C#實現圖片壓縮方法

這個是未經優化的簡單實現

public static System.Drawing.Image GetImageThumb(System.Drawing.Image sourceImg, int width, int height)
        {
            System.Drawing.Image targetImg = new System.Drawing.Bitmap(width, height);
            using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(targetImg))
            {
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
                g.DrawImage(sourceImg, new System.Drawing.Rectangle(0, 0, width, height), new System.Drawing
.Rectangle(0, 0, sourceImg.Width, sourceImg.Height), System.Drawing.GraphicsUnit.Pixel);
                g.Dispose();
            }
            return targetImg;
        }

這個方法比較簡單，用到的是高質量壓縮。經過這個方法壓縮後，200K的圖片只能壓縮到160k左右。

經過改寫代碼實現了如下的方法

public Bitmap GetImageThumb(Bitmap mg, Size newSize)
        {
            double ratio = 0d;
            double myThumbWidth = 0d;
            double myThumbHeight = 0d;
            int x = 0;
            int y = 0;

            Bitmap bp;

            if ((mg.Width / Convert.ToDouble(newSize.Width)) > (mg.Height /
            Convert.ToDouble(newSize.Height)))
                ratio = Convert.ToDouble(mg.Width) / Convert.ToDouble(newSize.Width);
            else
                ratio = Convert.ToDouble(mg.Height) / Convert.ToDouble(newSize.Height);
            myThumbHeight = Math.Ceiling(mg.Height / ratio);
            myThumbWidth = Math.Ceiling(mg.Width / ratio);

            Size thumbSize = new Size((int)newSize.Width, (int)newSize.Height);
            bp = new Bitmap(newSize.Width, newSize.Height);
            x = (newSize.Width - thumbSize.Width) / 2;
            y = (newSize.Height - thumbSize.Height);
            System.Drawing.Graphics g = Graphics.FromImage(bp);
            g.SmoothingMode = SmoothingMode.HighQuality;
            g.InterpolationMode = InterpolationMode.HighQualityBicubic;
            g.PixelOffsetMode = PixelOffsetMode.HighQuality;
            Rectangle rect = new Rectangle(x, y, thumbSize.Width, thumbSize.Height);
            g.DrawImage(mg, rect, 0, 0, mg.Width, mg.Height, GraphicsUnit.Pixel);

            return bp;
        }

這樣實現的壓縮使壓縮率大幅度上升。其實代碼並沒有變多少，最主要的是在保存的時候要是用jpg格式，

如果不指定格式，默認使用的是png格式。

下面這個是園友寫的根據設置圖片質量數值來壓縮圖片的方法：

public static bool GetPicThumbnail(string sFile, string outPath, int flag)
        {
            System.Drawing.Image iSource = System.Drawing.Image.FromFile(sFile);
            ImageFormat tFormat = iSource.RawFormat;

            //以下代碼為保存圖片時，設置壓縮質量  
            EncoderParameters ep = new EncoderParameters();
            long[] qy = new long[1];
            qy[0] = flag;//設置壓縮的比例1-100  
            EncoderParameter eParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, qy);
            ep.Param[0] = eParam;
            try
            {
                ImageCodecInfo[] arrayICI = ImageCodecInfo.GetImageEncoders();
                ImageCodecInfo jpegICIinfo = null;
                for (int x = 0; x < arrayICI.Length; x++)
                {
                    if (arrayICI[x].FormatDescription.Equals("JPEG"))
                    {
                        jpegICIinfo = arrayICI[x];
                        break;
                    }
                }
                if (jpegICIinfo != null)
                {
                    iSource.Save(outPath, jpegICIinfo, ep);//dFile是壓縮後的新路徑  
                }
                else
                {
                    iSource.Save(outPath, tFormat);
                }
                return true;
            }
            catch
            {
                return false;
            }
            finally
            {
                iSource.Dispose();
                iSource.Dispose();
            }
        }