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\n"); fflush(param_trace);#endifreturn -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_PARAMfprintf(param_trace,"Use decode 1×1 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_PARAMfprintf(param_trace,"Use decode 1×2 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 2×2 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_PARAMfprintf(param_trace,"Use decode 2×1 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 8×8, 8×16, or 16×16) */ //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_PARAMfprintf(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_PARAMfprintf(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;}
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走马观花之外，这才是深入体验，探索自我的最佳时间，
