This article demonstrates how to use a radial gradient brush in Silverlight using XAML and VB.NET.
Radial Gradient Brush
A radial gradient brush paints an area with a radial gradient that has a circle, along with a focal point, to define the gradient behavior. The focal point defines the center of the gradient and has default value 0.0. The RadialGradientBrush object represents a radial gradient brush.
Figure 1 shows a radial gradient.
Figure 1
Figure 2 taken from MSDN shows a rectangle filled with a radial gradient brush with multiple gradient stops at various points.
Figure 2
Creating a Radial Gradient Brush
The RadialGradientBrush element in XAML creates a radial gradient brush.
The Center property of the RadialGradientBrush represents the center of the outermost circle of the radial gradient. The default center point of the gradient circle is (0.5, 0.5).
The GradientOrigin property of the RadialGradientBrush represents the location of the focal point of the gradient. The default focal point of the gradient circle is (0.5, 0.5).
The RadiusX and the RadiusY properties of the RadialGradientBrush represent the X and Y radius of the radial gradient.
The following code snippet creates a radial gradient brush with blue and red colors by setting GradientStops. The GradientOrigin and Center properties are default properties. The code snippet also sets the RadiusX and RadiusY properties.
<RadialGradientBrush
GradientOrigin="0.5,0.5"
Center="0.5,0.5"
RadiusX="0.5" RadiusY="0.5">>
<RadialGradientBrush.GradientStops>
<GradientStop Color="Blue" Offset="0" />
<GradientStop Color="Red" Offset="1.0" />
</RadialGradientBrush.GradientStops>
</RadialGradientBrush>
We can fill a shape with a radial gradient brush by setting the Fill property of a shape to a gradient brush. The code snippet in Listing 1 creates a rectangle shape sets the Fill property to a RadialGradientBrush with blue and red colors where center of the radial gradient starts with the blue color.
<Rectangle Width="200" Height="100" Stroke="Black" >
<Rectangle.Fill>
<RadialGradientBrush
GradientOrigin="0.5,0.5"
Center="0.5,0.5" >
<RadialGradientBrush.GradientStops>
<GradientStop Color="Blue" Offset="0" />
<GradientStop Color="Red" Offset="1.0" />
</RadialGradientBrush.GradientStops>
</RadialGradientBrush>
</Rectangle.Fill>
</Rectangle>
Listing 1
The output looks like Figure 3.
Figure 3
Now let us apply multiple stops with multiple colors. The code snippet in Listing 2 creates a radial gradient brush with five stops.
<Rectangle Width="200" Height="100" Stroke="Black" >
<Rectangle.Fill>
<RadialGradientBrush
GradientOrigin="0.5,0.5"
Center="0.5,0.5" >
<RadialGradientBrush.GradientStops>
<GradientStop Color="Blue" Offset="0.1" />
<GradientStop Color="Orange" Offset="0.25" />
<GradientStop Color="Yellow" Offset="0.50" />
<GradientStop Color="Green" Offset="0.75" />
<GradientStop Color="Red" Offset="1.0" />
</RadialGradientBrush.GradientStops>
</RadialGradientBrush>
</Rectangle.Fill>
</Rectangle>
Listing 2
The new output generated by Listing 96 looks like Figure 4.
Figure 4
The CreateARectangleWithRGBrush method listed in Listing 3 draws same rectangle in Figure 4 dynamically.
Public Sub CreateARectangleWithRGBrush()
' Create a Rectangle
Dim blueRectangle As New Rectangle()
blueRectangle.Height = 100
blueRectangle.Width = 200
' Create a radial gradient brush with five stops
Dim fiveColorRGB As New RadialGradientBrush()
fiveColorRGB.GradientOrigin = New Point(0.5, 0.5)
fiveColorRGB.Center = New Point(0.5, 0.5)
' Create and add Gradient stops
Dim blueGS As New GradientStop()
blueGS.Color = Colors.Blue
blueGS.Offset = 0.0R
fiveColorRGB.GradientStops.Add(blueGS)
Dim orangeGS As New GradientStop()
orangeGS.Color = Colors.Orange
orangeGS.Offset = 0.25
fiveColorRGB.GradientStops.Add(orangeGS)
Dim yellowGS As New GradientStop()
yellowGS.Color = Colors.Yellow
yellowGS.Offset = 0.5R
fiveColorRGB.GradientStops.Add(yellowGS)
Dim greenGS As New GradientStop()
greenGS.Color = Colors.Green
greenGS.Offset = 0.75
fiveColorRGB.GradientStops.Add(greenGS)
Dim redGS As New GradientStop()
redGS.Color = Colors.Red
redGS.Offset = 1.0R
fiveColorRGB.GradientStops.Add(redGS)
' Set Fill property of rectangle
blueRectangle.Fill = fiveColorRGB
' Add Rectangle to the page
LayoutRoot.Children.Add(blueRectangle)
End Sub
Listing 3
The following code snippet changes the GradientOrigin and Center properties and now new output looks like Figure 5.
<RadialGradientBrush
GradientOrigin="0.2,0.5"
Center="0.1,0.5"
RadiusX="0.5" RadiusY="0.5">
Figure 5
Summary
In this article, we saw how to create and use a radial gradient brush in Silverlight using VB.NET and XAML.