• libjpeg-turbo的使用


    平时都用OpenCV的imwrite存图,保存为bmp格式的图片,速度快但占用空间大;存为jpg格式,占用空间小但存图时间长。
    了解到libjpeg这个开源库存图,实验了一下,老版的libjpeg与OpenCV相比,没有优势。新版的libjpeg-turbo就不一样了,存图效率很高。
    (1)从官网下载最新版本的libjpeg-turbo
    https://libjpeg-turbo.org/
    在这里插入图片描述
    (2)安装.exe文件,解压压缩包
    在这里插入图片描述
    (3)应用
    参考doc文件夹中的example.txt,了解使用步骤
    注意:
    要求的图片指针数据为 RGB!
    要求的图片指针数据为 RGB!
    要求的图片指针数据为 RGB!

    参数解析:
    char filename 要保存的图像文件名
    int quality jpg图像的压缩率(0-100)
    unsigned char
     image_buffer 图像指针(RGB排序)

    int image_width 图像宽
    int image_height 图像高

      1 void write_JPEG_file(char *filename, int quality, unsigned char* image_buffer, int image_width, int image_height)
      2 {
      3     /* This struct contains the JPEG compression parameters and pointers to
      4     * working space (which is allocated as needed by the JPEG library).
      5     * It is possible to have several such structures, representing multiple
      6     * compression/decompression processes, in existence at once.  We refer
      7     * to any one struct (and its associated working data) as a "JPEG object".
      8     */
      9     struct jpeg_compress_struct cinfo;
     10     /* This struct represents a JPEG error handler.  It is declared separately
     11     * because applications often want to supply a specialized error handler
     12     * (see the second half of this file for an example).  But here we just
     13     * take the easy way out and use the standard error handler, which will
     14     * print a message on stderr and call exit() if compression fails.
     15     * Note that this struct must live as long as the main JPEG parameter
     16     * struct, to avoid dangling-pointer problems.
     17     */
     18     struct jpeg_error_mgr jerr;
     19     /* More stuff */
     20     FILE *outfile;                /* target file */
     21     JSAMPROW row_pointer[1];      /* pointer to JSAMPLE row[s] */
     22     int row_stride;               /* physical row width in image buffer */
     23 
     24     /* Step 1: allocate and initialize JPEG compression object */
     25 
     26     /* We have to set up the error handler first, in case the initialization
     27     * step fails.  (Unlikely, but it could happen if you are out of memory.)
     28     * This routine fills in the contents of struct jerr, and returns jerr's
     29     * address which we place into the link field in cinfo.
     30     */
     31     cinfo.err = jpeg_std_error(&jerr);
     32     /* Now we can initialize the JPEG compression object. */
     33     jpeg_create_compress(&cinfo);
     34 
     35     /* Step 2: specify data destination (eg, a file) */
     36     /* Note: steps 2 and 3 can be done in either order. */
     37 
     38     /* Here we use the library-supplied code to send compressed data to a
     39     * stdio stream.  You can also write your own code to do something else.
     40     * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
     41     * requires it in order to write binary files.
     42     */
     43     if ((outfile = fopen(filename, "wb")) == NULL) {
     44         fprintf(stderr, "can't open %s
    ", filename);
     45         exit(1);
     46     }
     47     jpeg_stdio_dest(&cinfo, outfile);
     48 
     49     /* Step 3: set parameters for compression */
     50 
     51     /* First we supply a description of the input image.
     52     * Four fields of the cinfo struct must be filled in:
     53     */
     54     cinfo.image_width = image_width;      /* image width and height, in pixels */
     55     cinfo.image_height = image_height;
     56     cinfo.input_components = 3;           /* # of color components per pixel */
     57     cinfo.in_color_space = JCS_RGB;       /* colorspace of input image */
     58     /* Now use the library's routine to set default compression parameters.
     59     * (You must set at least cinfo.in_color_space before calling this,
     60     * since the defaults depend on the source color space.)
     61     */
     62     jpeg_set_defaults(&cinfo);
     63     /* Now you can set any non-default parameters you wish to.
     64     * Here we just illustrate the use of quality (quantization table) scaling:
     65     */
     66     jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);
     67 
     68     /* Step 4: Start compressor */
     69 
     70     /* TRUE ensures that we will write a complete interchange-JPEG file.
     71     * Pass TRUE unless you are very sure of what you're doing.
     72     */
     73     jpeg_start_compress(&cinfo, TRUE);
     74 
     75     /* Step 5: while (scan lines remain to be written) */
     76     /*           jpeg_write_scanlines(...); */
     77 
     78     /* Here we use the library's state variable cinfo.next_scanline as the
     79     * loop counter, so that we don't have to keep track ourselves.
     80     * To keep things simple, we pass one scanline per call; you can pass
     81     * more if you wish, though.
     82     */
     83     row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */
     84 
     85     while (cinfo.next_scanline < cinfo.image_height) {
     86         /* jpeg_write_scanlines expects an array of pointers to scanlines.
     87         * Here the array is only one element long, but you could pass
     88         * more than one scanline at a time if that's more convenient.
     89         */
     90         row_pointer[0] = &image_buffer[cinfo.next_scanline * row_stride];
     91         (void)jpeg_write_scanlines(&cinfo, row_pointer, 1);
     92     }
     93 
     94     /* Step 6: Finish compression */
     95 
     96     jpeg_finish_compress(&cinfo);
     97     /* After finish_compress, we can close the output file. */
     98     fclose(outfile);
     99 
    100     /* Step 7: release JPEG compression object */
    101 
    102     /* This is an important step since it will release a good deal of memory. */
    103     jpeg_destroy_compress(&cinfo);
    104 
    105     /* And we're done! */
    106 }
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  • 原文地址:https://www.cnblogs.com/ybqjymy/p/13722494.html
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