Question

I want to draw a line on a panel and then move it as the mouse moves. To do so, I draw the line and when the mouse moves I redraw the line to the new location and remove the previous line by drawing a line with the background color on it. It works fine if I do not use the high quality smoothing mode. But if use high quality smoothing mode, it leave traces on the panel. Does anybody know how to fix this? Thank you.
Here is the code

        int x_previous = 0;
        int y_previous = 0;

        private void panel1_MouseMove(object sender, MouseEventArgs e)
        {
            Pen pen1 = new System.Drawing.Pen(Color.Black, 3);
            Pen pen2 = new System.Drawing.Pen(panel1.BackColor, 3);
            Graphics g = panel1.CreateGraphics();
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            g.DrawLine(pen2, new Point(0, 0), new Point(x_previous, y_previous));
            g.DrawLine(pen1, new Point(0, 0), new Point(e.Location.X, e.Location.Y));
            x_previous = e.Location.X;
            y_previous = e.Location.Y;
        }

Here is the snapshot with SmoothingMode

Here is the snapshot without SmoothingMode

Answer 1

Instead of drawing a line over a line, the safest option would be to clear the graphics using g.Clear(panel1.BackColor). This will wipe everything off that has been drawn, so that you can safely draw a new line:

private void panel1_MouseMove(object sender, MouseEventArgs e)
{
    Pen pen1 = new System.Drawing.Pen(Color.Black, 3);
    Pen pen2 = new System.Drawing.Pen(panel1.BackColor, 3);
    Graphics g = panel1.CreateGraphics();
    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
    // Clear the graphics, creating a blank area to draw on
    g.Clear(panel1.BackColor);
    g.DrawLine(pen1, new Point(0, 0), new Point(e.Location.X, e.Location.Y));
    x_previous = e.Location.X;
    y_previous = e.Location.Y;
}

Hope this helps!