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 2：解码数据
===================
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\n");
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\n");
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)\n");
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\n");
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\n");
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\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]);
//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\n",y,x/xstride_by_mcu);
for(j=0;j<64;j++){
fprintf(fp,"%d ",priv->component_infos[cY].DCT[j]);
}
fprintf(fp,"\n");
fclose(fp);
}
#if TRACE_PARAM
fprintf(param_trace,"\n第3行，第4列\n");
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,"\n");
}
fprintf(fp,"\n-----------------------\n");
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\n",y,x/xstride_by_mcu);
for(j=0;j<64*4;j++){
fprintf(fp,"%d ",priv->Y[j]);
}
fprintf(fp,"\n");
fclose(fp);
}
#if TRACE_PARAM
fprintf(param_trace,"第3行，第4列\n");
for(j=0;j<8;j++){
for(k=0;k<8;k++){
fprintf(param_trace,"%d ",priv->Y[j*8+k]);
}
fprintf(param_trace,"\n");
}
fprintf(fp,"\n-----------------------\n");
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\n", priv->stream_length+2);
fprintf(param_trace,"Input bytes actually read: %d\n", 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