Irregular PNG button's click event in WPF

Question

I need an irregularly shaped button in WPF. I am doing it in this way using XAML:

<Button Name="toggleButton" Click="toggleButton_Click" Canvas.Left="177" Canvas.Top="0">
  <Button.Template>
    <ControlTemplate>
      <Image Source="ball.png" />
    </ControlTemplate>
  </Button.Template>
</Button>

My ball.png image is a PNG image with a ball with transparent area around it. The button displays correctly, but Click event handler is executed even when I clik on the transparent part of the image.

Is there any way to create irregular buttons using transparent PNGs?

Thanks, Michal

Answer 1

Michal, I wouldn't create a real "button". Just go to blend, get your image as an image brush into a rectangle, right-click make it a Button. If you are using Blend 4, this will generate the appropriate states in the Visual State Manager. You can get your mouse over, pressed, etc., to look like a button. Even disabled state.

Here's an example:

<Window.Resources>
        <Style x:Key="CrazyButtonStyle" TargetType="{x:Type Button}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type Button}">
                        <Grid>
                            <VisualStateManager.VisualStateGroups>
                                <VisualStateGroup x:Name="CommonStates">
                                    <VisualState x:Name="Normal">
                                        <Storyboard>
                                            <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[1].(GradientStop.Color)" Storyboard.TargetName="path">
                                                <EasingColorKeyFrame KeyTime="0" Value="#FFBEBEBE"/>
                                            </ColorAnimationUsingKeyFrames>
                                            <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[0].(GradientStop.Color)" Storyboard.TargetName="path">
                                                <EasingColorKeyFrame KeyTime="0" Value="#FF919191"/>
                                            </ColorAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="MouseOver">
                                        <Storyboard>
                                            <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[0].(GradientStop.Color)" Storyboard.TargetName="path">
                                                <EasingColorKeyFrame KeyTime="0" Value="#FF782B2B"/>
                                            </ColorAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Pressed">
                                        <Storyboard>
                                            <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[0].(GradientStop.Color)" Storyboard.TargetName="path">
                                                <EasingColorKeyFrame KeyTime="0" Value="#FF1D0C0C"/>
                                            </ColorAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Disabled">
                                        <Storyboard>
                                            <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[0].(GradientStop.Color)" Storyboard.TargetName="path">
                                                <EasingColorKeyFrame KeyTime="0" Value="#FF720505"/>
                                            </ColorAnimationUsingKeyFrames>
                                            <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[1].(GradientStop.Color)" Storyboard.TargetName="path">
                                                <EasingColorKeyFrame KeyTime="0" Value="#FF6E0000"/>
                                            </ColorAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                </VisualStateGroup>
                            </VisualStateManager.VisualStateGroups>
                            <Path x:Name="path" Data="M95.5,33.499981 C71.500031,31.499967 76.5,68.5 76.5,68.5 L112.5,75.500276 107.5,92.500393 144.5,105.50048 152.5,79.500301 132.5,63.50019 154.5,54.500128 173.5,68.500225 168.5,87.500356 172.5,97.500425 185.5,96.500418 197.12084,75.753906 200.12084,44.753692 176.5,34.49999 143.5,32.499975 142.5,13.499841 130.5,28.499946 137.5,41.500036 135.5,51.500106 117.5,52.500113 99.5,54.500126 116.5,39.500022 z" Stretch="Fill" Stroke="{x:Null}">
                                <Path.Fill>
                                    <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                                        <GradientStop Color="Black" Offset="0"/>
                                        <GradientStop Color="White" Offset="1"/>
                                    </LinearGradientBrush>
                                </Path.Fill>
                            </Path>
                            <ContentPresenter HorizontalAlignment="Right" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Top" Margin="0,24.52,18.715,0"/>
                        </Grid>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsFocused" Value="True"/>
                            <Trigger Property="IsDefaulted" Value="True"/>
                            <Trigger Property="IsMouseOver" Value="True"/>
                            <Trigger Property="IsPressed" Value="True"/>
                            <Trigger Property="IsEnabled" Value="False"/>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>

Now use the button as follows:

<Button Content="This is my crazy button" Style="{DynamicResource CrazyButtonStyle}"/>

I know this is not using the transparent png as the button but it would be a much better alternative if you have the ability to convert the png to xaml.

Answer 2

You can create a class that inherits from Image and overrides HitTestCore so that it does not respond to hit testing over the transparent parts of an image, and then use that class in your template instead of a plain Image.

Here is an example, although the code to check for transparent pixels isn't very robust since it makes some assumptions about the image source and pixel format. If you already have code to check for transparent pixels then you should plug that in instead.

public class TransparentImage
    : Image
{
    protected override HitTestResult HitTestCore(
        PointHitTestParameters hitTestParameters)
    {
        // Get value of current pixel
        var source = (BitmapSource)Source;
        var x = (int)(hitTestParameters.HitPoint.X /
            ActualWidth * source.PixelWidth);
        var y = (int)(hitTestParameters.HitPoint.Y /
            ActualHeight * source.PixelHeight);
        var pixels = new byte[4];
        source.CopyPixels(new Int32Rect(x, y, 1, 1), pixels, 4, 0);
        // Check alpha channel
        if (pixels[3] < 10)
        {
            return new PointHitTestResult(this, hitTestParameters.HitPoint);
        }
        else
        {
            return null;
        }
    }

    protected override GeometryHitTestResult HitTestCore(
        GeometryHitTestParameters hitTestParameters)
    {
        // Do something similar here, possibly checking every pixel within
        // the hitTestParameters.HitGeometry.Bounds rectangle
        return base.HitTestCore(hitTestParameters);
    }
}