c# - Resizing an image in asp.net without losing the image quality

Question

I am developing an ASP.NET 3.5 web application in which I am allowing my users to upload either jpeg,gif,bmp or png images. If the uploaded image dimensions are greater then 103 x 32 the I want to resize the uploaded image to 103 x 32. I have read some blog posts and articles, and have also tried some of the code samples but nothing seems to work right. Has anyone succeed in doing this?

Answer 1

This is the code I use. It supports rotation, and also sets the image resolution to the JPEG standards of 72dpi@24-bit color (by default GDI+ saves images at 96dpi@32-bit color). It also fixes the black/gray border problem that some people experience when resizing images.

/// <summary>
/// Resizes and rotates an image, keeping the original aspect ratio. Does not dispose the original
/// Image instance.
/// </summary>
/// <param name="image">Image instance</param>
/// <param name="width">desired width</param>
/// <param name="height">desired height</param>
/// <param name="rotateFlipType">desired RotateFlipType</param>
/// <returns>new resized/rotated Image instance</returns>
public static Image Resize(Image image, int width, int height, RotateFlipType rotateFlipType)
{
    // clone the Image instance, since we don't want to resize the original Image instance
    var rotatedImage = image.Clone() as Image;
    rotatedImage.RotateFlip(rotateFlipType);
    var newSize = CalculateResizedDimensions(rotatedImage, width, height);

    var resizedImage = new Bitmap(newSize.Width, newSize.Height, PixelFormat.Format32bppArgb);
    resizedImage.SetResolution(72, 72);

    using (var graphics = Graphics.FromImage(resizedImage))
    {
        // set parameters to create a high-quality thumbnail
        graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphics.SmoothingMode = SmoothingMode.AntiAlias;
        graphics.CompositingQuality = CompositingQuality.HighQuality;
        graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;

        // use an image attribute in order to remove the black/gray border around image after resize
        // (most obvious on white images), see this post for more information:
        // http://www.codeproject.com/KB/GDI-plus/imgresizoutperfgdiplus.aspx
        using (var attribute = new ImageAttributes())
        {
            attribute.SetWrapMode(WrapMode.TileFlipXY);

            // draws the resized image to the bitmap
            graphics.DrawImage(rotatedImage, new Rectangle(new Point(0, 0), newSize), 0, 0, rotatedImage.Width, rotatedImage.Height, GraphicsUnit.Pixel, attribute);
        }
    }

    return resizedImage;
}

/// <summary>
/// Calculates resized dimensions for an image, preserving the aspect ratio.
/// </summary>
/// <param name="image">Image instance</param>
/// <param name="desiredWidth">desired width</param>
/// <param name="desiredHeight">desired height</param>
/// <returns>Size instance with the resized dimensions</returns>
private static Size CalculateResizedDimensions(Image image, int desiredWidth, int desiredHeight)
{
    var widthScale = (double)desiredWidth / image.Width;
    var heightScale = (double)desiredHeight / image.Height;

    // scale to whichever ratio is smaller, this works for both scaling up and scaling down
    var scale = widthScale < heightScale ? widthScale : heightScale;

    return new Size
                   {
                       Width = (int) (scale * image.Width),
                       Height = (int) (scale * image.Height)
                   };
}