How to Eliminate Visual Artifacts When Zooming a UserControl with Rounded Corners?

Eliminating visual artifacts when zooming a UserControl with rounded corners

Zooming a UserControl with rounded corners can result in visual artifacts, such as disappearing or distorted borders. This issue arises when the control's Region, which defines its paintable area, is not updated properly during the zoom operation.

Solution:

To resolve this issue, consider implementing the following approach:

Apply a scale and translate matrix to the control's Region. This will effectively shrink the region's boundaries, ensuring that when the borders are painted, they fall within the Region and are properly anti-aliased.

Set the UserControl's background color to transparent. This allows the control's content to be drawn directly onto the parent container.

Improved Control Implementation:

Here's an enhanced version of the UserControl code that incorporates the above solution:

using System.Drawing;
using System.Drawing.Drawing2D;

public class RoundControl : UserControl
{
    private GraphicsPath GraphicsPathWithBorder;
    private float MyBaseWidth;
    private float m_PenSize = 2f;
    private Color m_BorderColor = Color.Yellow;
    private Color m_FillColor = Color.Green;

    public RoundControl()
    {
        ResizeRedraw = true;
        InitializeComponent();
        MyBaseWidth = Width;
        BackColor = Color.Transparent;
    }

    // ... (other properties and methods)

    protected override void OnPaint(PaintEventArgs e)
    {
        e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
        RectangleF rect = GraphicsPathWithBorder.GetBounds();
        float scaleX = 1 - ((m_PenSize + 1) / rect.Width);
        float scaleY = 1 - ((m_PenSize + 1) / rect.Height);
        using (Pen pen = new Pen(m_BorderColor, m_PenSize))
        using (Brush brush = new SolidBrush(m_FillColor))
        using (Matrix mx = new Matrix(scaleX, 0, 0, scaleY, pen.Width / 2, pen.Width / 2))
        {
            e.Graphics.Transform = mx;
            e.Graphics.FillPath(brush, GraphicsPathWithBorder);
            e.Graphics.DrawPath(pen, GraphicsPathWithBorder);
        }
        base.OnPaint(e);
    }

    // ... (other methods)
}

By implementing these changes, you can ensure that the UserControl's rounded corners remain visually consistent and free of artifacts during zoom operations.

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