C# 缩略图算法

 代码写多了，有些使用过的方法和技巧会一时半会想不起来，平日记录下来，方便自己和有需要的人日后查阅。

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();

        }

    }

}