Quality problem when resizing image?

Question

I'am using this code to resize image. But result is not fine, I want to best quality. I know its low quality because I also resize same image with photoshop and result is different so better. How do I fix it?

private static Image resizeImage(Image imgToResize, Size size)
    {
        int sourceWidth = imgToResize.Width;
        int sourceHeight = imgToResize.Height;

        float nPercent = 0;
        float nPercentW = 0;
        float nPercentH = 0;

        nPercentW = ((float)size.Width / (float)sourceWidth);
        nPercentH = ((float)size.Height / (float)sourceHeight);

        if (nPercentH < nPercentW)
            nPercent = nPercentH;
        else
            nPercent = nPercentW;

        int destWidth = (int)(sourceWidth * nPercent);
        int destHeight = (int)(sourceHeight * nPercent);

        Bitmap b = new Bitmap(destWidth, destHeight);
        Graphics g = Graphics.FromImage((Image)b);
        g.InterpolationMode = InterpolationMode.HighQualityBicubic;

        g.DrawImage(imgToResize, 0, 0, destWidth, destHeight);
        g.Dispose();

        return (Image)b;
    }

Answer 1

This is the routine I use. Perhaps you'll find it useful. It is an extension method, to boot. The only difference is that I omit the code to preserve the aspect ratio, which you could just as easily plug in.

public static Image GetImageHiQualityResized(this Image image, int width, int height)
{
    var thumb = new Bitmap(width, height);
    using (var g = Graphics.FromImage(thumb))
    {
        g.SmoothingMode = SmoothingMode.HighQuality;
        g.CompositingQuality = CompositingQuality.HighQuality;
        g.InterpolationMode = InterpolationMode.High;
        g.DrawImage(image, new Rectangle(0, 0, thumb.Width, thumb.Height));
        return thumb;
    }
}

Example usage of this extension method could include:

// Load the original image
using(var original = Image.FromFile(@"C:\myimage.jpg"))
using(var thumb = image.GetImageHiQualityResized(120, 80))
{
    thumb.Save(@"C:\mythumb.png", ImageFormat.Png);
}

Notes

• See http://msdn.microsoft.com/en-us/library/84767bxk.aspx for additional methods of saving and loading images.

The difference between the default JPG encoding and the default PNG encoding is, indeed, very different. Below are two thumbs using your example, one saved with ImageFormat.Png and one with ImageFormat.Jpeg.

PNG Image

JPEG Image

You may find the work done by the original poster in this question to be helpful if you determine that you absolutely must use JPEG. It involves configuring the image codec and encoding parameters to high-quality settings. http://stackoverflow.com/questions/1072897/net-saving-jpeg-with-the-same-quality-as-it-was-loaded

Were it me, I'd just as soon use the PNG format, as it is lossless.