JPEG编解码原理及解码器的调试
一、实验目的
掌握JPEG编解码系统的基本原理。初步掌握复杂的数据压缩算法实现，并能根据理论分析需要实现所对应数据的输出。
二、JPEG编解码原理
1、编码原理
（1）0偏置电平下移
例如从（0，255）变为（-128，127）,对于灰度级为2^n 的像素，通过减去2^n-1无符号整数变为有符号数，将绝对值大的数出现的概率减小，提高编码效率。
（2）8*8DCT变换
将最小编码单元MCU定为8X8的块，每一个单独的分量图像（Y、U、V）据此划分，每一块进行两维离散余弦DCT变换，这是为了使得能量集中，去冗余和去相关性，提高编码效率。**
（3）量化
采用中平型均匀量化器,根据人眼视觉的敏感特性，分为亮度量化值和色差量化值两种量化表，对低频部分细量化，对高频部分粗量化，该过程会产生一定的误差。
给出建议量化表： 真正量化表=缩放因子*基本量化表
（4）DC直流系数差分编码
DCT变换后，每一块的能量集中导致DC系数非常大；且相邻图像块的DC系数变化不大，即存在冗余。
因此使用差分脉冲调制编码DPCM技术，对相邻图像块之间量化DC系数的差值DIFF编码：
（5）AC交流系数Z字扫描与RLE
DCT变换后系数集中在左上角的低频分量区，因此采用Z字形按频率的高低读出，可使用游程编码RLE，都是零的最后可以给出EOB。
（6）Huffman编码
- 对DIFF用Huffman编码：
分成类别，类别ID为一元码编码；类内索引采用定长码 - AC系数游程编码（run,level)联合用Huffman编码：
类内索引采用定长码
2、解码原理
即编码的逆过程:
- 解码Huffman编码
- 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）
三、JPEG文件格式
SOI，Start of Image，图像开始
APP0，Application，应用程序保留标记0
DQT，Define Quantization Table，定义量化表
SOF0，Start of Frame，帧图像开始
DHT，Define Huffman Table，定义哈夫曼表
SOS，Start of Scan，扫描开始 12字节
EOI，End of Image，图像结束 2字节
1、segment的形式组织
JPEG文件以segment的形式组织，其中每个segment以一个marker开始，而每个marker均以0xFF和一个marker的标识符开始，随后为2字节的marker长度（不包含marker的起始两字节）和对应的payload（SOI和EOI marker只有2字节的标识符）。连续的0xFF字节并不是marker的起始标志，而是用来填充的特殊字符。
2、格式举例分析
test.jpg 1024*1024
使用FlexHEX进行解析：
1、SOI&EOI
- SOI ,Start of Image， 图像开始
标记代码 2字节 固定值0xFFD8 - EOI，End of Image， 图像结束 2字节
标记代码 2字节 固定值0xFFD9
2、APP0
标记代码 2字节 固定值0xFFE0
包含9个具体字段：
① 数据长度 2字节 ①~⑨9个字段的总长度
② 标识符 5字节 固定值0x4A46494600，即字符串“JFIF0”
③ 版本号 2字节 一般是0x0102，表示JFIF的版本号1.2
④ X和Y的密度单位 1字节 只有三个值可选
0：无单位；1：点数/英寸；2：点数/厘米
⑤ X方向像素密度 2字节 取值范围未知
⑥ Y方向像素密度 2字节 取值范围未知
⑦ 缩略图水平像素数目 1字节 取值范围未知
⑧ 缩略图垂直像素数目 1字节 取值范围未知
⑨ 缩略图RGB位图 长度可能是3的倍数 缩略图RGB位图数据
“00 10”数据总长度16字节； “4A 46 49 46 00"标识符对应字符串“JFIF0”；版本号0x0101；”00“ X和Y的密度单位为0；”00 01 00 01“X和Y方向像素密度；”00 00“缩略图水平和垂直像素数目为0
3、DQT
标记代码 2字节 固定值0xFFDB
包含9个具体字段：
① 数据长度 2字节 字段①和多个字段②的总长度
② 量化表 数据长度-2字节
a) 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
b) 表项 (64×(精度+1))字节
例如8位精度的量化表,其表项长度为64×（0+1）=64字节
本标记段中，字段②可以重复出现，表示多个量化表，但最多只能出现4次
以”FF DB“为标记的量化表有两个，分别长度为67字节，量化精度为8bit，量化表ID为0和1，表项长度都为64字节。
4、SOF0
标记代码 2字节 固定值0xFFC0
包含9个具体字段：
① 数据长度 2字节 ①~⑥六个字段的总长度
② 精度 1字节 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
③ 图像高度 2字节 图像高度（单位：像素）
④ 图像宽度 2字节 图像宽度（单位：像素）
⑤ 颜色分量数 1字节 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
⑥颜色分量信息 颜色分量数×3字节（通常为9字节）
a)颜色分量ID 1字节
b)水平/垂直采样因子 1字节
高4位：水平采样因子
低4位：垂直采样因子
c) 量化表 1字节
当前分量使用的量化表的ID
"00 11"表示总长度11字节； "08"每个数据样本为8位；"04 00 04 00"图像高宽为1024*1024；“03”颜色分量数YCrCb或YIQ；分量Y使用的量化表ID为0，CbCr使用的量化表ID为1。
5、DHT
标记代码 2字节 固定值0xFFC4
包含2个具体字段：
① 数据长度 2字节
② huffman表 数据长度-2字节
- 表ID和表类型 1字节
高4位：类型，只有两个值可选
0：DC直流；1：AC交流
低4位：哈夫曼表ID，
注意，DC表和AC表分开编码 - 不同位数的码字数量 16字节
- 编码内容 16个不同位数的码字数量之和（字节）
本标记段中，字段②可以重复出现（一般4次），也可以只出现1次。
4张Huffman表的长度分别为29字节、62字节、30字节、47字节；表类型和ID分别为“00”DC表0、 “10”AC表0、“01”DC表1、“11”AC表1；”00 03 01 01 01 01 01 01 01 00…00"表示DC表0中2位的码字3个，3位-9位的码字各1个，其余位数的无码字。"04 05 …07 08"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
6、SOS
标记代码 2字节 固定值0xFFDA
包含2个具体字段：
①数据长度 2字节 ①~④两个字段的总长度
②颜色分量数 1字节 应该和SOF中的字段⑤的值相同，即：
1：灰度图是；3： YCrCb或YIQ；4：CMYK。
③颜色分量信息
a) 颜色分量ID 1字节
b) 直流/交流系数表号 1字节
高4位：直流分量使用的哈夫曼树编号
低4位：交流分量使用的哈夫曼树编号
④ 压缩图像数据
a)谱选择开始 1字节 固定值0x00
b)谱选择结束 1字节 固定值0x3F
c)谱选择 1字节 在基本JPEG中总为00
“00 0C”表示长度为12字节，“03”颜色分量数YCrCb或YIQ；“00” 、“11”、“11”三分量的直流和交流分别对应哈夫曼树编号。
四、JPEG解码流程
int convert_one_image(const char *infilename, const char *outfilename, int output_format)
{
FILE *fp;
unsigned int length_of_file;
unsigned int width, height;
unsigned char *buf;
struct jdec_private *jdec;
unsigned char *components[3];
/* Load the Jpeg into memory */
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();
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)
exitmessage(tinyjpeg_get_errorstring(jdec));
/* Get the size of the image */
tinyjpeg_get_size(jdec, &width, &height);
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 */
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;
}
/* Only called this if the buffers were allocated by tinyjpeg_decode() */
tinyjpeg_free(jdec);
/* else called just free(jdec); */
free(buf);
return 0;
}
解析JPEG文件头:
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;
}
建立huffman表：
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 */
}
}
}
解析marker标识”:
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;
}
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];
}
}
}
解析量化表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;
}
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;
}
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;
}
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;
}
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;
}
解析JPEG实际数据：
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;
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
fprintf(p_trace,"Bad pixel format\n");
fflush(p_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
fprintf(p_trace,"Use decode 1x1 sampling\n");
fflush(p_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
fprintf(p_trace,"Use decode 1x2 sampling (not supported)\n");
fflush(p_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
fprintf(p_trace,"Use decode 2x2 sampling\n");
fflush(p_trace);
#endif
} else {
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
}
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) */
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;
}
}
}
}
#if TRACE
fprintf(p_trace,"Input file size: %d\n", priv->stream_length+2);
fprintf(p_trace,"Input bytes actually read: %d\n", priv->stream - priv->stream_begin + 2);
fflush(p_trace);
#endif
return 0;
}
五、解码器调试
将输入的JPG文件进行解码:
1.三个结构体设计
-
struct huffman_table
存储Huffman码表
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
储存当前8×8像块中有关解码的信息
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
JPEG数据流结构体，用于存储JPEG图像宽高、数据流指针、Huffman码表等内容，并包含struct huffman_table和struct component
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];
};
2.输出文件保存为YUV文件
static void write_yuv(const char *filename, int width, int height, unsigned char **components)
{
FILE *F;
char temp[1024];
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);
//输出yuv文件//
snprintf(temp, 1024, "%s.YUV", filename);
F = fopen(temp, "wb");
fwrite(components[0], width, height, F);
fwrite(components[1], width*height/4, 1, F);
fwrite(components[2], width*height/4, 1, F);
fclose(F);
}
得到了output.YUV:（未显示完全）
3.视音频编解码调试中TRACE的目的和含义
- 打开和关闭TRACE
- 修改TRACE
1为打开TRACE：
#define TRACE 1//add by nxn
#define TRACEFILE "trace_jpeg.txt"//add by nxn
0为关闭TRACE：
#define TRACE 0//add by nxn
#define TRACEFILE "trace_jpeg.txt"//add by nxn
trace_jpeg.txt文件包含Huffman表的解析
4．以txt文件输出所有的量化矩阵和所有的HUFFMAN码表
已有Huffman码表，需增加DCT矩阵的输出，
在建立量化表函数build_quantization_table中添加输出量化表的部分：
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;
int temp;
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;
//修改使其输出//
for (i=0; i<8; i++) {
for (j=0; j<8; j++) {
temp=ref_table[*zz++];
*qtable++ = temp * aanscalefactor[i] * aanscalefactor[j];
#if TRACE
fprintf(p_trace,"%-6d",temp);
#endif
}
fprintf(p_trace, "\n");
}
}