Tiny Jpeg Decoder （JPEG解码程序） 源代码分析 2：解码数据

注：分析Tiny Jpeg Decoder源代码的文章：

Tiny Jpeg Decoder （JPEG解码程序） 源代码分析 1：解码文件头

Tiny Jpeg Decoder （JPEG解码程序） 源代码分析 2：解码数据

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Tiny Jpeg Decoder是一个可以用于嵌入式系统的JPEG解码器。也可以在Windows上编译通过。在此分析一下它部分的源代码，辅助学习JPEG解码知识。

通过TinyJpeg可以将JPEG（*.jpg）文件解码为YUV（*.yuv）或者RGB（*.tga）文件。

真正的解码数据开始于tinyjpeg_decode()函数：

注意：本代码中包含部分自己写的代码，用于提取DCT系数表，解码后亮度数据表等数据。

/**
 * Decode and convert the jpeg image into @pixfmt@ image
 *解码函数
 * Note: components will be automaticaly allocated if no memory is attached.
 */
int tinyjpeg_decode(struct jdec_private *priv, int pixfmt)
{
  unsigned int x, y, xstride_by_mcu, ystride_by_mcu;
  unsigned int bytes_per_blocklines[3], bytes_per_mcu[3];
  decode_MCU_fct decode_MCU;
  const decode_MCU_fct *decode_mcu_table;
  const convert_colorspace_fct *colorspace_array_conv;
  convert_colorspace_fct convert_to_pixfmt;

  //-------------------------------------------
	FILE *fp;
	char *temp;
	int j,k;
  //-------------------------------------------

  if (setjmp(priv->jump_state))
    return -1;

  /* To keep gcc happy initialize some array */
  bytes_per_mcu[1] = 0;
  bytes_per_mcu[2] = 0;
  bytes_per_blocklines[1] = 0;
  bytes_per_blocklines[2] = 0;

  decode_mcu_table = decode_mcu_3comp_table;
  switch (pixfmt) {
     case TINYJPEG_FMT_YUV420P:
       colorspace_array_conv = convert_colorspace_yuv420p;
       if (priv->components[0] == NULL)
	 priv->components[0] = (uint8_t *)malloc(priv->width * priv->height);
       if (priv->components[1] == NULL)
	 priv->components[1] = (uint8_t *)malloc(priv->width * priv->height/4);
       if (priv->components[2] == NULL)
	 priv->components[2] = (uint8_t *)malloc(priv->width * priv->height/4);
       bytes_per_blocklines[0] = priv->width;
       bytes_per_blocklines[1] = priv->width/4;
       bytes_per_blocklines[2] = priv->width/4;
       bytes_per_mcu[0] = 8;
       bytes_per_mcu[1] = 4;
       bytes_per_mcu[2] = 4;
       break;

     case TINYJPEG_FMT_RGB24:
       colorspace_array_conv = convert_colorspace_rgb24;
       if (priv->components[0] == NULL)
	 priv->components[0] = (uint8_t *)malloc(priv->width * priv->height * 3);
       bytes_per_blocklines[0] = priv->width * 3;
       bytes_per_mcu[0] = 3*8;
       break;

     case TINYJPEG_FMT_BGR24:
       colorspace_array_conv = convert_colorspace_bgr24;
       if (priv->components[0] == NULL)
	 priv->components[0] = (uint8_t *)malloc(priv->width * priv->height * 3);
       bytes_per_blocklines[0] = priv->width * 3;
       bytes_per_mcu[0] = 3*8;
       break;

     case TINYJPEG_FMT_GREY:
       decode_mcu_table = decode_mcu_1comp_table;
       colorspace_array_conv = convert_colorspace_grey;
       if (priv->components[0] == NULL)
	 priv->components[0] = (uint8_t *)malloc(priv->width * priv->height);
       bytes_per_blocklines[0] = priv->width;
       bytes_per_mcu[0] = 8;
       break;

     default:
#if TRACE_PARAM
		 fprintf(param_trace,"Bad pixel format
");
		 fflush(param_trace);
#endif
       return -1;
  }

  xstride_by_mcu = ystride_by_mcu = 8;
  if ((priv->component_infos[cY].Hfactor | priv->component_infos[cY].Vfactor) == 1) {
     decode_MCU = decode_mcu_table[0];
     convert_to_pixfmt = colorspace_array_conv[0];
#if TRACE_PARAM
     fprintf(param_trace,"Use decode 1x1 sampling
");
	 fflush(param_trace);
#endif
  } else if (priv->component_infos[cY].Hfactor == 1) {
     decode_MCU = decode_mcu_table[1];
     convert_to_pixfmt = colorspace_array_conv[1];
     ystride_by_mcu = 16;
#if TRACE_PARAM
     fprintf(param_trace,"Use decode 1x2 sampling (not supported)
");
	 fflush(param_trace);
#endif
  } else if (priv->component_infos[cY].Vfactor == 2) {
     decode_MCU = decode_mcu_table[3];
     convert_to_pixfmt = colorspace_array_conv[3];
     xstride_by_mcu = 16;
     ystride_by_mcu = 16;
#if TRACE_PARAM 
	 fprintf(param_trace,"Use decode 2x2 sampling
");
	 fflush(param_trace);
#endif
  } else {
     decode_MCU = decode_mcu_table[2];
     convert_to_pixfmt = colorspace_array_conv[2];
     xstride_by_mcu = 16;
#if TRACE_PARAM
     fprintf(param_trace,"Use decode 2x1 sampling
");
	 fflush(param_trace);
#endif
  }

  resync(priv);

  /* Don't forget to that block can be either 8 or 16 lines */
  bytes_per_blocklines[0] *= ystride_by_mcu;
  bytes_per_blocklines[1] *= ystride_by_mcu;
  bytes_per_blocklines[2] *= ystride_by_mcu;

  bytes_per_mcu[0] *= xstride_by_mcu/8;
  bytes_per_mcu[1] *= xstride_by_mcu/8;
  bytes_per_mcu[2] *= xstride_by_mcu/8;

  /* Just the decode the image by macroblock (size is 8x8, 8x16, or 16x16) */
  //纵向
  for (y=0; y < priv->height/ystride_by_mcu; y++)
   {
     //trace("Decoding row %d
", 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]);
	 //横向（循环的写法还不一样？）
     for (x=0; x < priv->width; x+=xstride_by_mcu)
      {
		decode_MCU(priv);
		convert_to_pixfmt(priv);

//DCT系数-----------------------------------------------------------
		//temp=(char *)priv->component_infos->DCT;
		//if(y==4&&x==xstride_by_mcu*3){
		if(priv->dlg->m_vijpgoutputdct.GetCheck()==1){
			fp = fopen("DCT系数表.txt", "a+");
			//fwrite(temp,64,1,fp);
			fprintf(fp,"第%d行，第%d列
",y,x/xstride_by_mcu);
			for(j=0;j<64;j++){
			fprintf(fp,"%d ",priv->component_infos[cY].DCT[j]);
			}
			fprintf(fp,"
");
			fclose(fp);
		}
#if TRACE_PARAM
		fprintf(param_trace,"
第3行，第4列
");
		for(j=0;j<8;j++){
	 		for(k=0;k<8;k++){
				fprintf(param_trace,"%d ",priv->component_infos[cY].DCT[j*8+k]);
			}
			fprintf(param_trace,"
");
		}
		fprintf(fp,"
-----------------------
");
		fflush(param_trace);
#endif
		//}

//解码后系数（Y）---------------------------------------------------
		//temp=(char *)priv->Y;
		//if(y==4&&x==xstride_by_mcu*3){
		if(priv->dlg->m_vijpgoutputy.GetCheck()==1){
			fp = fopen("解码后Y系数表.txt", "a+");
			//fwrite(temp,64*4,1,fp);
			fprintf(fp,"第%d行，第%d列
",y,x/xstride_by_mcu);
			for(j=0;j<64*4;j++){
			fprintf(fp,"%d ",priv->Y[j]);
			}
			fprintf(fp,"
");
			fclose(fp);
		}
#if TRACE_PARAM
		fprintf(param_trace,"第3行，第4列
");
		for(j=0;j<8;j++){
	 		for(k=0;k<8;k++){
				fprintf(param_trace,"%d ",priv->Y[j*8+k]);
			}
			fprintf(param_trace,"
");
		}
		fprintf(fp,"
-----------------------
");
		fflush(param_trace);
#endif
		//}

//------------------------------------------------------------------
		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;
			}
		 }
      }
   }
#if TRACE_PARAM
  fprintf(param_trace,"Input file size: %d
", priv->stream_length+2);
  fprintf(param_trace,"Input bytes actually read: %d
", priv->stream - priv->stream_begin + 2);
  fflush(param_trace);
#endif

  return 0;
}

主页：http://www.saillard.org/programs_and_patches/tinyjpegdecoder/

源代码下载：http://download.csdn.net/detail/leixiaohua1020/6383115