概述：前一篇部落格中說到libjpeg-turbo的使用，有一些錯誤也在前篇部落格中糾正了，現在呢，就用TurboJPEG API實現圖片的解壓，縮放，壓縮。

1，直接上程式碼（簡單粗暴,重要程式碼都加了註釋，我就不多費口舌了）

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <unistd.h>
#include <sys/stat.h>
#include <turbojpeg.h>

typedef unsigned char uchar;

typedef
 
 struct tjp_info {
  int outwidth;
  int outheight;
  unsigned long jpg_size;
}tjp_info_t;

/*獲取當前ms數*/
static int get_timer_now ()
{
    struct timeval now;
    gettimeofday(&now, NULL);
    return(now.tv_sec * 1000 + now.tv_usec / 1000);
}

/*讀取檔案到記憶體*/
uchar *read_file2buffer(char *filepath, tjp_info_t *tinfo)
{
  FILE *fd;
  struct
 
 stat fileinfo;
  stat(filepath,&fileinfo);
  tinfo->jpg_size = fileinfo.st_size;

  fd = fopen(filepath,"rb");
  if (NULL == fd) {
    printf("file not open\n");
    return NULL;
  }
  uchar *data = (uchar *)malloc(sizeof(uchar) * fileinfo.st_size);
  fread(data,1,fileinfo.st_size,fd);
  fclose(fd);
  return
 
 data;
}

/*寫記憶體到檔案*/
void write_buffer2file(char *filename, uchar *buffer, int size)
{
  FILE *fd = fopen(filename,"wb");
  if (NULL == fd) {
    return;
  }
  fwrite(buffer,1,size,fd);
  fclose(fd);
}

/*圖片解壓縮*/
uchar *tjpeg_decompress(uchar *jpg_buffer, tjp_info_t *tinfo)
{
  tjhandle handle = NULL;
  int img_width,img_height,img_subsamp,img_colorspace;
  int flags = 0, pixelfmt = TJPF_RGB;
  /*建立一個turbojpeg控制代碼*/
  handle = tjInitDecompress();
  if (NULL == handle)  {
    return NULL;
  }
  /*獲取jpg圖片相關資訊但並不解壓縮*/
  int ret = tjDecompressHeader3(handle,jpg_buffer,tinfo->jpg_size,&img_width,&img_height,&img_subsamp,&img_colorspace);
  if (0 != ret) {
    tjDestroy(handle);
    return NULL;
  }
  /*輸出圖片資訊*/
  printf("jpeg width:%d\n",img_width);
  printf("jpeg height:%d\n",img_height);
  printf("jpeg subsamp:%d\n",img_subsamp);
  printf("jpeg colorspace:%d\n",img_colorspace);
  /*計算1/4縮放後的影象大小,若不縮放，那麼直接將上面的尺寸賦值給輸出尺寸*/
  tjscalingfactor sf;
  sf.num = 1;
  sf.denom = 4;
  tinfo->outwidth = TJSCALED(img_width, sf);
  tinfo->outheight = TJSCALED(img_height, sf);
  printf("w:%d,h:%d\n",tinfo->outwidth,tinfo->outheight);
  /*解壓縮時，tjDecompress2（）會自動根據設定的大小進行縮放，但是設定的大小要在它的支援範圍，如1/2 1/4等*/
  flags |= 0;
  int size = tinfo->outwidth * tinfo->outheight * 3;
  uchar *rgb_buffer = (uchar *)malloc(sizeof(uchar) * size);
  ret = tjDecompress2(handle, jpg_buffer, tinfo->jpg_size, rgb_buffer, tinfo->outwidth, 0,tinfo->outheight, pixelfmt, flags);
  if (0 != ret) {
    tjDestroy(handle);
    return NULL;
  }
  tjDestroy(handle);
  return rgb_buffer;
}

/*壓縮圖片*/
int tjpeg_compress(uchar *rgb_buffer, tjp_info_t *tinfo, int quality, uchar **outjpg_buffer, unsigned long *outjpg_size)
{
  tjhandle handle = NULL;
  int flags = 0;
  int subsamp = TJSAMP_422;
  int pixelfmt = TJPF_RGB;
  /*建立一個turbojpeg控制代碼*/
  handle=tjInitCompress();
  if (NULL == handle) {
    return -1;
  }
  /*將rgb圖或灰度圖等壓縮成jpeg格式圖片*/
  int ret = tjCompress2(handle, rgb_buffer,tinfo->outwidth,0,tinfo->outheight,pixelfmt,outjpg_buffer,outjpg_size,subsamp,quality, flags);
  if (0 != ret) {
    tjDestroy(handle);
    return -1;
  }
  tjDestroy(handle);
  return 0;
}

/*測試程式*/
int tj_test()
{
  tjp_info_t tinfo;
    char *filename = "./test.jpg";
    int start = get_timer_now();
  /*讀影象*/
    uchar *jpeg_buffer = read_file2buffer(filename,&tinfo);
    int rend = get_timer_now();
    printf("loadfile make time:%d\n",rend-start);
    if (NULL == jpeg_buffer) {
        printf("read file failed\n");
        return 1;
    }

    int dstart = get_timer_now();
  /*解壓縮*/
    uchar *rgb = tjpeg_decompress(jpeg_buffer,&tinfo);
    if (NULL == rgb) {
        printf("error\n");
    free(jpeg_buffer);
    return -1;
    }
    int dend = get_timer_now();
    printf("decompress make time:%d\n",dend-dstart);
    uchar *outjpeg=NULL;
    unsigned long outjpegsize;
    int cstart = get_timer_now();
  /*壓縮*/
    tjpeg_compress(rgb,&tinfo,80,&outjpeg,&outjpegsize);
    printf("out jpeg size = %lu\n",outjpegsize);
    int cend = get_timer_now();
    printf("compress make time:%d\n",cend-cstart);
    char *outfile = "./tjout.jpg";
    int wstart = get_timer_now();
    write_buffer2file(outfile,outjpeg,outjpegsize);
    int wend = get_timer_now();
    printf("write file make time:%d\n",wend-wstart);
    int end = get_timer_now();
    printf("tj all make time:%d\n",end-start);
  free(jpeg_buffer);
  free(rgb);
  return 0;
}

int main()
{
    tj_test();
}

2，我們看下執行結果和上篇中使用libjpeg-turbo中的libjpeg API實現的相同功能速度對比。我們可以看到花費的時間基本沒啥大差別，因為他們都是基於SIMD實現，TurboJPEG只不過是更高一層的封裝而已。