImageBrush: Used to create texture and soft or torn edge effects.

  The following image shows an ImageBrush used as an opacity mask.

  LinearGradientBrush opacity masking example
  
  

• DrawingBrush: Used to create complex opacity masks from patterns of shapes, images, and gradients.

  The following image shows a DrawingBrush used as an opacity mask.

  DrawingBrush opacity masking example
  
  

The gradient brushes (LinearGradientBrush and RadialGradientBrush) are particularly well-suited for use as an opacity mask. Because a SolidColorBrush fills an area with a uniform color, they make poor opacity masks; using a SolidColorBrush is equivalent to setting the element's or visual's OpacityMask property.

To create a gradient fill, you specify two or more gradient stops. Each gradient stop contains describes a color and a position (see Painting with Solid Colors and Gradients Overview for more information about creating and using gradients). The process is the same when using a gradient as an opacity mask, except that, instead of blending colors, the opacity mask gradient blends alpha channel values. So the actual color of the gradient's contents do not matter; only the alpha channel, or opacity, of each color matters. The following is an example.

<!--With the opacity mask:--> 
<Image
  Width="200" Height="150"
  Source="sampleImages\Waterlilies.jpg"
  Margin="10"
  HorizontalAlignment="Left"
  Grid.Column="2" Grid.Row="3">
  <Image.OpacityMask>
    <LinearGradientBrush StartPoint="0.1,0.1" EndPoint="0.75,0.75">
      <LinearGradientBrush.GradientStops>
        <GradientStop Offset="0" Color="Black"/>
        <GradientStop Offset="1" Color="Transparent"/>
      </LinearGradientBrush.GradientStops>
    </LinearGradientBrush>
  </Image.OpacityMask>
</Image>

In the previous example, the system-defined color Black is used as the starting color of the gradient. Because all of the colors in the Colors class, except Transparent, are fully opaque, they can be used to simply define a starting color for a gradient opacity mask.

For additional control over alpha values when defining an opacity mask, you can specify the alpha channel of colors using ARGB hexadecimal notation in markup or using the Color..::.FromScRgb method.

Specifying Color Opacity in "XAML"

<Canvas.OpacityMask>
  <RadialGradientBrush>
    <RadialGradientBrush.GradientStops>
      <GradientStop Offset="0" Color="#FF000000"/>
      <GradientStop Offset="1" Color="#00000000"/>
  </RadialGradientBrush.GradientStops>
</RadialGradientBrush>
</Canvas.OpacityMask>

Images can also be used as an opacity mask. The following image shows an example. A checkered background is used to show the transparent portions of the mask.

To use an image as an opacity mask, use an ImageBrush to contain the image. When creating an image to be used as an opacity mask, save the image in a format that supports multiple levels of transparency, such as Portable Network Graphics (PNG). The following example shows the code used to create the previous illustration.

<!-- With the Opacity Mask-->
<Image
  Height="150"
  Width="200"
  Source="sampleImages/Waterlilies.jpg"
  HorizontalAlignment="Left"
  Margin="10"
  Grid.Column="2" Grid.Row="1">
  <Image.OpacityMask>
    <ImageBrush ImageSource="sampleImages/tornedges.png"/>
  </Image.OpacityMask>
</Image>

Using a Tiled Image as an Opacity Mask

<!-- With the Opacity Mask -->
<Image
  Height="150"
  Width="200"
  Source="sampleImages/Waterlilies.jpg"
  HorizontalAlignment="Left"
  Margin="10"
  Grid.Column="2" Grid.Row="2">

  <Image.OpacityMask>
    <ImageBrush
      Viewport="0,0,50,50"
      ViewportUnits="Absolute"
      TileMode="Tile" 
      ImageSource="sampleImages/tornedges.png"/>
  </Image.OpacityMask>
</Image>

Drawings can be used an opacity mask. The shapes contained within the drawing can themselves be filled with gradients, solid colors, images, or even other drawings. The following image shows an example of a drawing used as an opacity mask. A checkered background is used to show the transparent portions of the mask.

To use a drawing as an opacity mask, use a DrawingBrush to contain the drawing. The following example shows the code used to create the previous illustration:

<!-- With the Opacity Mask-->
<Image 
      Grid.Row="4" Grid.Column="5"
      Height="150"
      Width="200"
      Source="sampleImages/Waterlilies.jpg">
  <Image.OpacityMask>
    <DrawingBrush>
      <DrawingBrush.Drawing>
        <GeometryDrawing>
          <GeometryDrawing.Brush>
            <RadialGradientBrush>
              <RadialGradientBrush.GradientStops>
                <GradientStop Offset="0" Color="Black" />
                <GradientStop Offset="1" Color="Transparent" />
              </RadialGradientBrush.GradientStops>
            </RadialGradientBrush>
          </GeometryDrawing.Brush>
          <GeometryDrawing.Geometry>
            <RectangleGeometry Rect="0.05,0.05 0.9,0.9" />
          </GeometryDrawing.Geometry>
          <GeometryDrawing.Pen>
            <Pen Thickness="0.1" Brush="Black" />
          </GeometryDrawing.Pen>
        </GeometryDrawing>
      </DrawingBrush.Drawing>
    </DrawingBrush>
  </Image.OpacityMask>
</Image>

Using a Tiled Drawing as an Opacity Mask

<!-- With the Opacity Mask-->
<Button
   Grid.Row="8" Grid.Column="5"
  Height="100"
  Width="200"
  FontFamily="MS Gothic"
  FontSize="16">
  A Button
  <Button.OpacityMask>
    <DrawingBrush Viewport="0,0,0.25,0.25" TileMode="Tile">
      <DrawingBrush.Drawing>
        <GeometryDrawing>
          <GeometryDrawing.Brush>
            <RadialGradientBrush>
              <RadialGradientBrush.GradientStops>
                <GradientStop Offset="0" Color="Black" />
                <GradientStop Offset="1" Color="Transparent" />
              </RadialGradientBrush.GradientStops>
            </RadialGradientBrush>
          </GeometryDrawing.Brush>
          <GeometryDrawing.Geometry>
            <RectangleGeometry Rect="0.05,0.05 0.9,0.9" />
          </GeometryDrawing.Geometry>
          <GeometryDrawing.Pen>
            <Pen Thickness="0.1" Brush="Black" />
          </GeometryDrawing.Pen>
        </GeometryDrawing>
      </DrawingBrush.Drawing>
    </DrawingBrush>
  </Button.OpacityMask>
</Button>

Concepts