WPF: Color under the pointer - wpf

Question

I have a control with a gradiant background. On the MouseDown or MouseUp event I want to capture what color the pixel is immeidately under the mouse pointer. How would I do that?

Answer 1

I created a behavior that can be attached to an Image object to grab the color, which is 100% WPF. Here is the behavior; it could be tweaked to work with any "Visual", not just an Image.
[Note: I hardcoded 96dpi for the creation of RenderTargetBitmap... your milage may vary]
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Interactivity;
using System.Windows.Media;
using System.Windows.Media.Imaging;
namespace Company.Solution.Project.Utilities.Behaviors
{
[Description("Used to sample the color under mouse for the image when the mouse is pressed. ")]
public class ImageBehaviorMouseDownPointSampleToColor : Behavior<Image>
{
public static readonly DependencyProperty SelectedColorProperty =
DependencyProperty.Register("SelectedColor", typeof(Color),
typeof(ImageBehaviorMouseDownPointSampleToColor),
new UIPropertyMetadata(Colors.White));
public Color SelectedColor
{
get { return (Color)GetValue(SelectedColorProperty); }
set { SetValue(SelectedColorProperty, value); }
}
protected override void OnAttached()
{
base.OnAttached();
AssociatedObject.MouseMove += AssociatedObject_MouseMove;
AssociatedObject.MouseDown += AssociatedObject_MouseDown;
}
protected override void OnDetaching()
{
base.OnDetaching();
AssociatedObject.MouseMove -= AssociatedObject_MouseMove;
AssociatedObject.MouseDown -= AssociatedObject_MouseDown;
}
private void AssociatedObject_MouseDown(object sender, MouseButtonEventArgs e)
{
SamplePixelForColor();
}
private void AssociatedObject_MouseMove(object sender, MouseEventArgs e)
{
if (e.LeftButton == MouseButtonState.Pressed)
{
SamplePixelForColor();
}
}
private void SamplePixelForColor()
{
// Retrieve the coordinate of the mouse position in relation to the supplied image.
Point point = Mouse.GetPosition(AssociatedObject);
// Use RenderTargetBitmap to get the visual, in case the image has been transformed.
var renderTargetBitmap = new RenderTargetBitmap((int)AssociatedObject.ActualWidth,
(int)AssociatedObject.ActualHeight,
96, 96, PixelFormats.Default);
renderTargetBitmap.Render(AssociatedObject);
// Make sure that the point is within the dimensions of the image.
if ((point.X <= renderTargetBitmap.PixelWidth) && (point.Y <= renderTargetBitmap.PixelHeight))
{
// Create a cropped image at the supplied point coordinates.
var croppedBitmap = new CroppedBitmap(renderTargetBitmap,
new Int32Rect((int)point.X, (int)point.Y, 1, 1));
// Copy the sampled pixel to a byte array.
var pixels = new byte[4];
croppedBitmap.CopyPixels(pixels, 4, 0);
// Assign the sampled color to a SolidColorBrush and return as conversion.
SelectedColor = Color.FromArgb(255, pixels[2], pixels[1], pixels[0]);
}
}
}
}