４、上面的工作准备完成后，就可以压缩了，压缩过程非常简单，首先调用jpeg_start_compress，然后可以对每一行进行压缩，也可以对若干行进行压缩，甚至可以对整个的图像进行一次压缩，压缩完成后，记得要调用jpeg_finish_compress函数，如下：

	jpeg_start_compress(&jcs, TRUE);

 	JSAMPROW row_pointer[1];   // 一行位图
 	int row_stride;      // 每一行的字节数 

	 row_stride = jcs.image_width;  // 如果不是索引图,此处需要乘以3

 	// 对每一行进行压缩
 	while (jcs.next_scanline < jcs.image_height) {
     		row_pointer[0] = & pDataConv[jcs.next_scanline * row_stride];
     		jpeg_write_scanlines(&jcs, row_pointer, 1);
 	}

 	jpeg_finish_compress(&jcs);

jpeg_destroy_compress(&jcs);

	struct jpeg_decompress_struct cinfo;
	struct jpeg_error_mgr jerr;

	cinfo.err = jpeg_std_error(&jerr);
	jpeg_create_decompress(&cinfo);

	FILE *f = fopen(strSourceFileName,"rb");
	if (f==NULL)
	{
		printf("Open file error!\n");
		return;
	}
	// 
	jpeg_stdio_src(&cinfo, f);

   jpeg_read_header(&cinfo, TRUE);

   data = new BYTE cinfo.image_width*cinfo.image_height*cinfo.num_components];

	jpeg_start_decompress(&cinfo);

	JSAMPROW row_pointer[1];
	while (cinfo.output_scanline < cinfo.output_height)
	{
		row_pointer[0] = &data[(cinfo.output_height - cinfo.output_scanline-1)*cinfo.image_width*cinfo.num_components];
		jpeg_read_scanlines(&cinfo,row_pointer ,
					1);
	}
	jpeg_finish_decompress(&cinfo);

  jpeg_destroy_decompress(&cinfo);

	fclose(f);