Resizing an image in asp.net without losing the image quality

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)
                   };
}

I had the same problem a while back and dealt with it this way:

private Image RezizeImage(Image img, int maxWidth, int maxHeight)
{
    if(img.Height < maxHeight && img.Width < maxWidth) return img;
    using (img)
    {
        Double xRatio = (double)img.Width / maxWidth;
        Double yRatio = (double)img.Height / maxHeight;
        Double ratio = Math.Max(xRatio, yRatio);
        int nnx = (int)Math.Floor(img.Width / ratio);
        int nny = (int)Math.Floor(img.Height / ratio);
        Bitmap cpy = new Bitmap(nnx, nny, PixelFormat.Format32bppArgb);
        using (Graphics gr = Graphics.FromImage(cpy))
        {
            gr.Clear(Color.Transparent);

            // This is said to give best quality when resizing images
            gr.InterpolationMode = InterpolationMode.HighQualityBicubic;

            gr.DrawImage(img,
                new Rectangle(0, 0, nnx, nny),
                new Rectangle(0, 0, img.Width, img.Height),
                GraphicsUnit.Pixel);
        }
        return cpy;
    }

}

private MemoryStream BytearrayToStream(byte[] arr)
{
    return new MemoryStream(arr, 0, arr.Length);
}

private void HandleImageUpload(byte[] binaryImage)
{
    Image img = RezizeImage(Image.FromStream(BytearrayToStream(binaryImage)), 103, 32);
    img.Save("IMAGELOCATION.png", System.Drawing.Imaging.ImageFormat.Gif);
}

I just read that this was the the way to get highest quality.

The code associated with the actual resizing of the bitmap is as follows.

public static Bitmap ResizeBitmap( Bitmap originalBitmap, int requiredHeight, int requiredWidth )
{
   int[] heightWidthRequiredDimensions;

   // Pass dimensions to worker method depending on image type required
   heightWidthRequiredDimensions = WorkDimensions(originalBitmap.Height, originalBitmap.Width, requiredHeight, requiredWidth);


   Bitmap resizedBitmap = new Bitmap( heightWidthRequiredDimensions[1],
                                      heightWidthRequiredDimensions[0] );

   const float resolution = 72;

   resizedBitmap.SetResolution( resolution, resolution );

   Graphics graphic = Graphics.FromImage( (Image) resizedBitmap );

   graphic.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
   graphic.DrawImage( originalBitmap, 0, 0, resizedBitmap.Width, resizedBitmap.Height );

   graphic.Dispose();
   originalBitmap.Dispose();
   //resizedBitmap.Dispose(); // Still in use


   return resizedBitmap;
}


private static int[] WorkDimensions(int originalHeight, int originalWidth, int requiredHeight, int requiredWidth )
{
   int imgHeight = 0;
   int imgWidth = 0;

   imgWidth = requiredHeight;
   imgHeight = requiredWidth;


   int requiredHeightLocal = originalHeight;
   int requiredWidthLocal = originalWidth;

   double ratio = 0;

   // Check height first
   // If original height exceeds maximum, get new height and work ratio.
   if ( originalHeight > imgHeight )
   {
       ratio = double.Parse( ( (double) imgHeight / (double) originalHeight ).ToString() );
       requiredHeightLocal = imgHeight;
       requiredWidthLocal = (int) ( (decimal) originalWidth * (decimal) ratio );
   }

   // Check width second. It will most likely have been sized down enough
   // in the previous if statement. If not, change both dimensions here by width.
   // If new width exceeds maximum, get new width and height ratio.
   if ( requiredWidthLocal >= imgWidth )
   {
       ratio = double.Parse( ( (double) imgWidth / (double) originalWidth ).ToString() );
       requiredWidthLocal = imgWidth;
       requiredHeightLocal = (int) ( (double) originalHeight * (double) ratio );
   }

   int[] heightWidthDimensionArr = { requiredHeightLocal, requiredWidthLocal };

   return heightWidthDimensionArr;
}
}

This blog post contains full source code for image resizing, and compression (if required)

http://blog.bombdefused.com/2010/08/bulk-image-optimizer-in-c-full-source.html