WPF RadialGradientBrush in VB.NET

This article demonstrates how to use LinearGradientBrush in WPF using XAML and VB.NET.
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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 21 shows a radial gradient.

RGB21.jpg
 

Figure 21. Radial Gradient

RGB22.jpg
 

Figure 22. Radial Gradient with Stops

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 a shape's Fill property to the gradient brush. The code snippet in Listing 18 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 18

The output looks like Figure 23.

RGB23.jpg
 

Figure 23. A shape filled with a radial gradient brush

Now let's apply multiple stops with multiple colors. The code snippet in Listing 19 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 19

The new output generated by Listing 96 looks like Figure 34.

RGB24.jpg
 

Figure 24. A radial gradient brush with 5 stops

The CreateARectangleWithRGBrush method listed in Listing 20 draws same rectangle in Figure 24 dynamically.

    Public Sub CreateARectangleWithRGBrush()
        ' Create a Rectangle
        Dim blueRectangle As New Rectangle()
        blueRectangle.Height = 100
        blueRectangle.Width = 20

        ' 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 = 0.0
        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 = 0.5
        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 = 1.0
        fiveColorRGB.GradientStops.Add(redGS)

        ' Set Fill property of rectangle
        blueRectangle.Fill = fiveColorRGB

        ' Add Rectangle to the page
        LayoutRoot.Children.Add(blueRectangle)
    End Sub

Listing 20

The following code snippet changes the GradientOrigin and Center properties and now new output looks like Figure 25.

<RadialGradientBrush
            GradientOrigin="0.2,0.5"
            Center="0.1,0.5"
            RadiusX="0.5" RadiusY="0.5">

RGB25.jpg
 

Figure 25. Radial gradient

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