C# 缩略图算法

using (var stream = new FileStream(physicalPath, FileMode.Open, FileAccess.Read))
{
    Bitmap target = null;
    int targetWidth, targetHeight;
    try
    {
        using (var bitmap = new Bitmap(stream))
        {
            var size = bitmap.PhysicalDimension; 
            if (size.Width > size.Height)
            {
                if (size.Width > _maximumWidth)
                {
                    targetWidth = _maximumWidth;
                    targetHeight = (int)size.Height * _maximumWidth / (int)size.Width;
                }
                else
                {
                    targetWidth = (int)size.Width;
                    targetHeight = (int)size.Height;
                }

                if (targetHeight >= _maximumHeight)
                {
                    targetWidth = targetWidth * _maximumHeight / targetHeight;
                    targetHeight = _maximumHeight;
                }
            }
            else
            {
                if (size.Height > _maximumHeight)
                {
                    targetHeight = _maximumHeight;
                    targetWidth = (int)size.Width * _maximumHeight / (int)size.Height;
                }
                else
                {
                    targetHeight = (int)size.Height;
                    targetWidth = (int)size.Width;
                }

                if (targetWidth >= _maximumWidth)
                {
                    targetHeight = targetHeight * _maximumWidth / targetWidth;
                    targetWidth = _maximumWidth;
                }
            }

            SizeF targetSize = SizeF.Empty;

            targetSize = new SizeF(targetWidth, targetHeight);

            target = new Bitmap(targetWidth, targetHeight);
            using (var graphics = Graphics.FromImage(target))
            {
                graphics.CompositingQuality = compositingQuality;
                graphics.InterpolationMode = interpolationMode;
                graphics.CompositingMode = CompositingMode.SourceCopy;

                graphics.DrawImage(bitmap, 0, 0, targetSize.Width, targetSize.Height);
            }
        }

        context.Response.ContentType = "image/jpg";
        using (MemoryStream memoryStream = new MemoryStream())
        {
            target.Save(memoryStream, ImageFormat.Jpeg);
            memoryStream.WriteTo(context.Response.OutputStream);
        }
    }
    finally
    {
        if (target != null)
        {
            target.Dispose();
        }
    }
}