/// <summary>
/// 生成jpg縮略圖字節,本人的小軟件中需要用到的功能，所以自己做了一個函數，和大家分享
/// 為什么要生成字節而不是文件，這是為了方便后續處理啦^_^
/// </summary>
/// <param name="originalImagePath">原始路徑</param>
/// <param name="quality">質量0-100</param>
/// <param name="width">寬度</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"://指定高寬縮放（可能變形）
break;
case "W"://指定寬，高按比例
toheight = originalImage.Height * width / originalImage.Width;
break;
case "H"://指定高，寬按比例
towidth = originalImage.Width * height / originalImage.Height;
break;
case "Cut"://指定高寬裁減（不變形）
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;
}

//新建一個bmp圖片
Image bitmap = new System.Drawing.Bitmap(towidth, toheight);

//新建一個畫板
Graphics g = System.Drawing.Graphics.FromImage(bitmap);

//設置高質量插值法
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

//設置高質量,低速度呈現平滑程度
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

//清空畫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繪制原圖片的指定部分
g.DrawImage(originalImage, new Rectangle(0, 0, towidth, toheight),
new Rectangle(x, y, ow, oh),
GraphicsUnit.Pixel);

try
{
//以jpg格式保存縮略圖
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();
}
}

/**/
/// <summary>
/// 保存JPG時用
/// </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;
}