Question

I am making a bar chart and I want two separate gradients for each bar. First I want a gradient to go from top to bottom solid red to transparent red. I want to paint over the top of that a gradient that goes from right to left, black to opaque.

So – In the bottom left we should have;

• Bottom left – Alpha 0
• Bottom right – Alpha 0
• Top left – Alpha 255 Colour Red
• Top Right – Alpha 255 Colour Black

So in effect I want to take a solid colour, add a left to right gradient to black then take the output of that and add a top to bottom gradient to transparency.

All this and I want it to be in a single brush, is this even possible?

Answer 1

Yes. Use a VisualBrush whose Visual is a Rectangle inside a Border to combine the other two brushes.

Something like this:

<LinearGradientBrush x:Key="UnderBrush" EndPoint="0,1"> 
  <GradientStop Color="#FFFF0000" Offset="0" /> 
  <GradientStop Color="#00FF0000" Offset="1" /> 
</LinearGradientBrush> 

<LinearGradientBrush x:Key="OverBrush" EndPoint="1,0"> 
  <GradientStop Color="#00000000" Offset="0" /> 
  <GradientStop Color="#FF000000" Offset="1" /> 
</LinearGradientBrush> 

<VisualBrush x:Key="CombinedBrush">
  <VisualBrush.Visual>
    <Border Background="{StaticResource UnderBrush}">
      <Rectangle Fill="{StaticResource OverBrush}" Width="1" Height="1" />
    </Border>
  </VisualBrush.Visual>
</VisualBrush>

CombinedBrush can be used to paint your bars, and you will get the effect you describe.

Silverlight version

Since Silverlight has no VisualBrush you must build a WritableBitmap in code and use it with an ImageBrush:

<ImageBrush x:Key="CombinedBrush">
  <my:VisualBrushSimulator.Visual>
    <Border Background="{StaticResource UnderBrush}">
      <Rectangle Fill="{StaticResource OverBrush}" Width="1" Height="1" />
    </Border>
  </my:VisualBrushSimulator.Visual>
</ImageBrush>

Here is how the VisualBrushSimulator might be implemented:

public class VisualBrushSimulator : DependencyObject
{
  public Visual GetVisual(DependencyObject obj) { return (Visual)obj.GetValue(VisualProperty); }
  public void SetVisual(DependencyObject obj, Visual value) { obj.SetValue(VisualProperty, value); }
  public static readonly DependencyProperty VisualProperty = DependencyProperty.RegisterAttached("Visual", typeof(Visual), typeof(VisualBrushSimulator), new PropertyMetadata
  {
    PropertyChangedCallback = (obj, e) =>
    {
      int width=1000;
      int height=1000;
      var bitmap = new WritableBitmap(width, height);
      bitmap.Render((Visual)e.NewValue, new ScaleTransform { ScaleX = width, ScaleY = height });
      ((ImageBrush)obj).ImageSource = bitmap;
    }
  });
}

Note that this is not a true VisualBrush simulation, since changes to the Visual do not affect the brush.