在面板上移动高质量线c＃

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!

Answer 2

Instead of drawing the line in the event handler for the mouse movement you should use it to Invalidate the panel and perform the line drawing in a handler for its Paint event. There will be no need to erase the old line.

Answer 3

I haven't used WinForms in a while, so you'll have to forgive me if this doesn't work.

I assume that anti-aliasing has slightly blurred the edges of the line so that the initial line extends slightly further out than the width of the pen. That also means that the edges of the white pen won't be completely opaque when drawing over the black line.

Try increasing the width of the white pen until it completely covers the black, and maybe see if you can leave the Graphics object with lower quality rendering for the white pen, and use the smoothing only for the black pen.