数据压缩原理 实验五 JPEG原理分析及JPEG解码器的调试

实验原理

JPEG编码的过程如下图所示。解码是编码的逆过程。

JPEG文件解码流程：
①读入文件的相关信息
②初步了解图像数据流的结构
③颜色分量单元的内部解码
④直流系数的差分编码
⑤反量化&反zig-zag编码
⑥反离散余弦变换

JPEG文件格式：

实验流程

1.读取文件
2.解析文件
SOI、APP0、DQT、SOF0、DHT、SOS
3.根据每个分量的水平和垂直采样因子计算最小编码单元的大小，并得到每个最小编码单元内8×8宏块的个数
4.对每个最小编码单元解码
5.解析到EOI结束
6.将Y、Cb、Cr转换为所需的色彩空间保存

代码分析

main函数

int main(int argc, char *argv[]){  int output_format = TINYJPEG_FMT_YUV420P;  char *output_filename, *input_filename;  clock_t start_time, finish_time;  unsigned int duration;  int current_argument;  int benchmark_mode = 0;  Q_table = fopen("Q_table.txt","w");       //量化表、DC AC表  DC_table = fopen("DC_table.yuv","w");  AC_table = fopen("AC_table.yuv","w");#if TRACE  p_trace = fopen(TRACEFILE,"w");               //路径文件  if (p_trace == NULL)  {      printf("trace file open error!");  }#endif  if (argc < 3)    usage();  current_argument = 1;  while (1)   {     if (strcmp(argv[current_argument], "--benchmark")==0)       benchmark_mode = 1;     else       break;     current_argument++;   }  if (argc < current_argument+2)    usage();  input_filename = argv[current_argument];  //设置变换方式  if (strcmp(argv[current_argument+1],"yuv420p")==0)    output_format = TINYJPEG_FMT_YUV420P;  else if (strcmp(argv[current_argument+1],"rgb24")==0)    output_format = TINYJPEG_FMT_RGB24;  else if (strcmp(argv[current_argument+1],"bgr24")==0)    output_format = TINYJPEG_FMT_BGR24;  else if (strcmp(argv[current_argument+1],"grey")==0)    output_format = TINYJPEG_FMT_GREY;  else if (strcmp(argv[current_argument+1],"yuv")==0)    output_format = TINYJPEG_FMT_YUV;  else    exitmessage("Bad format: need to be one of yuv420p, rgb24, bgr24, grey\n");  output_filename = argv[current_argument+2];  start_time = clock();  if (benchmark_mode)    load_multiple_times(input_filename, output_filename, output_format);    //加载多次  else    convert_one_image(input_filename, output_filename, output_format);    //转换一个图像  finish_time = clock();  duration = finish_time - start_time;  snprintf(error_string, sizeof(error_string),"Decoding finished in %u ticks\n", duration);  fclose(Q_table);          //关文件  fclose(DC_table);  fclose(AC_table);#if TRACE  fclose(p_trace);#endif  return 0;}

输出量化表

static int parse_DQT(struct jdec_private *priv, const unsigned char *stream){  int i;  int qi;  float *table;  const unsigned char *dqt_block_end;#if TRACE  fprintf(p_trace,"> DQT marker\n");  fflush(p_trace);#endif  dqt_block_end = stream + be16_to_cpu(stream);  stream += 2;  /* Skip length */   while (stream < dqt_block_end)   {     qi = *stream++;#if SANITY_CHECK     if (qi>>4)       snprintf(error_string, sizeof(error_string),"16 bits quantization table is not supported\n");     if (qi>4)       snprintf(error_string, sizeof(error_string),"No more 4 quantization table is supported (got %d)\n", qi);#endif     table = priv->Q_tables[qi];     build_quantization_table(table, stream);     /***************************************************/    for(i=0;i<64;i++)    {         if((!(i%8)))     {         fprintf(Q_table,"\n%f",table[i]);     }     else      {         fprintf(Q_table," %f",table[i]);     }    }     fprintf(Q_table,"\n");     /***************************************************/     stream += 64;        }#if TRACE  fprintf(p_trace,"< DQT marker\n");  fflush(p_trace);#endif  return 0;}

输出DC、AC系数

for (y=0; y < priv->height/ystride_by_mcu; y++)   {     //trace("Decoding row %d\n", y);     priv->plane[0] = priv->components[0] + (y * bytes_per_blocklines[0]);     priv->plane[1] = priv->components[1] + (y * bytes_per_blocklines[1]);     priv->plane[2] = priv->components[2] + (y * bytes_per_blocklines[2]);     for (x=0; x < priv->width; x+=xstride_by_mcu)      {    decode_MCU(priv);    convert_to_pixfmt(priv);    priv->plane[0] += bytes_per_mcu[0];    priv->plane[1] += bytes_per_mcu[1];    priv->plane[2] += bytes_per_mcu[2];    if (priv->restarts_to_go>0)     {       priv->restarts_to_go--;       if (priv->restarts_to_go == 0)        {          priv->stream -= (priv->nbits_in_reservoir/8);          resync(priv);          if (find_next_rst_marker(priv) < 0)        return -1;        }     }     DC[0]=(priv->component_infos->DCT[0]+512.0)/4;     DCimage[0]=(unsigned char)(DC[0]+0.5);     fwrite(DCimage,1,1,DC_table);     //按块写DC系数     ACimage[0]=(unsigned char)(priv->component_infos->DCT[1]+128);     fwrite(ACimage,1,1,AC_table);     //按块写AC系数      }   }

实验结果

原始图像

部分输出文件

DC图像

AC图像