Question

My problem is with clipping an image via GraphicsPath. I have a fairly complex clipping path (composed of various arcs) and I need to save only that part of the image that is within this path.

I have a class like this:

class A 
{
    private GraphicsPath GetGraphicsPath()
    { 
        ... some stuff ...
        return gp;
    }

    public void Draw(Graphics g)
    {
         g.DrawPath(pen, GetGraphicsPath());
    }

    public Save(Image img, string fileName)
    {
        Bitmap b = new Bitmap(img.Width, img.Height);
        Graphics bg = Graphics.FromImage(b);
        bg.Clip = new Region(GetGraphicsPath());
        bg.DrawImage(img, 0, 0);
        b.Save(fileName + ".png");
    }
}

and then I have a form with a panel pnMain, with a background image. I have two buttons with the following functions:

    private void button1_Click(object sender, EventArgs e)
    {
        Graphics g = pnMain.CreateGraphics();
        a.Draw(g);
    }

    private void button2_Click(object sender, EventArgs e)
    {
        Image img = pnMain.BackgroundImage;
        a.Save(img, "test");
    }

(in both cases a is an instance of A). My problem is that the clipping region drawn on screen (using first button and method Draw) is what I would like to save, however, the clipped image saved through second button and method Save is distorted, smaller and shifted. What am I missing?

Answer 1

I figured it out — it is the resolution of the background image in the panel. The properties HorizontalResolution and VerticalResolution of the Bitmap img have to match the DPIX and DPIY properties of the Graphics. I did not found out how to set any of these via C#, but I was able to change the resolution of the bitmap in Photoshop and now everything works.