c# 图片按比例缩放

public static Bitmap GetThumbnail(Bitmap b, int destHeight,int destWidth)  

        {          

            System.Drawing.Image imgSource = b;     

            System.Drawing.Imaging.ImageFormat thisFormat = imgSource.RawFormat; 

            int sW = 0, sH = 0;         

            // 按比例缩放          

            int sWidth = imgSource.Width;

            int sHeight = imgSource.Height;

            if (sHeight > destHeight || sWidth > destWidth)

            {              

                if ((sWidth * destHeight) > (sHeight * destWidth))   

                {                

                    sW = destWidth;   

                    sH = (destWidth * sHeight) / sWidth; 

                }              

                else           

                {          

                    sH = destHeight;   

                    sW = (sWidth * destHeight) / sHeight;   

                }          

            }          

            else        

            {         

                sW = sWidth; 

                sH = sHeight; 

            }   

            Bitmap outBmp = new Bitmap(destWidth, destHeight); 

            Graphics g = Graphics.FromImage(outBmp);     

            g.Clear(Color.Transparent);        

            // 设置画布的描绘质量        

            g.CompositingQuality = CompositingQuality.HighQuality;

            g.SmoothingMode = SmoothingMode.HighQuality;      

            g.InterpolationMode = InterpolationMode.HighQualityBicubic;   

            g.DrawImage(imgSource,new Rectangle((destWidth - sW) / 2, (destHeight - sH) / 2, sW, sH), 0, 0, imgSource.Width, imgSource.Height, GraphicsUnit.Pixel);    

            g.Dispose();     

            // 以下代码为保存图片时，设置压缩质量    

            EncoderParameters encoderParams = new EncoderParameters(); 

            long[] quality = new long[1];     

            quality[0] = 100;     

            EncoderParameter encoderParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);  

            encoderParams.Param[0] = encoderParam;  

            imgSource.Dispose();        

            return outBmp;     

        }