BorderClipConverter.cs

using System.Globalization;
using System.Windows.Data;
using System.Windows.Media;

namespace MaterialDesignThemes.Wpf.Converters;

internal class BorderClipConverter : IMultiValueConverter
{
    public object Convert(object?[]? values, Type targetType, object? parameter, CultureInfo culture)
    {
        if (values is [double width, double height, ..])
        {
            if (width < 1.0 || height < 1.0)
            {
                return Geometry.Empty;
            }

            CornerRadius cornerRadius = default;
            Thickness borderThickness = default;
            if (values.Length > 2 && values[2] is CornerRadius radius)
            {
                cornerRadius = radius;
                if (values.Length > 3 && values[3] is Thickness thickness)
                {
                    borderThickness = thickness;
                }
            }

            var geometry = GetRoundRectangle(new Rect(0, 0, width, height), borderThickness, cornerRadius);
            geometry.Freeze();

            return geometry;
        }

        return DependencyProperty.UnsetValue;
    }

    public object?[]? ConvertBack(object? value, Type[] targetTypes, object? parameter, CultureInfo culture)
        => throw new NotImplementedException();

    // https://wpfspark.wordpress.com/2011/06/08/clipborder-a-wpf-border-that-clips/
    private static Geometry GetRoundRectangle(Rect baseRect, Thickness borderThickness, CornerRadius cornerRadius)
    {
        // Normalizing the corner radius
        if (cornerRadius.TopLeft < double.Epsilon)
        {
            cornerRadius.TopLeft = 0.0;
        }

        if (cornerRadius.TopRight < double.Epsilon)
        {
            cornerRadius.TopRight = 0.0;
        }

        if (cornerRadius.BottomLeft < double.Epsilon)
        {
            cornerRadius.BottomLeft = 0.0;
        }

        if (cornerRadius.BottomRight < double.Epsilon)
        {
            cornerRadius.BottomRight = 0.0;
        }

        // Taking the border thickness into account
        double leftHalf = borderThickness.Left * 0.5;
        if (leftHalf < double.Epsilon)
        {
            leftHalf = 0.0;
        }

        double topHalf = borderThickness.Top * 0.5;
        if (topHalf < double.Epsilon)
        {
            topHalf = 0.0;
        }

        double rightHalf = borderThickness.Right * 0.5;
        if (rightHalf < double.Epsilon)
        {
            rightHalf = 0.0;
        }

        double bottomHalf = borderThickness.Bottom * 0.5;
        if (bottomHalf < double.Epsilon)
        {
            bottomHalf = 0.0;
        }

        // Create the rectangles for the corners that needs to be curved in the base rectangle 
        // TopLeft Rectangle 
        var topLeftRect = new Rect(
            baseRect.Location.X,
            baseRect.Location.Y,
            Math.Max(0.0, cornerRadius.TopLeft - leftHalf),
            Math.Max(0.0, cornerRadius.TopLeft - rightHalf));

        // TopRight Rectangle 
        var topRightRect = new Rect(
            baseRect.Location.X + baseRect.Width - cornerRadius.TopRight + rightHalf,
            baseRect.Location.Y,
            Math.Max(0.0, cornerRadius.TopRight - rightHalf),
            Math.Max(0.0, cornerRadius.TopRight - topHalf));

        // BottomRight Rectangle
        var bottomRightRect = new Rect(
            baseRect.Location.X + baseRect.Width - cornerRadius.BottomRight + rightHalf,
            baseRect.Location.Y + baseRect.Height - cornerRadius.BottomRight + bottomHalf,
            Math.Max(0.0, cornerRadius.BottomRight - rightHalf),
            Math.Max(0.0, cornerRadius.BottomRight - bottomHalf));

        // BottomLeft Rectangle 
        var bottomLeftRect = new Rect(
            baseRect.Location.X,
            baseRect.Location.Y + baseRect.Height - cornerRadius.BottomLeft + bottomHalf,
            Math.Max(0.0, cornerRadius.BottomLeft - leftHalf),
            Math.Max(0.0, cornerRadius.BottomLeft - bottomHalf));

        // Adjust the width of the TopLeft and TopRight rectangles so that they are proportional to the width of the baseRect 
        if (topLeftRect.Right > topRightRect.Left)
        {
            double newWidth = (topLeftRect.Width / (topLeftRect.Width + topRightRect.Width)) * baseRect.Width;
            topLeftRect = new Rect(topLeftRect.Location.X, topLeftRect.Location.Y, newWidth, topLeftRect.Height);
            topRightRect = new Rect(
                baseRect.Left + newWidth,
                topRightRect.Location.Y,
                Math.Max(0.0, baseRect.Width - newWidth),
                topRightRect.Height);
        }

        // Adjust the height of the TopRight and BottomRight rectangles so that they are proportional to the height of the baseRect
        if (topRightRect.Bottom > bottomRightRect.Top)
        {
            double newHeight = (topRightRect.Height / (topRightRect.Height + bottomRightRect.Height)) * baseRect.Height;
            topRightRect = new Rect(topRightRect.Location.X, topRightRect.Location.Y, topRightRect.Width, newHeight);
            bottomRightRect = new Rect(
                bottomRightRect.Location.X,
                baseRect.Top + newHeight,
                bottomRightRect.Width,
                Math.Max(0.0, baseRect.Height - newHeight));
        }

        // Adjust the width of the BottomLeft and BottomRight rectangles so that they are proportional to the width of the baseRect
        if (bottomRightRect.Left < bottomLeftRect.Right)
        {
            double newWidth = (bottomLeftRect.Width / (bottomLeftRect.Width + bottomRightRect.Width)) * baseRect.Width;
            bottomLeftRect = new Rect(bottomLeftRect.Location.X, bottomLeftRect.Location.Y, newWidth, bottomLeftRect.Height);
            bottomRightRect = new Rect(
                baseRect.Left + newWidth,
                bottomRightRect.Location.Y,
                Math.Max(0.0, baseRect.Width - newWidth),
                bottomRightRect.Height);
        }

        // Adjust the height of the TopLeft and BottomLeft rectangles so that they are proportional to the height of the baseRect
        if (bottomLeftRect.Top < topLeftRect.Bottom)
        {
            double newHeight = (topLeftRect.Height / (topLeftRect.Height + bottomLeftRect.Height)) * baseRect.Height;
            topLeftRect = new Rect(topLeftRect.Location.X, topLeftRect.Location.Y, topLeftRect.Width, newHeight);
            bottomLeftRect = new Rect(
                bottomLeftRect.Location.X,
                baseRect.Top + newHeight,
                bottomLeftRect.Width,
                Math.Max(0.0, baseRect.Height - newHeight));
        }

        StreamGeometry roundedRectGeometry = new();

        using (var context = roundedRectGeometry.Open())
        {
            // Begin from the Bottom of the TopLeft Arc and proceed clockwise
            context.BeginFigure(topLeftRect.BottomLeft, true, true);

            // TopLeft Arc
            context.ArcTo(topLeftRect.TopRight, topLeftRect.Size, 0, false, SweepDirection.Clockwise, true, true);

            // Top Line
            context.LineTo(topRightRect.TopLeft, true, true);

            // TopRight Arc
            context.ArcTo(topRightRect.BottomRight, topRightRect.Size, 0, false, SweepDirection.Clockwise, true, true);

            // Right Line
            context.LineTo(bottomRightRect.TopRight, true, true);

            // BottomRight Arc
            context.ArcTo(bottomRightRect.BottomLeft, bottomRightRect.Size, 0, false, SweepDirection.Clockwise, true, true);

            // Bottom Line
            context.LineTo(bottomLeftRect.BottomRight, true, true);

            // BottomLeft Arc
            context.ArcTo(bottomLeftRect.TopLeft, bottomLeftRect.Size, 0, false, SweepDirection.Clockwise, true, true);
        }

        return roundedRectGeometry;
    }
}