数据压缩_实验五_JPEG解码

2020/5/31 15:44:03 人评论 次浏览 分类:学习教程

文章目录

  • JPEG解码实验
    • 一、实验原理(JPEG文件格式)
      • JPEG编码原理
      • 1、Segment的组织形式
      • 2、DQT(定义量化表)
      • 3、SOF0
      • 4、DHT(定义Huffman树表)
      • 5、SOS
    • 二、实验步骤
      • 1、程序设计整体框架
      • 2、结构体设计
        • (1)struct huffman_table
        • (2)struct component
        • (3)struct jdec_private
      • 3、TRACE的目的和含义
      • 4、关键位置代码
        • (1)解析DQT
        • (2)解析SOF0
        • (3)解析DHT
        • (4)解析SOS
        • (5)依据每个分量的水平、垂直采样因子计算MCU的大小,并得到每个MCU中8x8宏块的个数
        • (6)对每个MCU解码
          • 基本框架
          • 进行Huffman解码
      • 5、将输出文件保存为YUV文件
      • 6、输出量化矩阵和HUFFMAN码表
        • 输出量化矩阵
        • 输出Huffman码表
      • 7、输出DC图像并统计概率分布
      • 8、输出某一个AC值图像并统计其概率分布

JPEG解码实验

一、实验原理(JPEG文件格式)

JPEG编码原理

截屏2020-05-31下午3.24.33

1、Segment的组织形式

JPEG 在文件中以 Segment 的形式组织,它具有以下特点:

  • 均以 0xFF 开始,后跟 1 byte的Marker和2byte的Segment length(包含表示Length本身所占用的2byte,不含“0xFF” + “Marker” 所占用的2byte)
  • 采用 Motorola 序(相对于Intel 序),即保存时高位在前,低位在后
  • Data 部分中,0xFF 后若为 0x00,则跳过此字节不予处理

2、DQT(定义量化表)

Define Quantization Table,定义量化表。

截屏2020-05-30下午5.44.50
  • 量化表标记代码:0xFFDB
  • 量化表长度:0x43,指包含Segment length的2byte在内共有67byte
  • 精度及量化表ID:低位0为量化表的ID索引值,其取值范围为 0~3,所以说最多可能有四张量化表(亮度量化表,色度量化表,可能会有 R、G、B 各一张量化表)。高位0为量化精度,有两个可选值,0:8 位;1:16 位,这里是 0,代表量化精度为 8bit,即用一个字节来表示一个量化系数。
  • 数据:64字节的量化系数,(一个字节对应一个量化系数,对于 8x8 的宏块来说,共 8x8=64 个量化系数)

JPEG标准中采用线性均匀量化器,对64个DCT系数除以量化步长并四舍五入取整,量化步长由量化表决定。量化表为8x8矩阵,与DCT变换系数一一对应,亮度分量和彩色分量使用不同的量化表。

量化后系数的读取顺序按Z字形排列。

3、SOF0

  • 得到每个sample的比特数、长宽、颜色分量数
  • 得到每个颜色分量的ID、水平采样因子、垂直采样因子、使用的量化表序号(与DQT中序号对应)
截屏2020-05-30下午7.20.56
  • 数据长度:长度本身占 2byte,length=17
  • 图像精度 :1byte, 这里是 08,即精度为一个字节。
  • 图像高度:2 byte, 以像素为单位,lines=1024。
  • 图像宽度:2 byte, 以像素为单位,samples_per_lines=1024。
  • 颜色分量数:一个字节,这里是 03,代表有三个分量,YCrCb。
  • 颜色分量信息:每个分量有三个字节,第一个为分量的 ID,01:Y 、02:U、 03:V;第二个字节高位为水平采样因子,低位为垂直采样因子,这里三个分量的采样率相同,所以采样格式为 4:4:4;第三个字节代表这个分量对应的量化表 ID,可以看出,Y 对应的量化表 ID 索引值为 00,而 UV 对应的量化表 ID都为 01,即它们共用一张量化表。所以,除了标记外,总共的长度就为 2+1+5+3*3=17

4、DHT(定义Huffman树表)

截屏2020-05-30下午8.52.38
  • 数据长度:2 byte,这里是0x1D,length=29

  • Huffman表ID号和类型:1 byte,高 4 位为表的类型,0:DC 直流;1:AC交流。

    0x00表示DC直流0号表;
    0x01表示DC直流1号表;
    0x10表示AC交流0号表;
    0x11表示AC交流1号表

  • 不同长度 Huffman 的码字数量:固定为 16 个字节,每个字节代表从长度为 1到长度为16的码字的个数。

  • 每个符号码字对应的权值:由前一部分数据知道,此哈夫曼树有0+3+1+1+1+1+1+1+1=10个叶子结点,即本字段应该有10个字节。这段数据表示10个叶子结点按从小到大排列,其权值依次为04、 05、 06、 03、 02、 01、 00、09、 07、 08 。

    对于直流系数,权值就是解码时再需要读入的bit位数。这个再次读入的位数通过查表得到真正的码值。注意:直流系数是差分编码的!

    对于交流系数,权值的高4位表示当前数值前面有多少个连续的零,低4位表示该交流分量数值的二进制位数,也就是接下来需要读入的位数。

5、SOS

得到解析每个颜色分量的DC、AC值所使用的Huffman表序号(与DHT中序号对应)

截屏2020-05-31上午12.33.59
  • 数据长度:2 byte,这里是0x0C,length=12
  • 颜色分量数:1 byte,0x03三个分量,与SOF中的颜色分量相同
  • 颜色分量信息:每个分量对应 3 个字节,第一个字节是颜色分量 ID,1,2,3对应 YUV,第二个字节高位为直流分量使用的哈夫曼树编号,这里 Y 的直流分量用的是 DC 的第 0 张表,低四位代表交流分量使用的哈夫曼树编号,这里 Y的交流分量用的是 AC 的第 0 张表,而两个色度信号的直流分量都用的是 DC的第 1 张表,交流分量用的是 AC 的第 1 张表.

二、实验步骤

1、程序设计整体框架

jpeg解码

2、结构体设计

本实验设计三个关键结构体,具体分析如下。

(1)struct huffman_table

快速查找,加快程序运行速度。

// 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.
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];
};

lookup[]为快速查找表,用来加快查表速度。如果失败,则使用slowtable[]做慢速查找。

codesize为码字长度。

(2)struct component

存储一个8x8宏块的Y/U/V分量。

struct component 
{
  unsigned int Hfactor;		/* 水平方向采样信息 */
  unsigned int Vfactor;		/* 垂直方向采样信息 */
  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 前一个DC系数*/
  short int DCT[64];		/* DCT coef 8x8宏块内的DCT系数*/
#if SANITY_CHECK
  unsigned int cid;
#endif
};

假设水平采样因子:垂直采样因子= 2 :2,水平方向每两个分量采样一次,垂直方向每两个分量采样一次。

(3)struct jdec_private

存储整个jpeg文件信息,其中嵌套了以上两种结构体。

struct jdec_private
{
  /* Public variables */
  uint8_t *components[COMPONENTS];	/* #define COMPONENTS	3 */
  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];

};

3、TRACE的目的和含义

提供一组方法和属性,帮助跟踪代码的执行。可以使用Trace类中的属性和方法来检测发布版本。 借助检测功能,可以监视应用程序在真实设置中运行的运行状况。

#define TRACE 1//add by nxn

利用宏定义,当TRACE的值为1时,TRACE开启。当TRACE的值为0时,TRACE关闭。

具体使用例子如下:

#if TRACE
  p_trace=fopen(TRACEFILE,"w");
  if (p_trace==NULL)
  {
	  printf("trace file open error!");
  }
#endif

TRACE=1时,执行内部语句,打开TRACEFILE。

程序执行完毕后,打开trace_jpeg.txt即可查看追踪内容。
trace_jpeg.txt

4、关键位置代码

(1)解析DQT

/* 解析量化表 */
/* 得到量化表长度(可能含多张量化表) */
/* 得到量化表精度 */
/* 得到及检查量化表的序号(只能是0-3) */
/* 得到量化表的内容(8x8=64) */
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); /* 指向量化表结尾 */
  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];    /* table指向第qi个量化表 */
     build_quantization_table(table, stream);   /* 创建量化表 */
     stream += 64;
   }
#if TRACE
  fprintf(p_trace,"< DQT marker\n");
  fflush(p_trace);
#endif
  return 0;
}

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++) {
       // 量化后系数的读取顺序按Z字形排列
       *qtable++ = ref_table[*zz++] * aanscalefactor[i] * aanscalefactor[j];
     }
   }

(2)解析SOF0

/* 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);   // 获取当前图片的高度
  width  = be16_to_cpu(stream+5);   // 获取当前图片的宽度
  nr_components = stream[7];    //  颜色分量数
#if SANITY_CHECK    //  检验图像是否符合要求
  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;  // 将指针移至颜色分量信息处
  for (i=0; i<nr_components; i++) {
     cid = *stream++;   // 分量ID
     sampling_factor = *stream++;   // 水平竖直采样因子
     Q_table = *stream++;   // 量化表序号
     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;  // 取低4位赋值
     c->Hfactor = sampling_factor>>4;   // 取高8位赋值
     c->Q_table = priv->Q_tables[Q_table];  //  量化表序号赋值
#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;  // 赋宽度
  priv->height = height;    // 赋高度
#if TRACE
  fprintf(p_trace,"< SOF marker\n");
  fflush(p_trace);
#endif

  return 0;
}

(3)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; // 读入数据长度
  stream += 2;	/* Skip length */
#if TRACE
  fprintf(p_trace,"> DHT marker (length=%d)\n", length);
  fflush(p_trace);
#endif

  while (length>0) {	// 是否还有表
     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++) { /* 计算Huffman树共有多少叶子节点 */
	    huff_bits[i] = *stream++;
	    count += huff_bits[i];
     }
#if SANITY_CHECK    /* 检查数据格式 */
     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 ) // 判断建立AC表orDC表
       /* HTAC[index&0xf]确定为第几张表 */
       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;
}
/*
 * 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;   // 按码字顺序保存每个码字的长度
   }
  *hz = 0;

  /* 将某一块内存中的内容全部设置为0xff */
  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;    // Huffcode数组里按码字顺序保存每个码字
  hz = huffsize;    // huffsize数组中按码字顺序保存每个码字的长度
  nbits = *hz;      // 码字长度
  while (*hz)
   {
      /* 码长相同+1 码长增加码字左移一位 */
     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]; // 权值 
     code = huffcode[i];    // 码长
     code_size = huffsize[i];   // 码字长度
	#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;	// 建立权值和码长的关系
     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;	// 建立查找表和权值的关系
      }
     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 */
      }
   }
}

(4)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];   // 得到颜色分量数
#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++) {   // 依次处理三个component
     cid = *stream++;   // 得到ID
     table = *stream++;     // Huffman表序号
#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
     /* 指向所用的Huffman表 */
     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;
}

(5)依据每个分量的水平、垂直采样因子计算MCU的大小,并得到每个MCU中8x8宏块的个数

  xstride_by_mcu = ystride_by_mcu = 8;  //  初始化为444情况
  /* 水平垂直采样因子都为1 */
  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
     fprintf(p_trace,"Use decode 1x1 sampling\n");
	 fflush(p_trace);
#endif
  } else if (priv->component_infos[cY].Hfactor == 1) {  /* 水平方向采样因子为1 */
     decode_MCU = decode_mcu_table[1];
     convert_to_pixfmt = colorspace_array_conv[1];
     ystride_by_mcu = 16;
#if TRACE
     fprintf(p_trace,"Use decode 1x2 sampling (not supported)\n");
	 fflush(p_trace);
#endif
  } else if (priv->component_infos[cY].Vfactor == 1) {  /* 垂直方向采样因子为1 */
     decode_MCU = decode_mcu_table[2];
     convert_to_pixfmt = colorspace_array_conv[2];
     xstride_by_mcu = 16;   
#if TRACE 
	 fprintf(p_trace,"Use decode 2x1 sampling\n");
	 fflush(p_trace);
#endif
  } else {   /* 水平垂直采样因子都为2 */
     decode_MCU = decode_mcu_table[3];
     convert_to_pixfmt = colorspace_array_conv[3];
     xstride_by_mcu = 16;
     ystride_by_mcu = 16;
#if TRACE
     fprintf(p_trace,"Use decode 2x2 sampling\n");
	 fflush(p_trace);
#endif
  }

(6)对每个MCU解码

  1. 对每个宏块进行Huffman解码,得到DCT系数
  2. 对每个宏块的 DCT 系数进行 IDCT,得到 Y、Cb、Cr
  3. 遇到 Segment Marker RST 时,清空之前的 DC DCT 系数
基本框架
/*
 * Decode a 1x1 directly in 1 color
 */
static void decode_MCU_1x1_1plane(struct jdec_private *priv)
{
  // Y
  process_Huffman_data_unit(priv, cY);  // 以8x8宏块为单位进行Huffman解码
  IDCT(&priv->component_infos[cY], priv->Y, 8); // 对得到的DCT系数进行IDCT
  
  // Cb
  process_Huffman_data_unit(priv, cCb);
  IDCT(&priv->component_infos[cCb], priv->Cb, 8);

  // Cr
  process_Huffman_data_unit(priv, cCr);
  IDCT(&priv->component_infos[cCr], priv->Cr, 8);
}

若MCU为其他格式,则:

1x2 2x1

  // Y
  process_Huffman_data_unit(priv, cY);
  IDCT(&priv->component_infos[cY], priv->Y, 16);
  process_Huffman_data_unit(priv, cY);
  IDCT(&priv->component_infos[cY], priv->Y+8, 16);

2x2

  // Y
  process_Huffman_data_unit(priv, cY);
  IDCT(&priv->component_infos[cY], priv->Y, 16);
  process_Huffman_data_unit(priv, cY);
  IDCT(&priv->component_infos[cY], priv->Y+8, 16);
进行Huffman解码
/**
 *
 * 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];    // 存放DCT系数


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

  /* DC coefficient decoding */
  huff_code = get_next_huffman_code(priv, c->DC_table); // 找到下一个完整Huffman码字
  //trace("+ %x\n", huff_code);
  if (huff_code) { 
     get_nbits(priv->reservoir, priv->nbits_in_reservoir, priv->stream, huff_code, DCT[0]); //查表的DC DCT系数
     DCT[0] += c->previous_DC;  // DC系数采用差分编码,恢复原值
     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);  // 找到下一个完整Huffman码字
     //trace("- %x\n", huff_code);

     size_val = huff_code & 0xF;    // 取出低8位:权值
     count_0 = huff_code >> 4;      // 取出高8位:数值前有多少个连0

     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;
	     }
        // 译码
	    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]]; // z字形扫描顺序转换为矩阵顺序
}

5、将输出文件保存为YUV文件

补充后代码如下:

static void write_yuv(const char *filename, int width, int height, unsigned char **components)
{
  FILE *F;
  char temp[1024];
 	/* 将YUV写入同一个文件 */
  FILE* output;
  output = fopen("output.yuv", "wb");
  fwrite(components[0], width, height, output);
  fwrite(components[1], width * height / 4, 1, output);
  fwrite(components[2], width * height / 4, 1, output);
  fclose(output);

  snprintf(temp, 1024, "%s.Y", filename);
  F = fopen(temp, "wb");
  fwrite(components[0], width, height, F);
  fclose(F);
  snprintf(temp, 1024, "%s.U", filename);
  F = fopen(temp, "wb");
  fwrite(components[1], width*height/4, 1, F);
  fclose(F);
  snprintf(temp, 1024, "%s.V", filename);
  F = fopen(temp, "wb");
  fwrite(components[2], width*height/4, 1, F);
  fclose(F);
}

使用YUVViewer观看到的YUV文件(与原图一致)

截屏2020-05-31下午1.29.06

6、输出量化矩阵和HUFFMAN码表

以txt文件形式输出。

输出量化矩阵

补充后代码如下:

static void build_quantization_table(float *qtable, const unsigned char *ref_table)
{
  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++) {
       fprintf(DQT_File, "%d\t", ref_table[*zz]);
       fflush(DQT_File);
       *qtable++ = ref_table[*zz++] * aanscalefactor[i] * aanscalefactor[j];
     }
     fprintf(DQT_File, "\n");
     fflush(DQT_File);
   }
}

结果:

截屏2020-05-31下午1.15.52

输出Huffman码表

补充后代码如下

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; // 读入数据长度
  stream += 2;	/* Skip length */
#if TRACE
  fprintf(p_trace,"> DHT marker (length=%d)\n", length);
  fflush(p_trace);
#endif
  fprintf(DHT_File, "> DHT marker (length=%d)\n", length);
  fflush(DHT_File);

  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++) { /* 计算Huffman树共有多少叶子节点 */
	    huff_bits[i] = *stream++;
	    count += huff_bits[i];
     }
#if SANITY_CHECK    /* 检查数据格式 */
     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

     fprintf(DHT_File, "Huffman table %s[%d] length=%d\n", (index & 0xf0) ? "AC" : "DC", index & 0xf, count);
     fflush(DHT_File);

     if (index & 0xf0 ) // 判断建立AC表orDC表
       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;
  }
  fprintf(DHT_File, "< DHT marker\n");
  fflush(DHT_File);
#if TRACE
  fprintf(p_trace,"< DHT marker\n");
  fflush(p_trace);
#endif
  return 0;
}
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;   // 按码字顺序保存每个码字的长度
   }
  *hz = 0;

  /* 将某一块内存中的内容全部设置为0xff */
  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;    // Huffcode数组里按码字顺序保存每个码字
  hz = huffsize;    // huffsize数组中按码字顺序保存每个码字的长度
  nbits = *hz;      // 码字长度
  while (*hz)
   {
      /* 码长相同+1 码长增加码字左移一位 */
     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]; // 权值 
     code = huffcode[i];    // 码长
     code_size = huffsize[i];   // 码字长度
	#if TRACE
     fprintf(p_trace,"val=%2.2x code=%8.8x codesize=%2.2d\n", val, code, code_size);
	 fflush(p_trace);
    #endif
     fprintf(DHT_File, "val=%2.2x code=%8.8x codesize=%2.2d\n", val, code, code_size);
     fflush(DHT_File);
     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 */
      }

   }
}

结果:

截屏2020-05-31下午1.14.27

7、输出DC图像并统计概率分布

由于MCU为8x8宏块,故DC值的个数为原图像大小的1/64。

在 convert_one_image() 函数中补充:

  /* 输出DC AC图像 */
  /* 写入图像的UV分量 全部置为128 */
  unsigned char temp = 128;
  for (int i = 0; i < width*height/32; i++) {
      fwrite(&temp, 1, 1, DC_File);
      fwrite(&temp, 1, 1, AC_File);
  }

在 tinyjpeg_decode() 函数中的循环中添加:

/* 输出DC AC图像 */
unsigned char DC_Value = 0;
unsigned char AC_Value = 0;
/* 读出DC AC系数并映射至0~255范围 */
DC_Value = (unsigned char)((priv->component_infos->DCT[0] + 512.0) / 4 + 0.5);
AC_Value = (unsigned char)(priv->component_infos->DCT[1] + 128);
fwrite(&DC_Value, 1, 1, DC_File);
fwrite(&AC_Value, 1, 1, AC_File);
/* 计算概率分布 */
count_DC[DC_Value]++;
count_AC[AC_Value]++;

在 tinyjpeg_decode() 函数末尾添加:

  /* 计算频率分布 */
  for (int i = 0; i < 256; i++) {
      Freq_DC[i] = count_DC[i] / (priv->width * priv->height / 64);
      Freq_AC[i] = count_AC[i] / (priv->width * priv->height / 64);
  }
  /* 输出至文件 */
  fprintf(freq_DC_File, "symbol\tfreq\n");
  fprintf(freq_AC_File, "symbol\tfreq\n");
  for (int i = 0; i < 256; i++) {
      fprintf(freq_DC_File, "%d\t%lf\n", i, Freq_DC[i]);
      fprintf(freq_AC_File, "%d\t%lf\n", i, Freq_AC[i]);
  }

结果

  • DC值图像
截屏2020-05-31下午3.09.37
  • DC值概率分布
截屏2020-05-31下午3.11.05 image-20200531151335869

8、输出某一个AC值图像并统计其概率分布

修改代码见上文

  • AC值图像
截屏2020-05-31下午3.15.41
  • AC值概率分布
截屏2020-05-31下午3.16.45 image-20200531151848339

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