c#实现的jpg缩略图函数

/// <summary>
/// 生成jpg缩略图字节
/// </summary>
/// <param name="originalImagePath">原始路径</param>
/// <param name="quality">质量0-100</param>
/// <param name="width">宽度</param>
/// <param name="height">高度</param>
/// <param name="mode">模式：HW,W,H,Cut</param>
/// <returns></returns>
public static byte[] MakeJPGThumbnailBytes(string originalImagePath, long quality, int width, int height, string mode)
{
Image originalImage = Image.FromFile(originalImagePath);
MemoryStream s = new MemoryStream();
int towidth = width;
int toheight = height;
 
int x = 0;
int y = 0;
int ow = originalImage.Width;
int oh = originalImage.Height;
 
switch (mode)
{
case "HW"://指定高宽缩放（可能变形）
break;
case "W"://指定宽，高按比例
toheight = originalImage.Height * width / originalImage.Width;
break;
case "H"://指定高，宽按比例
towidth = originalImage.Width * height / originalImage.Height;
break;
case "Cut"://指定高宽裁减（不变形）
if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)
{
oh = originalImage.Height;
ow = originalImage.Height * towidth / toheight;
y = 0;
x = (originalImage.Width - ow) / 2;
}
else
{
ow = originalImage.Width;
oh = originalImage.Width * height / towidth;
x = 0;
y = (originalImage.Height - oh) / 2;
}
break;
default:
break;
}
 
//新建一个bmp图片
Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
 
//新建一个画板
Graphics g = System.Drawing.Graphics.FromImage(bitmap);
 
//设置高质量插值法
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
 
//设置高质量,低速度呈现平滑程度
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
 
//清空画布并以透明背景色填充
g.Clear(Color.Transparent);
 
//在指定位置并且按指定大小绘制原图片的指定部分
g.DrawImage(originalImage, new Rectangle(0, 0, towidth, toheight),
new Rectangle(x, y, ow, oh),
GraphicsUnit.Pixel);
 
try
{
//以jpg格式保存缩略图
EncoderParameters eps = new EncoderParameters(1);
EncoderParameter ep = new EncoderParameter(Encoder.Quality,quality);
eps.Param[0] = ep;
bitmap.Save(s, GetCodecInfo("image/jpeg"), eps);
return s.GetBuffer();
}
catch (System.Exception e)
{
throw e;
}
finally
{
originalImage.Dispose();
bitmap.Dispose();
s.Dispose();
g.Dispose();
} www.jb200.com
}
 
/**/
/// <summary>
/// 保存JPG时用
/// </summary>
/// <param name="mimeType"></param>
/// <returns>得到指定mimeType的ImageCodecInfo</returns>
private static ImageCodecInfo GetCodecInfo(string mimeType)
{
ImageCodecInfo[] CodecInfo = ImageCodecInfo.GetImageEncoders();
foreach (ImageCodecInfo ici in CodecInfo)
{
if (ici.MimeType == mimeType) return ici;
}
return null;
}