Image resizing - sometimes very poor quality?!

Posted by eWolf on Stack Overflow See other posts from Stack Overflow or by eWolf
Published on 2009-04-29T19:52:00Z Indexed on 2010/05/08 10:48 UTC
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I'm resizing some images to the screen resolution of the user; if the aspect ratio is wrong, the image should be cut. My code looks like this:

protected void ConvertToBitmap(string filename)
    {
        var origImg = System.Drawing.Image.FromFile(filename);
        var widthDivisor = (double)origImg.Width / (double)System.Windows.Forms.Screen.PrimaryScreen.Bounds.Width;
        var heightDivisor = (double)origImg.Height / (double)System.Windows.Forms.Screen.PrimaryScreen.Bounds.Height;
        int newWidth, newHeight;

        if (widthDivisor < heightDivisor)
        {
            newWidth = (int)((double)origImg.Width / widthDivisor);
            newHeight = (int)((double)origImg.Height / widthDivisor);
        }
        else
        {
            newWidth = (int)((double)origImg.Width / heightDivisor);
            newHeight = (int)((double)origImg.Height / heightDivisor);
        }

         var newImg = origImg.GetThumbnailImage(newWidth, newHeight, null, IntPtr.Zero);
        newImg.Save(this.GetBitmapPath(filename), System.Drawing.Imaging.ImageFormat.Bmp);
    }

In most cases, this works fine. But for some images, the result has an extremely poor quality. It looks like the would have been resized to something very small (thumbnail size) and enlarged again.. But the resolution of the image is correct. What can I do?

Example orig image: alt text

Example resized image: alt text

Note: I have a WPF application but I use the WinForms function for resizing because it's easier and because I already need a reference to System.Windows.Forms for a tray icon.

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