JPEG是一种图像压缩算法，而JPEG在做压缩编码的时候往往需要设定一个压缩质量/质量因子的参数(quality)。而在编码完成以后我们有时候希望获取到JPEG压缩文件的这个quality，好在万恶的jpeg编码协议却并没有直接存储这个quality参数,而opencv libjpeg这样的第三方库也没有提供获取JPEG文件质量参数的接口。可是JPEG文件存储了一个量化表的参数(quantize table)，这里我们就是运用读取这个JPEG文件的quantize table然后反推这个JPEG文件的quality。

1. 关于JPEG具体的压缩算法请查看wiki
2. 需要用到第三方库文件:libjpeg(下载链接)
3. 安装方法：

   tar xvf jpegsrc.v9.tar.gz
   cd jpeg-9
   mkdir build
   cd build
   ../configure
   make -j
   make install
   

4. 实现代码

   #include <stdio.h>
   #include <math.h>
   
   #include "jpeglib.h"
   #include <setjmp.h>
   
   //标准明度量化表
   static const unsigned int std_luminance_quant_tbl[DCTSIZE2] = {
   16,  11,  10,  16,  24,  40,  51,  61,
   12,  12,  14,  19,  26,  58,  60,  55,
   14,  13,  16,  24,  40,  57,  69,  56,
   14,  17,  22,  29,  51,  87,  80,  62,
   18,  22,  37,  56,  68, 109, 103,  77,
   24,  35,  55,  64,  81, 104, 113,  92,
   49,  64,  78,  87, 103, 121, 120, 101,
   72,  92,  95,  98, 112, 100, 103,  99
   };
   //读取JPG文件的质量参数
   int ReadJpegQuality(const char *filename)
   {
     FILE * infile = fopen(filename, "rb");
     fseek(infile,0,SEEK_END);
     size_t sz = ftell(infile);
     fseek(infile,0,SEEK_SET);
     unsigned char* buffer = new unsigned char[sz];
     fread(buffer,1,sz,infile);
     fclose(infile);
     //如果不是JPG格式的文件返回-1
     if(buffer==NULL || sz <= 2 || 
     0xFF != (uint8_t)buffer[0] || 
     0xD8 != (uint8_t)buffer[1])
     {
       return -1;
     }
     struct jpeg_decompress_struct cinfo;
     struct jpeg_error_mgr jerr;
     cinfo.err = jpeg_std_error(&jerr);
     jpeg_create_decompress(&cinfo);
     jpeg_mem_src(&cinfo,(unsigned char*)buffer,sz);
     jpeg_read_header(&cinfo, TRUE);
     int tmp_quality = 0;
     int linear_quality = 0;
     const int aver_times = 3;
     int times = 0;
     int aver_quality = 0;
     //量化表反推3次，取平均值
     for(int i=0;i<DCTSIZE2;i++)
     {
       long temp = cinfo.quant_tbl_ptrs[0]->quantval[i];
       if(temp<32767L&&temp>0)
       {
         linear_quality = ceil((float)(temp*100L - 50L)/std_luminance_quant_tbl[i]);
         if(linear_quality==1) tmp_quality = 1;
         else if(linear_quality==100) tmp_quality = 100;
         else if(linear_quality>100)
         {
           tmp_quality = ceil((float)5000/linear_quality);
         }
       else
       {
         tmp_quality = 100 - ceil((float)linear_quality/2);
       }
       aver_quality += tmp_quality;
       if(aver_times==++times)
       {
         aver_quality /= aver_times;
         break;
       } 
       }
     }
     jpeg_destroy_decompress(&cinfo);
     return aver_quality;
   }
   
   int main(int argc,char** argv)
   {
     printf("quality: %d\n",ReadJpegQuality("test1.jpg"));
     return 0; 
   }
   

5. 编译链接的方法也很简单了
   g++ main.cpp -I/your_path_to/libjpeg-9b/build/include -L/your_path_to/libjpeg-9b/build/lib/ -ljpeg