JPEG图片的解码\压缩流程

一、解码：

1：创建并初始化一个JPEG解码对象（解码对象是一个结构图对象）

    /* Step 1: allocate and initialize JPEG decompression object */

    /* override error_exit. */
    /* Now we can initialize the JPEG decompression object. */
    jpeg_create_decompress(&cinfo);

2：创建错误对象

/* Step 2:  We set up the normal JPEG error routines */
cinfo.err = jpeg_std_error(&jerr);

3：打开指定带解码的JPEG图片

/* Step 3: specify data source (eg, a file) */

    jpeg_stdio_src(&cinfo, infile);

4：获取待解码图片的图片信息，解析文件头。JPEG文件中包含一些元数据，如图像尺寸、颜色空间信息等。解析文件头可以获得这些元数据，为后续处理做准备。

/* Step 4: read file parameters with jpeg_read_header() */

    (void)jpeg_read_header(&cinfo, TRUE);
    /* We can ignore the return value from jpeg_read_header since
     *   (a) suspension is not possible with the stdio data source, and
     *   (b) we passed TRUE to reject a tables-only JPEG file as an error.
     * See libjpeg.txt for more info.
     */

5：设置解码参数。（可根据自身要求选择对应参数，也可使用默认的参数）

/* Step 5: set parameters for decompression */

    /* In this example, we don't need to change any of the defaults set by
     * jpeg_read_header(), so we do nothing here.
     */

6：开始解码：

​ JPEG图像经过了压缩，解码流程需要先将压缩的数据进行解压缩。解压缩过程包括两个步骤：解码哈夫曼编码和逆量化。

• 解码哈夫曼编码：JPEG使用了哈夫曼编码对图像数据进行熵编码，解码过程就是根据哈夫曼编码表将压缩的数据解码成原始的频域系数。

• 逆量化：JPEG在压缩过程中对频域系数进行了量化，解码流程需要对这些量化后的系数进行逆量化，还原为未经量化的系数。

   /* Step 6: Start decompressor */
  
      (void)jpeg_start_decompress(&cinfo);
      /* We can ignore the return value since suspension is not possible
       * with the stdio data source.
       */
  
      /* We may need to do some setup of our own at this point before reading
       * the data.  After jpeg_start_decompress() we have the correct scaled
       * output image dimensions available, as well as the output colormap
       * if we asked for color quantization.
       * In this example, we need to make an output work buffer of the right size.
       */
      /* JSAMPLEs per row in output buffer */
      row_stride = cinfo.output_width * cinfo.output_components; // 计算一行的大小
  
      /* Make a one-row-high sample array that will go away when done with image */
      buffer = calloc(1, row_stride);
  

7：调用循环从JPEG图片中以行为单位进行解码

/* Step 7: while (scan lines remain to be read) */
    /*           jpeg_read_scanlines(...); */

    /* Here we use the library's state variable cinfo.output_scanline as the
     * loop counter, so that we don't have to keep track ourselves.
     */
    int data = 0;

    while (cinfo.output_scanline < cinfo.output_height)
    {
        /* jpeg_read_scanlines expects an array of pointers to scanlines.
         * Here the array is only one element long, but you could ask for
         * more than one scanline at a time if that's more convenient.
         */
        (void)jpeg_read_scanlines(&cinfo, &buffer, 1); // 从上到下，从左到右  RGB RGB RGB RGB

        for (int i = 0; i < cinfo.output_width; ++i) // 012 345
        {
            data |= buffer[3 * i] << 16;    // R
            data |= buffer[3 * i + 1] << 8; // G
            data |= buffer[3 * i + 2];      // B

            // 把像素点写入到LCD的指定位置
            lcd_mp[800 * start_y + start_x + 800 * (cinfo.output_scanline - 1) + i] = data;

            data = 0;
        }
    }

8： 完成解码

 /* Step 8: Finish decompression */

    (void)jpeg_finish_decompress(&cinfo);
    /* We can ignore the return value since suspension is not possible
     * with the stdio data source.
     */

9 ：释放解码对象

 /* Step 9: Release JPEG decompression object */

    /* This is an important step since it will release a good deal of memory. */
    jpeg_destroy_decompress(&cinfo);

    /* After finish_decompress, we can close the input file.
     * Here we postpone it until after no more JPEG errors are possible,
     * so as to simplify the setjmp error logic above.  (Actually, I don't
     * think that jpeg_destroy can do an error exit, but why assume anything...)
     */
    fclose(infile);

    /* At this point you may want to check to see whether any corrupt-data
     * warnings occurred (test whether jerr.pub.num_warnings is nonzero).
     */

    /* And we're done! */
    return 1;

二、压缩：

1：创建并初始化一个JPEG解码对象并创建错误对象

 /* Step 1: allocate and initialize JPEG compression object */

  /* We have to set up the error handler first, in case the initialization
   * step fails.  (Unlikely, but it could happen if you are out of memory.)
   * This routine fills in the contents of struct jerr, and returns jerr's
   * address which we place into the link field in cinfo.
   */
  cinfo.err = jpeg_std_error(&jerr);
  /* Now we can initialize the JPEG compression object. */
  jpeg_create_compress(&cinfo);

2：打开目标文件：

/* Step 2: specify data destination (eg, a file) */
  /* Note: steps 2 and 3 can be done in either order. */

  /* Here we use the library-supplied code to send compressed data to a
   * stdio stream.  You can also write your own code to do something else.
   * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
   * requires it in order to write binary files.
   */
  if ((outfile = fopen(filename, "wb")) == NULL) {
    fprintf(stderr, "can't open %s\n", filename);
    exit(1);
  }
  jpeg_stdio_dest(&cinfo, outfile);

3:设置压缩参数

/* Step 3: set parameters for compression */

  /* First we supply a description of the input image.
   * Four fields of the cinfo struct must be filled in:
   */
  cinfo.image_width = image_width; 	/* image width and height, in pixels */
  cinfo.image_height = image_height;
  cinfo.input_components = 3;		/* # of color components per pixel */
  cinfo.in_color_space = JCS_RGB; 	/* colorspace of input image */
  /* Now use the library's routine to set default compression parameters.
   * (You must set at least cinfo.in_color_space before calling this,
   * since the defaults depend on the source color space.)
   */
  jpeg_set_defaults(&cinfo);
  /* Now you can set any non-default parameters you wish to.
   * Here we just illustrate the use of quality (quantization table) scaling:
   */
  jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);

4：开始压缩

/* Step 4: Start compressor */

  /* TRUE ensures that we will write a complete interchange-JPEG file.
   * Pass TRUE unless you are very sure of what you're doing.
   */
  jpeg_start_compress(&cinfo, TRUE);

5：按行循环压缩JPEG图片

/* Step 5: while (scan lines remain to be written) */
  /*           jpeg_write_scanlines(...); */

  /* Here we use the library's state variable cinfo.next_scanline as the
   * loop counter, so that we don't have to keep track ourselves.
   * To keep things simple, we pass one scanline per call; you can pass
   * more if you wish, though.
   */
  row_stride = image_width * 3;	/* JSAMPLEs per row in image_buffer */

  while (cinfo.next_scanline < cinfo.image_height) {
    /* jpeg_write_scanlines expects an array of pointers to scanlines.
     * Here the array is only one element long, but you could pass
     * more than one scanline at a time if that's more convenient.
     */
    row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
    (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
  }

6：结束压缩：

/* Step 6: Finish compression */

  jpeg_finish_compress(&cinfo);
  /* After finish_compress, we can close the output file. */
  fclose(outfile);

7:释放压缩对象：

/* Step 7: release JPEG compression object */

  /* This is an important step since it will release a good deal of memory. */
  jpeg_destroy_compress(&cinfo);

  /* And we're done! */
}