一般在web应用中，对客户端提交上来的图片肯定需要进行压缩的。尤其是比较大的图片，如果不经过压缩会导致页面变的很大，打开速度比较慢，当然了如果是需要高质量的图片也得需要生产缩略图。
下面贴出我自己琢磨的图片压缩算法，首先这个是未经优化的简单实现：
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;
}
这个方法比较简单，用到的是高质量压缩。经过这个方法压缩后，200K的图片只能压缩到160k左右。经过改写代码实现了如下的方法：
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;
}
这样实现的压缩使压缩率大幅度上升。其实代码并没有变多少，最主要的是在保存的时候要是用jpg格式，如果不指定格式，默认使用的是png格式。
下面这个是园友写的根据设置图片质量数值来压缩图片的方法：
public static bool GetPicThumbnail(string sFile, string outPath, int flag)
{
System.Drawing.Image iSource = System.Drawing.Image.FromFile(sFile);
ImageFormat tFormat = iSource.RawFormat;
//以下代码为保存图片时，设置压缩质量
EncoderParameters ep = new EncoderParameters();
long[] qy = new long[1];
qy[0] = flag;//设置压缩的比例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是压缩后的新路径
}
else
{
iSource.Save(outPath, tFormat);
}
return true;
}
catch
{
return false;
}
finally
{
iSource.Dispose();
iSource.Dispose();
}
}
转载来源：http://www.cnblogs.com/lifeil/archive/2013/02/25/2931683.html