Scale and Rotate an image in iOS / MonoTouch, using the EXIF data.

scaleAndrotate.cs

//MIT license
//https://gist.github.com/nicwise/890460

public static UIImage ScaleImage(UIImage image, int maxSize)
{
    UIImage res;
    using (CGImage imageRef = image.CGImage)
    {
        CGImageAlphaInfo alphaInfo = imageRef.AlphaInfo;
        CGColorSpace colorSpaceInfo = CGColorSpace.CreateDeviceRGB();
        if (alphaInfo == CGImageAlphaInfo.None)
        {
            alphaInfo = CGImageAlphaInfo.NoneSkipLast;
        }
        
        int width, height;
        width = imageRef.Width;
        height = imageRef.Height;

        if (height >= width)
        {
            width = (int)Math.Floor((double)width * ((double)maxSize / (double)height));
            height = maxSize;
        }
        else
        {
            height = (int)Math.Floor((double)height * ((double)maxSize / (double)width));
            width = maxSize;
        }

        CGBitmapContext bitmap;
        if (image.Orientation == UIImageOrientation.Up || image.Orientation == UIImageOrientation.Down)
        {
            bitmap = new CGBitmapContext(IntPtr.Zero, width, height, imageRef.BitsPerComponent, imageRef.BytesPerRow, colorSpaceInfo, alphaInfo);
        }
        else
        {
            bitmap = new CGBitmapContext(IntPtr.Zero, height, width, imageRef.BitsPerComponent, imageRef.BytesPerRow, colorSpaceInfo, alphaInfo);
        }

        switch (image.Orientation)
        {
            case UIImageOrientation.Left:
                bitmap.RotateCTM((float)Math.PI / 2);
                bitmap.TranslateCTM(0, -height);
                break;
            case UIImageOrientation.Right:
                bitmap.RotateCTM(-((float)Math.PI / 2));
                bitmap.TranslateCTM(-width, 0);
                break;
            case UIImageOrientation.Up:
                break;
            case UIImageOrientation.Down:
                bitmap.TranslateCTM(width, height);
                bitmap.RotateCTM(-(float)Math.PI);
                break;
        }

        bitmap.DrawImage(new Rectangle(0, 0, width, height), imageRef);
        res = UIImage.FromImage(bitmap.ToImage());
        bitmap = null;
    }
    return res;
}