高质量缩略图（转载）

private static Size NewSize(int maxWidth, int maxHeight, int width, int height)
{
double w = 0.0;
double h = 0.0;
double sw = Convert.ToDouble(width);
double sh = Convert.ToDouble(height);
double mw = Convert.ToDouble(maxWidth);
double mh = Convert.ToDouble(maxHeight);

if ( sw < mw && sh < mh )
{
w = sw;
h = sh;
}
else if ( (sw/sh) > (mw/mh) )
{
w = maxWidth;
h = (w * sh)/sw;
}
else
{
h = maxHeight;
w = (h * sw)/sh;
}

return new Size(Convert.ToInt32(w), Convert.ToInt32(h));
}

public static void SendSmallImage(string fileName, string newFile, int maxHeight, int maxWidth)
{
System.Drawing.Image img = System.Drawing.Image.FromFile(fileName);
System.Drawing.Imaging.ImageFormat thisFormat = img.RawFormat;

Size newSize = NewSize(maxWidth, maxHeight, img.Width, img.Height);
Bitmap outBmp = new Bitmap(newSize.Width, newSize.Height);
Graphics g = Graphics.FromImage(outBmp);

// 设置画布的描绘质量
g.CompositingQuality = CompositingQuality.HighQuality;
g.SmoothingMode = SmoothingMode.HighQuality;
g.InterpolationMode = InterpolationMode.HighQualityBicubic;

g.DrawImage(img, new Rectangle(0, 0, newSize.Width, newSize.Height),
0, 0, img.Width, img.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;

//获得包含有关内置图像编码解码器的信息的ImageCodecInfo 对象。
ImageCodecInfo[] arrayICI = ImageCodecInfo.GetImageEncoders();
ImageCodecInfo jpegICI = null;
for (int x = 0; x < arrayICI.Length; x++)
{
if (arrayICI[x].FormatDescription.Equals("JPEG"))
{
jpegICI = arrayICI[x];//设置JPEG编码
break;
}
}

if (jpegICI != null)
{
outBmp.Save(newFile, jpegICI, encoderParams);
}
else
{
outBmp.Save(newFile, thisFormat);
}

img.Dispose();
outBmp.Dispose();
}