数据压缩实验五：JPEG原理分析及JPEG解码器的调试

一、JPEG原理分析

编码部分

1.零偏置（Level Offset）

对于灰度级是$\ 2^n$ 2n的像素，通过减去 $\ 2^{n-1}$ 2n−1，将无符号的整数值变成有符号数。
例：对于 n=8 ，即将 0~255 的值域，通过减去 128 ，转换为值域在[128 ~127] 内。

2.DCT变换

DCT变换的全称是离散余弦变换（DCT），能够将空域的信号转换到频域上。
图像的存储过程中,高频部分往往占用了很大的空间,而低频部分却占用的不多,因此,在图像处理的时候,牺牲部分高频,既可以提高利用率,也能保存下图像的大部分细节。
DCT变换本身是无损的且具有对称性。对原始图像进行离散余弦变换，变换后DCT系数能量主要集中在左上角，其余大部分系数接近于零。

3.量化

uniform scalar quantization
采用中平型均匀量化器
$\ l(k) = [θ(k)/Q(k)+0.5],k=0,1,2，……$ l(k)=[θ(k)/Q(k)+0.5],k=0,1,2，……
θ(k)：DCT系数
Q(k)：量化步长

量化表的确定：
因为人眼对亮度信号比对色差信号更敏感，因此使用了两
种量化表：亮度量化值和色差量化值
根据人眼的视觉特性（对低频敏感，对高频不太敏感）对
低频分量采取较细的量化，对高频分量采取较粗的量化
建议基本量化表：

4.量化后系数的编码

1）DC系数差分编码
8 × 8 图像块经过 DCT 变换之后得到的 DC 直流系数有两个特点：
a.系数的数值比较大
b.相邻 8 × 8 图像块的 DC 系数值变化不大：冗余
根据这个特点， JPEG 算法使用了差分脉冲调制编码( DPCM技术)，对相邻图像块之间量化 DC 系数的差值 DIFF进行编码：$\ DIFF_k = DC_k-DC_{k-1}$ DIFFk​=DCk​−DCk−1​
对差值DIFF进行Huffman编码

2）AC系数
a.由于经 DCT 变换后，系数大多数集中在左上角，即低频
分量区，因此采用 Z 字形按频率的高低顺序读出，可以出
现很多连零的机会。使用游程编码。尤其在最后，如果都是零，给出 EOB (End of Block) 即可。

b.AC系数编码
游程编码
在 JPEG 和 MPEG 编码中规定为：(run,level)
表示连续 run 个 0 ，后面跟值为 level 的系数。
如：−3 0 −3 −2 −6 2 −4 1 −4 1 1 5 1 2 −1 1 −1 2 0 0 0 0 0 −1 −1
EOB
表示为 (0,3) ; (1,-3) ; (0,-2) ; (0,-6) ; (0,2) ; (0,-4) ; (0,1) ; (0,-4) ;(0,1) (0,1) ; (0,5) ; ( 0,1);(0,2) ; (0,-1) ; (0,1) ; (0,-1) ; (0,2) ; (5,-1) ;(0,-1) ; EOB

Run: 最多 15 个，用 4 位表示 Z
Level ：类似 DC

• 分成 16 个类别，用 4 位表示表示类别号 C
• 类内索引
  对 (Z, C)联合用 Huffman 编码
  对类内索引用定长码编码

解码部分

与编码相反：
解码 Huffman 数据
解码 DC 差值
重构量化后的系数
DCT 逆变换
丢弃填充的行 列
反 0 偏置
对丢失的 CbCr 分量差值（下采样的逆过程）
YCbCr ——> RGB

二、JPEG文件格式

JPEG文件大体上可以分成两个部分:标记码(Tag)和压缩数据。
标记码由两个字节构成,前一个字节是固定值0xFF ,后一个字节则根据不同意义有不同数值。
在每个标记码之前还可以添加数目不限的无意义的0xFF填充,也就说连续的多个0xFF可以被理解为一个θxFF ,并表示一个标记码的开始。
在一个完整的两字节的标记码后，就是该标记码对应的压缩数据流，记录了关于文件的诸种信息。

1.图像起始SOI和EOI
一定以 0xFFD8 开始，表示图像开始 SOI (Start of Image) 标记
一定以 0xFFD9 结束，表示图像结束 EOI(End of Image) 标记

2.APP0
固定值0xFFE0

名称	字节数	描述
APPO长度(length)	2	9个字段的总长度,即不包括标记代码，但包括本字段
标识符(identifier)	5	固定值 0x4A46494600，即字符串“JFIF0”
版本号(version)	2	一般是 0x0102，表示 JFIF 的版本号 1.2,可能会有其他数值代表其他版本
X和Y的密度单位	1	units=0: 无单位; units=1: 点数/英寸; units=2:点数/厘米
X方向像素密度(X density)	2
Y方向像素密度(Y density)	2
缩略图水平像素数目(thumbnail horizontal pixels)	1
缩略图垂直像素数目(thumbnail vertical pixels)	1
缩略图RGB位图(thumbnail RGB bi tmap)	3的倍数	缩略图 RGB 位图数据

3.APPn
n取1-15（任选）
固定值0xFFE1-0XFFFF

名称	字节数	描述
APPn长度(length)	2	2 个字段的总长度,即不包括标记代码，但包括本字段
详细信息(application specific information)	数据长度-2

4.一个或多个量化表DQT
固定值0xFFDB

名称	字节数	描述
数据长度(length)	2	2 个字段的总长度,即不包括标记代码，但包括本字段
量化表(quantization table)	数据长度-2	精度及量化表id（1字节）高 4 位：精度，只有两个可选值 0：8位；1：16位; 低 4 位：量化表 ID，取值范围为 0～3
		表项（16*（精度+1）字节）

5.SOF帧开始
固定值0xFFC0

名称	字节数	描述
帧开始长度(start of frame length)	2	六个字段的总长度，即不包括标记代码，但包括本字段
精度(precision)	1	每个数据样本的位数，通常是 8 位，一般软件都不支持 12 位和 16 位
图像高度(image height)	2	图像高度（单位：像素），如果不支持 DNL 就必须 > 0
图像宽度(image width)	2	图像宽度（单位：像素），如果不支持 DNL 就必须 > 0
颜色分量数(number of color components)	1	只有 3 个数值可选 1：灰度图；3：YCrCb 或 YIQ；4：CMYK； 而 JFIF 中使用 YCrCb，故这里颜色分量数恒为 3
对每个颜色分量(for each component)	颜色分量数*3	《颜色分量信息表格》

对颜色分量：

• ID
• 垂直方向的样本因子(vertical sample factor)
• 水平方向的样本因子(hori zontal sample factor)
• 量化表号(quantization table#)

6.DHT霍夫曼表
固定值0xFFC4

7.SOS扫描开始
固定值0xFFDA

三、JPEG解码器调试

1．逐步调试JPEG解码器程序。将输入的JPG文件进行解码，将输出文件保存为可供YUVViewer观看的YUV文件。

2. 程序调试过程中，应做到：

理解程序设计的整体框架

loadjpeg.c

主要部分main、load_multiple_times和convert_one_image
以及不同的写出文件（tga、yuv、pgm）的函数

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;

  if (argc < 3)//如果输入的参数小于3，显示使用指南
    usage();

  current_argument = 1;//目前进行到的argument
  while (1)
   {
     if (strcmp(argv[current_argument], "--benchmark")==0)//调试界面中的可选项[option]
       benchmark_mode = 1;
     else
       break;
     current_argument++;//指向下一串字符
   }

  if (argc < current_argument+2)//参数不合要求
    usage();//再次显示使用指南

  input_filename = argv[current_argument];//输入文件名
  //if判断要求哪种format，选择不同的输出形式
  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
    exitmessage("Bad format: need to be one of yuv420p, rgb24, bgr24, grey\n");//format错误
  output_filename = argv[current_argument+2];//输出文件名
  start_time = clock();

  if (benchmark_mode)
    load_multiple_times(input_filename, output_filename, output_format);
  else
    convert_one_image(input_filename, output_filename, output_format);
    
 finish_time = clock();
 duration = finish_time - start_time;
 snprintf(error_string, sizeof(error_string),"Decoding finished in %u ticks\n", duration);
#if TRACE
  fclose(p_trace);
#endif
  return 0;
}

load_multiple_times

/**
 * Load one jpeg image, and try to decompress 1000 times, and save the result.
 * This is mainly used for benchmarking the decoder, or to test if between each
 * called of the library the DCT is corrected reset (a bug was found).
 */
int load_multiple_times(const char *filename, const char *outfilename, int output_format)
{
  FILE *fp;
  int cout, length_of_file;//定义计数和输入文件长度
  unsigned int width, height;//定义宽和高
  unsigned char *buf;//定义一个buf的缓冲区
  struct jdec_private *jdec;//定义一个jdec_private类型的jdec
  unsigned char *components[4];//定义指针数组

  jdec = tinyjpeg_init();
  count = 0;

  /* Load the Jpeg into memory */
  fp = fopen(filename, "rb");
  if (fp == NULL)
    exitmessage("Cannot open filename\n");
  length_of_file = filesize(fp);
  buf = (unsigned char *)malloc(length_of_file + 4);
  fread(buf, length_of_file, 1, fp);
  fclose(fp);

  while (count<1000)//图像解码1000次
   {
     if (tinyjpeg_parse_header(jdec, buf, length_of_file)<0)
       exitmessage(tinyjpeg_get_errorstring(jdec));

     tinyjpeg_decode(jdec, output_format);

     count++;
   }

  /* 
   * Get address for each plane (not only max 3 planes is supported), and
   * depending of the output mode, only some components will be filled 
   * RGB: 1 plane, YUV420P: 3 planes, GREY: 1 plane
   */
  tinyjpeg_get_components(jdec, components);
  tinyjpeg_get_size(jdec, &width, &height);

  /* Save it */
  switch (output_format)
   {
    case TINYJPEG_FMT_RGB24:
    case TINYJPEG_FMT_BGR24:
      write_tga(outfilename, output_format, width, height, components);
      break;
    case TINYJPEG_FMT_YUV420P:
      write_yuv(outfilename, width, height, components);
      break;
    case TINYJPEG_FMT_GREY:
      write_pgm(outfilename, width, height, components);
      break;
   }

  free(buf);
  tinyjpeg_free(jdec);
  return 0;
}

convert_one_image

int convert_one_image(const char *infilename, const char *outfilename, int output_format)
{
  FILE *fp;
  unsigned int length_of_file;//定义输入文件长度
  unsigned int width, height;//定义宽和高
  unsigned char *buf;//定义一个buf的缓冲区
  struct jdec_private *jdec;//定义一个jdec_private类型的jdec
  unsigned char *components[3];//定义指针数组

  /* Load the Jpeg into memory */
  //打开文件之后把图片的基本信息写入
  fp = fopen(infilename, "rb");
  if (fp == NULL)
    exitmessage("Cannot open filename\n");
  length_of_file = filesize(fp);
  buf = (unsigned char *)malloc(length_of_file + 4);
  if (buf == NULL)
    exitmessage("Not enough memory for loading file\n");
  fread(buf, length_of_file, 1, fp);
  fclose(fp);

  /* Decompress it */
  jdec = tinyjpeg_init();//初始化
  if (jdec == NULL)
    exitmessage("Not enough memory to alloc the structure need for decompressing\n");

  if (tinyjpeg_parse_header(jdec, buf, length_of_file)<0)//解压缩JPEG图像的头信息
    exitmessage(tinyjpeg_get_errorstring(jdec));

  /* Get the size of the image */
  tinyjpeg_get_size(jdec, &width, &height);

  DCT_0_mat = (short int*)malloc(width * height / 64 * sizeof(short int));
  DCT_1_mat = (short int*)malloc(width * height / 64 * sizeof(short int));
  DC_pos = 0;
  AC_pos = 0;


  snprintf(error_string, sizeof(error_string),"Decoding JPEG image...\n");
  if (tinyjpeg_decode(jdec, output_format) < 0)
    exitmessage(tinyjpeg_get_errorstring(jdec));

  /* 
   * Get address for each plane (not only max 3 planes is supported), and
   * depending of the output mode, only some components will be filled 
   * RGB: 1 plane, YUV420P: 3 planes, GREY: 1 plane
   */
  tinyjpeg_get_components(jdec, components);

  /* Save it */  //写输出文件
  switch (output_format)
   {
    case TINYJPEG_FMT_RGB24:
    case TINYJPEG_FMT_BGR24:
      write_tga(outfilename, output_format, width, height, components);
      break;
    case TINYJPEG_FMT_YUV420P:
      write_yuv(outfilename, width, height, components);
      write_DC_AC(width, height);
      break;
    case TINYJPEG_FMT_GREY:
      write_pgm(outfilename, width, height, components);
      break;
   }

  /* Only called this if the buffers were allocated by tinyjpeg_decode() */
  tinyjpeg_free(jdec);
  /* else called just free(jdec); */

  free(buf);
  return 0;
}

tinyjpeg.c

1.4个函数处理数据流
fill_ nbits：数据放入存储库。将任何0xff，0x00转换为0xff。
get_ nbits：从流中读取nbits，并写入结果，nbits从流中移除，寄存器被自动填满。
look_ nbits：从流中读取nbits，而不将其标记为已读。
Skip_nbits：从流中读取nbits，但不返回结果。
流：当前指针在JPEG数据(每字节读取字节)
reservoir：包含信息位的寄存器。只有nbits_ in_ reservoir
有效。

#define fill_nbits(reservoir,nbits_in_reservoir,stream,nbits_wanted) do { \
   while (nbits_in_reservoir<nbits_wanted) \
    { \
      unsigned char c; \
      if (stream >= priv->stream_end) \
        longjmp(priv->jump_state, -EIO); \
      c = *stream++; \
      reservoir <<= 8; \
      if (c == 0xff && *stream == 0x00) \
        stream++; \
      reservoir |= c; \
      nbits_in_reservoir+=8; \
    } \
}  while(0);

/* Signed version !!!! */
#define get_nbits(reservoir,nbits_in_reservoir,stream,nbits_wanted,result) do { \
   fill_nbits(reservoir,nbits_in_reservoir,stream,(nbits_wanted)); \
   result = ((reservoir)>>(nbits_in_reservoir-(nbits_wanted))); \
   nbits_in_reservoir -= (nbits_wanted);  \
   reservoir &= ((1U<<nbits_in_reservoir)-1); \
   if ((unsigned int)result < (1UL<<((nbits_wanted)-1))) \
       result += (0xFFFFFFFFUL<<(nbits_wanted))+1; \
}  while(0);

#define look_nbits(reservoir,nbits_in_reservoir,stream,nbits_wanted,result) do { \
   fill_nbits(reservoir,nbits_in_reservoir,stream,(nbits_wanted)); \
   result = ((reservoir)>>(nbits_in_reservoir-(nbits_wanted))); \
}  while(0);

/* To speed up the decoding, we assume that the reservoir have enough bit 
 * slow version:
 * #define skip_nbits(reservoir,nbits_in_reservoir,stream,nbits_wanted) do { \
 *   fill_nbits(reservoir,nbits_in_reservoir,stream,(nbits_wanted)); \
 *   nbits_in_reservoir -= (nbits_wanted); \
 *   reservoir &= ((1U<<nbits_in_reservoir)-1); \
 * }  while(0);
 */
#define skip_nbits(reservoir,nbits_in_reservoir,stream,nbits_wanted) do { \
   nbits_in_reservoir -= (nbits_wanted); \
   reservoir &= ((1U<<nbits_in_reservoir)-1); \
}  while(0);

2.创建霍夫曼表

/**
 *
 * Decode a single block that contains the DCT coefficients.
 * The table coefficients is already dezigzaged at the end of the operation.
 *
 */
static void process_Huffman_data_unit(struct jdec_private *priv, int component)
{
  unsigned char j;
  unsigned int huff_code;
  unsigned char size_val, count_0;

  struct component *c = &priv->component_infos[component];
  short int DCT[64];


  /* Initialize the DCT coef table */
  memset(DCT, 0, sizeof(DCT));

  /* DC coefficient decoding */
  huff_code = get_next_huffman_code(priv, c->DC_table);
  //trace("+ %x\n", huff_code);
  if (huff_code) {
     get_nbits(priv->reservoir, priv->nbits_in_reservoir, priv->stream, huff_code, DCT[0]);
     DCT[0] += c->previous_DC;
     c->previous_DC = DCT[0];
  } else {
     DCT[0] = c->previous_DC;
  }

  /* AC coefficient decoding */
  j = 1;
  while (j<64)
   {
     huff_code = get_next_huffman_code(priv, c->AC_table);
     //trace("- %x\n", huff_code);

     size_val = huff_code & 0xF;
     count_0 = huff_code >> 4;

     if (size_val == 0)
      { /* RLE */
	if (count_0 == 0)
	  break;	/* EOB found, go out */
	else if (count_0 == 0xF)
	  j += 16;	/* skip 16 zeros */
      }
     else
      {
	j += count_0;	/* skip count_0 zeroes */
	if (__unlikely(j >= 64))
	 {
	   snprintf(error_string, sizeof(error_string), "Bad huffman data (buffer overflow)");
	   break;
	 }
	get_nbits(priv->reservoir, priv->nbits_in_reservoir, priv->stream, size_val, DCT[j]);
	j++;
      }
   }

  for (j = 0; j < 64; j++)
    c->DCT[j] = DCT[zigzag[j]];
}

/*
 * Takes two array of bits, and build the huffman table for size, and code
 * 
 * lookup will return the symbol if the code is less or equal than HUFFMAN_HASH_NBITS.
 * code_size will be used to known how many bits this symbol is encoded.
 * slowtable will be used when the first lookup didn't give the result.
 */
static void build_huffman_table(const unsigned char *bits, const unsigned char *vals, struct huffman_table *table)
{
  unsigned int i, j, code, code_size, val, nbits;
  unsigned char huffsize[HUFFMAN_BITS_SIZE+1], *hz;
  unsigned int huffcode[HUFFMAN_BITS_SIZE+1], *hc;
  int next_free_entry;

  /*
   * Build a temp array 
   *   huffsize[X] => numbers of bits to write vals[X]
   */
  hz = huffsize;
  for (i=1; i<=16; i++)
   {
     for (j=1; j<=bits[i]; j++)
       *hz++ = i;
   }
  *hz = 0;

  memset(table->lookup, 0xff, sizeof(table->lookup));
  for (i=0; i<(16-HUFFMAN_HASH_NBITS); i++)
    table->slowtable[i][0] = 0;

  /* Build a temp array
   *   huffcode[X] => code used to write vals[X]
   */
  code = 0;
  hc = huffcode;
  hz = huffsize;
  nbits = *hz;
  while (*hz)
   {
     while (*hz == nbits)
      {
	*hc++ = code++;
	hz++;
      }
     code <<= 1;
     nbits++;
   }

  /*
   * Build the lookup table, and the slowtable if needed.
   */
  next_free_entry = -1;
  for (i=0; huffsize[i]; i++)
   {
     val = vals[i];
     code = huffcode[i];
     code_size = huffsize[i];
	#if TRACE
     fprintf(p_trace,"val=%2.2x code=%8.8x codesize=%2.2d\n", val, code, code_size);
	 fflush(p_trace);
    #endif
     table->code_size[val] = code_size;
     if (code_size <= HUFFMAN_HASH_NBITS)
      {
	/*
	 * Good: val can be put in the lookup table, so fill all value of this
	 * column with value val 
	 */
	int repeat = 1UL<<(HUFFMAN_HASH_NBITS - code_size);
	code <<= HUFFMAN_HASH_NBITS - code_size;
	while ( repeat-- )
	  table->lookup[code++] = val;

      }
     else
      {
	/* Perhaps sorting the array will be an optimization */
	uint16_t *slowtable = table->slowtable[code_size-HUFFMAN_HASH_NBITS-1];
	while(slowtable[0])
	  slowtable+=2;
	slowtable[0] = code;
	slowtable[1] = val;
	slowtable[2] = 0;
	/* TODO: NEED TO CHECK FOR AN OVERFLOW OF THE TABLE */
      }

   }
}

static void build_default_huffman_tables(struct jdec_private *priv)
{
  if (   (priv->flags & TINYJPEG_FLAGS_MJPEG_TABLE) 
      && priv->default_huffman_table_initialized)
    return;

  build_huffman_table(bits_dc_luminance, val_dc_luminance, &priv->HTDC[0]);
  build_huffman_table(bits_ac_luminance, val_ac_luminance, &priv->HTAC[0]);

  build_huffman_table(bits_dc_chrominance, val_dc_chrominance, &priv->HTDC[1]);
  build_huffman_table(bits_ac_chrominance, val_ac_chrominance, &priv->HTAC[1]);

  priv->default_huffman_table_initialized = 1;
}

3.建立量化表

static void build_quantization_table(float *qtable, const unsigned char *ref_table)
{
  /* Taken from libjpeg. Copyright Independent JPEG Group's LLM idct.
   * For float AA&N IDCT method, divisors are equal to quantization
   * coefficients scaled by scalefactor[row]*scalefactor[col], where
   *   scalefactor[0] = 1
   *   scalefactor[k] = cos(k*PI/16) * sqrt(2)    for k=1..7
   * We apply a further scale factor of 8.
   * What's actually stored is 1/divisor so that the inner loop can
   * use a multiplication rather than a division.
   */
  int i, j;
  static const double aanscalefactor[8] = {
     1.0, 1.387039845, 1.306562965, 1.175875602,
     1.0, 0.785694958, 0.541196100, 0.275899379
  };
  const unsigned char *zz = zigzag;

  for (i=0; i<8; i++) {
     for (j=0; j<8; j++) {
       *qtable++ = ref_table[*zz++] * aanscalefactor[i] * aanscalefactor[j];
     }
   }

}

4.解析DQT

static int parse_DQT(struct jdec_private *priv, const unsigned char *stream)
{
  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);
     stream += 64;
   }
#if TRACE
  fprintf(p_trace,"< DQT marker\n");
  fflush(p_trace);
#endif
  return 0;
}

5.解析SOF

static int parse_SOF(struct jdec_private *priv, const unsigned char *stream)
{
  int i, width, height, nr_components, cid, sampling_factor;
  int Q_table;
  struct component *c;
#if TRACE
  fprintf(p_trace,"> SOF marker\n");
  fflush(p_trace);
#endif
  print_SOF(stream);

  height = be16_to_cpu(stream+3);
  width  = be16_to_cpu(stream+5);
  nr_components = stream[7];
#if SANITY_CHECK
  if (stream[2] != 8)
    snprintf(error_string, sizeof(error_string),"Precision other than 8 is not supported\n");
  if (width>JPEG_MAX_WIDTH || height>JPEG_MAX_HEIGHT)
    snprintf(error_string, sizeof(error_string),"Width and Height (%dx%d) seems suspicious\n", width, height);
  if (nr_components != 3)
    snprintf(error_string, sizeof(error_string),"We only support YUV images\n");
  if (height%16)
    snprintf(error_string, sizeof(error_string),"Height need to be a multiple of 16 (current height is %d)\n", height);
  if (width%16)
    snprintf(error_string, sizeof(error_string),"Width need to be a multiple of 16 (current Width is %d)\n", width);
#endif
  stream += 8;
  for (i=0; i<nr_components; i++) {
     cid = *stream++;
     sampling_factor = *stream++;
     Q_table = *stream++;
     c = &priv->component_infos[i];
#if SANITY_CHECK
     c->cid = cid;
     if (Q_table >= COMPONENTS)
       snprintf(error_string, sizeof(error_string),"Bad Quantization table index (got %d, max allowed %d)\n", Q_table, COMPONENTS-1);
#endif
     c->Vfactor = sampling_factor&0xf;
     c->Hfactor = sampling_factor>>4;
     c->Q_table = priv->Q_tables[Q_table];
#if TRACE
     fprintf(p_trace,"Component:%d  factor:%dx%d  Quantization table:%d\n",
           cid, c->Hfactor, c->Hfactor, Q_table );
	 fflush(p_trace);
#endif

  }
  priv->width = width;
  priv->height = height;
#if TRACE
  fprintf(p_trace,"< SOF marker\n");
  fflush(p_trace);
#endif

  return 0;
}

6.解析SOS

static int parse_SOS(struct jdec_private *priv, const unsigned char *stream)
{
  unsigned int i, cid, table;
  unsigned int nr_components = stream[2];
#if TRACE
  fprintf(p_trace,"> SOS marker\n");
  fflush(p_trace);
#endif

#if SANITY_CHECK
  if (nr_components != 3)
    snprintf(error_string, sizeof(error_string),"We only support YCbCr image\n");
#endif

  stream += 3;
  for (i=0;i<nr_components;i++) {
     cid = *stream++;
     table = *stream++;
#if SANITY_CHECK
     if ((table&0xf)>=4)
	snprintf(error_string, sizeof(error_string),"We do not support more than 2 AC Huffman table\n");
     if ((table>>4)>=4)
	snprintf(error_string, sizeof(error_string),"We do not support more than 2 DC Huffman table\n");
     if (cid != priv->component_infos[i].cid)
        snprintf(error_string, sizeof(error_string),"SOS cid order (%d:%d) isn't compatible with the SOF marker (%d:%d)\n",
	      i, cid, i, priv->component_infos[i].cid);
#if TRACE
     fprintf(p_trace,"ComponentId:%d  tableAC:%d tableDC:%d\n", cid, table&0xf, table>>4);
	 fflush(p_trace);
#endif
#endif
     priv->component_infos[i].AC_table = &priv->HTAC[table&0xf];
     priv->component_infos[i].DC_table = &priv->HTDC[table>>4];
  }
  priv->stream = stream+3;
#if TRACE
  fprintf(p_trace,"< SOS marker\n");
  fflush(p_trace);
#endif
  return 0;
}

7.解析DHT

static int parse_DHT(struct jdec_private *priv, const unsigned char *stream)
{
  unsigned int count, i;
  unsigned char huff_bits[17];
  int length, index;

  length = be16_to_cpu(stream) - 2;
  stream += 2;	/* Skip length */
#if TRACE
  fprintf(p_trace,"> DHT marker (length=%d)\n", length);
  fflush(p_trace);
#endif

  while (length>0) {
     index = *stream++;

     /* We need to calculate the number of bytes 'vals' will takes */
     huff_bits[0] = 0;
     count = 0;
     for (i=1; i<17; i++) {
	huff_bits[i] = *stream++;
	count += huff_bits[i];
     }
#if SANITY_CHECK
     if (count >= HUFFMAN_BITS_SIZE)
       snprintf(error_string, sizeof(error_string),"No more than %d bytes is allowed to describe a huffman table", HUFFMAN_BITS_SIZE);
     if ( (index &0xf) >= HUFFMAN_TABLES)
       snprintf(error_string, sizeof(error_string),"No more than %d Huffman tables is supported (got %d)\n", HUFFMAN_TABLES, index&0xf);
#if TRACE
     fprintf(p_trace,"Huffman table %s[%d] length=%d\n", (index&0xf0)?"AC":"DC", index&0xf, count);
	 fflush(p_trace);
#endif
#endif

     if (index & 0xf0 )
       build_huffman_table(huff_bits, stream, &priv->HTAC[index&0xf]);
     else
       build_huffman_table(huff_bits, stream, &priv->HTDC[index&0xf]);

     length -= 1;
     length -= 16;
     length -= count;
     stream += count;
  }
#if TRACE
  fprintf(p_trace,"< DHT marker\n");
  fflush(p_trace);
#endif
  return 0;
}

8.解析DRI

static int parse_DRI(struct jdec_private *priv, const unsigned char *stream)
{
  unsigned int length;

#if TRACE
  fprintf(p_trace,"> DRI marker\n");
  fflush(p_trace);
#endif

  length = be16_to_cpu(stream);

#if SANITY_CHECK
  if (length != 4)
    snprintf(error_string, sizeof(error_string),"Length of DRI marker need to be 4\n");
#endif

  priv->restart_interval = be16_to_cpu(stream+2);

#if TRACE
  fprintf(p_trace,"Restart interval = %d\n", priv->restart_interval);
  fprintf(p_trace,"< DRI marker\n");
  fflush(p_trace);
#endif
  return 0;
}

9.parse_JFIF
完成标记解析

static int parse_JFIF(struct jdec_private *priv, const unsigned char *stream)
{
  int chuck_len;
  int marker;
  int sos_marker_found = 0;
  int dht_marker_found = 0;
  const unsigned char *next_chunck;

  /* Parse marker */
  while (!sos_marker_found)
   {
     if (*stream++ != 0xff)
       goto bogus_jpeg_format;
     /* Skip any padding ff byte (this is normal) */
     while (*stream == 0xff)
       stream++;

     marker = *stream++;
     chuck_len = be16_to_cpu(stream);
     next_chunck = stream + chuck_len;
     switch (marker)
      {
       case SOF:
	 if (parse_SOF(priv, stream) < 0)
	   return -1;
	 break;
       case DQT:
	 if (parse_DQT(priv, stream) < 0)
	   return -1;
	 break;
       case SOS:
	 if (parse_SOS(priv, stream) < 0)
	   return -1;
	 sos_marker_found = 1;
	 break;
       case DHT:
	 if (parse_DHT(priv, stream) < 0)
	   return -1;
	 dht_marker_found = 1;
	 break;
       case DRI:
	 if (parse_DRI(priv, stream) < 0)
	   return -1;
	 break;
       default:
#if TRACE
	fprintf(p_trace,"> Unknown marker %2.2x\n", marker);
	fflush(p_trace);
#endif
	 break;
      }

     stream = next_chunck;
   }

  if (!dht_marker_found) {
#if TRACE
	  fprintf(p_trace,"No Huffman table loaded, using the default one\n");
	  fflush(p_trace);
#endif
    build_default_huffman_tables(priv);
  }

#ifdef SANITY_CHECK
  if (   (priv->component_infos[cY].Hfactor < priv->component_infos[cCb].Hfactor)
      || (priv->component_infos[cY].Hfactor < priv->component_infos[cCr].Hfactor))
    snprintf(error_string, sizeof(error_string),"Horizontal sampling factor for Y should be greater than horitontal sampling factor for Cb or Cr\n");
  if (   (priv->component_infos[cY].Vfactor < priv->component_infos[cCb].Vfactor)
      || (priv->component_infos[cY].Vfactor < priv->component_infos[cCr].Vfactor))
    snprintf(error_string, sizeof(error_string),"Vertical sampling factor for Y should be greater than vertical sampling factor for Cb or Cr\n");
  if (   (priv->component_infos[cCb].Hfactor!=1) 
      || (priv->component_infos[cCr].Hfactor!=1)
      || (priv->component_infos[cCb].Vfactor!=1)
      || (priv->component_infos[cCr].Vfactor!=1))
    snprintf(error_string, sizeof(error_string),"Sampling other than 1x1 for Cr and Cb is not supported");
#endif

  return 0;
bogus_jpeg_format:
#if TRACE
  fprintf(p_trace,"Bogus jpeg format\n");
  fflush(p_trace);
#endif
  return -1;
}

10.解析文件头
初始化准备

int tinyjpeg_parse_header(struct jdec_private *priv, const unsigned char *buf, unsigned int size)
{
  int ret;

  /* Identify the file */
  if ((buf[0] != 0xFF) || (buf[1] != SOI))
    snprintf(error_string, sizeof(error_string),"Not a JPG file ?\n");

  priv->stream_begin = buf+2;
  priv->stream_length = size-2;
  priv->stream_end = priv->stream_begin + priv->stream_length;

  ret = parse_JFIF(priv, priv->stream_begin);

  return ret;
}

11.指向宏块解码

typedef void (*decode_MCU_fct) (struct jdec_private *priv);

static const decode_MCU_fct decode_mcu_3comp_table[4] = {
   decode_MCU_1x1_3planes,
   decode_MCU_1x2_3planes,
   decode_MCU_2x1_3planes,
   decode_MCU_2x2_3planes,
};

static const decode_MCU_fct decode_mcu_1comp_table[4] = {
   decode_MCU_1x1_1plane,
   decode_MCU_1x2_1plane,
   decode_MCU_2x1_1plane,
   decode_MCU_2x2_1plane,
};

12.指向彩色空间转换

typedef void (*convert_colorspace_fct) (struct jdec_private *priv);

static const convert_colorspace_fct convert_colorspace_yuv420p[4] = {
   YCrCB_to_YUV420P_1x1,
   YCrCB_to_YUV420P_1x2,
   YCrCB_to_YUV420P_2x1,
   YCrCB_to_YUV420P_2x2,
};

static const convert_colorspace_fct convert_colorspace_rgb24[4] = {
   YCrCB_to_RGB24_1x1,
   YCrCB_to_RGB24_1x2,
   YCrCB_to_RGB24_2x1,
   YCrCB_to_RGB24_2x2,
};

static const convert_colorspace_fct convert_colorspace_bgr24[4] = {
   YCrCB_to_BGR24_1x1,
   YCrCB_to_BGR24_1x2,
   YCrCB_to_BGR24_2x1,
   YCrCB_to_BGR24_2x2,
};

static const convert_colorspace_fct convert_colorspace_grey[4] = {
   YCrCB_to_Grey_1x1,
   YCrCB_to_Grey_1x2,
   YCrCB_to_Grey_2x1,
   YCrCB_to_Grey_2x2,
};

理解三个结构体的设计目的

• struct huffman_table
快速查找表

struct huffman_table
{
  /* Fast look up table, using HUFFMAN_HASH_NBITS bits we can have directly the symbol,
   * if the symbol is <0, then we need to look into the tree table */
  short int lookup[HUFFMAN_HASH_SIZE];
  /* code size: give the number of bits of a symbol is encoded */
  unsigned char code_size[HUFFMAN_HASH_SIZE];
  /* some place to store value that is not encoded in the lookup table 
   * FIXME: Calculate if 256 value is enough to store all values
   */
  uint16_t slowtable[16-HUFFMAN_HASH_NBITS][256];
};

• struct component
保存宏块的信息

struct component 
{
  unsigned int Hfactor;
  unsigned int Vfactor;
  float *Q_table;		/* Pointer to the quantisation table to use */
  struct huffman_table *AC_table;
  struct huffman_table *DC_table;
  short int previous_DC;	/* Previous DC coefficient */
  short int DCT[64];		/* DCT coef */
#if SANITY_CHECK
  unsigned int cid;
#endif
};

• struct jdec_private
保存图片各类信息

struct jdec_private
{
  /* Public variables */
  uint8_t *components[COMPONENTS];
  unsigned int width, height;	/* Size of the image */
  unsigned int flags;

  /* Private variables */
  const unsigned char *stream_begin, *stream_end;
  unsigned int stream_length;

  const unsigned char *stream;	/* Pointer to the current stream */
  unsigned int reservoir, nbits_in_reservoir;

  struct component component_infos[COMPONENTS];
  float Q_tables[COMPONENTS][64];		/* quantization tables */
  struct huffman_table HTDC[HUFFMAN_TABLES];	/* DC huffman tables   */
  struct huffman_table HTAC[HUFFMAN_TABLES];	/* AC huffman tables   */
  int default_huffman_table_initialized;
  int restart_interval;
  int restarts_to_go;				/* MCUs left in this restart interval */
  int last_rst_marker_seen;			/* Rst marker is incremented each time */

  /* Temp space used after the IDCT to store each components */
  uint8_t Y[64*4], Cr[64], Cb[64];

  jmp_buf jump_state;
  /* Internal Pointer use for colorspace conversion, do not modify it !!! */
  uint8_t *plane[COMPONENTS];

};

理解在视音频编解码调试中TRACE的目的和含义

trace：输出中间过程中的某些变量，或者错误信息。
将TRACE的值置为1，打开，进行上述说明的信息的输出
将TRACE的值置为0，关闭，跳过。

#if TRACE
	//添加希望看到的信息代码
#endif

3．以txt文件输出所有的量化矩阵和所有的HUFFMAN码表。

4． 输出DC图像并统计其概率分布。

5． 输出某一个AC值图像并统计其概率分布。