jpeg_start_decompress (j_decompress_ptr cinfo)分析

最新推荐文章于 2024-07-22 15:50:44 发布

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最新推荐文章于 2024-07-22 15:50:44 发布

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分类专栏： libjpeg

本文链接：https://blog.youkuaiyun.com/xieqb/article/details/45499099

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本文详细解析了JPEG图像解压缩的过程，包括初始化控制模块、选择活动模块、处理多扫描文件等关键步骤，并介绍了如何设置输出通道为最终图像展示做准备。

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GLOBAL(boolean)
jpeg_start_decompress (j_decompress_ptr cinfo)
{
  if (cinfo->global_state == DSTATE_READY)

 {
    /* First call: initialize master control, select active modules ；初始化参数*/
    jinit_master_decompress(cinfo);
    if (cinfo->buffered_image)

    {
      /* No more work here; expecting jpeg_start_output next */
      cinfo->global_state = DSTATE_BUFIMAGE;
      return TRUE;
    }
    cinfo->global_state = DSTATE_PRELOAD;
  }
  if (cinfo->global_state == DSTATE_PRELOAD) {
    /* If file has multiple scans, absorb them all into the coef buffer */
    if (cinfo->inputctl->has_multiple_scans) {
#ifdef D_MULTISCAN_FILES_SUPPORTED
      for (;;) {
	int retcode;
	/* Call progress monitor hook if present */
	if (cinfo->progress != NULL)
	  (*cinfo->progress->progress_monitor) ((j_common_ptr) cinfo);
	/* Absorb some more input */
	retcode = (*cinfo->inputctl->consume_input) (cinfo);
	if (retcode == JPEG_SUSPENDED)
	  return FALSE;
	if (retcode == JPEG_REACHED_EOI)
	  break;
	/* Advance progress counter if appropriate */
	if (cinfo->progress != NULL &&
	    (retcode == JPEG_ROW_COMPLETED || retcode == JPEG_REACHED_SOS)) {
	  if (++cinfo->progress->pass_counter >= cinfo->progress->pass_limit) {
	    /* jdmaster underestimated number of scans; ratchet up one scan */
	    cinfo->progress->pass_limit += (long) cinfo->total_iMCU_rows;
	  }
	}
      }
#else
      ERREXIT(cinfo, JERR_NOT_COMPILED);
#endif /* D_MULTISCAN_FILES_SUPPORTED */
    }
    cinfo->output_scan_number = cinfo->input_scan_number;
  } else if (cinfo->global_state != DSTATE_PRESCAN)
    ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);
  /* Perform any dummy output passes, and set up for the final pass */
  return output_pass_setup(cinfo);
}