LinearGradientBrush with a fixed-width gradient on both ends

Question

I want to use a LinearGradientBrush to fill a rectangular range on a canvas with a gradient. The catch is, I want the gradient to appear the same on both sides of the rectangle, no matter how large the rectangle is.

What I'm trying to draw is something like this:

Here's my first attempt:

LinearGradientBrush brush = new LinearGradientBrush();
brush.SpreadMethod = GradientSpreadMethod.Reflect;
brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#FF0000FF"), 0.0));
brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#22FFFFFF"), 0.25));
brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#22FFFFFF"), 0.75));
brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#FF0000FF"), 1.00));
brush.StartPoint = new Point(0, 0);
brush.EndPoint = new Point(1, 0);

Rectangle rect = new Rectangle();            
rect.Width = 100;
rect.Height = 100;
rect.Fill = brush;
Canvas.SetLeft(rect, 50);
Canvas.SetTop(rect, 50);
MyCanvas.Children.Add(rect);

Rectangle rect2 = new Rectangle();
rect2.Width = 300;
rect2.Height = 100;
rect2.Fill = brush;
Canvas.SetLeft(rect2, 50);
Canvas.SetTop(rect2, 200);
MyCanvas.Children.Add(rect2);

Not quite right; the bigger the rectangle, the wider the gradients appear:

I tried setting MappingMode to Absolute, but that doesn't work either.

LinearGradientBrush brush = new LinearGradientBrush();
brush.MappingMode = BrushMappingMode.Absolute;
brush.SpreadMethod = GradientSpreadMethod.Reflect;

brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#FF0000FF"), 0.0));
brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#22FFFFFF"), 0.25));

brush.StartPoint = new Point(0, 0);
brush.EndPoint = new Point(50, 0);   //I want the gradient to be across 50 pixels

Rectangle rect = new Rectangle();    
//Everything else is the same

This give me something that looks like this:

which is way off. Any suggestions?

Answer 1

One way to accomplish this is to create a custom control.
Compose it with 3 rectangles on a Grid;

• 1 Column, 50px wide, with a rectangle filled with the Linear Gradient
• 1 Column "Content" rectangle filling the remainder
• 1 Column, 50px wide, with a rectangle filled with the Linear Gradient.

Answer 2

You can use bind the Background of the rectangle to its ActualHeight and use a converter, calculating the percent of that width represents the intended width (using absolute values you could also set the pixels directly):

class HeightToBrushComplexPropertyConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        double height = (double)value;
        double percent = 50.0 / height;

        LinearGradientBrush brush = new LinearGradientBrush();
        brush.StartPoint = new Point(0, 0);
        brush.EndPoint = new Point(1, 0);

        brush.MappingMode = BrushMappingMode.RelativeToBoundingBox;
        brush.SpreadMethod = GradientSpreadMethod.Pad;

        brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#FF0000FF"), 0.0));
        brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#22FFFFFF"), percent));
        brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#22FFFFFF"), 1 - percent));
        brush.GradientStops.Add(new GradientStop((Color)ColorConverter.ConvertFromString("#FF0000FF"), 1));

        return brush;

    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Improvements to make on this converter: accept the number of pixels and color as parameters.