Why Do Rounded Corners on a Zoomed UserControl Exhibit Visual Artifacts, and How Can They Be Resolved?

Visual Artifacts While Zooming UserControl with Rounded Corners

Problem:

A UserControl with rounded corners and a colored border exhibits visual artifacts when zoomed. The border becomes invisible on one side during zooming out and multiple borders appear on the same side when zooming in.

Solution:

Instead of painting the Region directly, apply Scale and Translate transformations to move the painted area slightly inward within the control's Region. This ensures that the anti-aliased pixels of the border are within the Region, preserving the anti-aliasing effect.

Implementation:

• Set the User Control's BackgroundColor to Color.Transparent to support color transparency.
• Create GraphicsPathWithBorder and Region to define the control's shape.

• In the OnPaint event:

  • Set the SmoothingMode to AntiAlias.
  • Calculate scale and translate values based on the Pen size.
  • Apply the transformations using a Matrix.
  • Fill and draw the path using a Brush and Pen, respectively.
• Handle zooming by updating the Width and recalculating the Region.

Example:

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

public class RoundControl : UserControl
{
    // ...

    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);
    }

    // ...
}

Result:

This approach eliminates the visual artifacts and provides a seamless and visually appealing zooming effect.

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