PPM/PBM/PGM图像格式

http://blog.csdn.net/xiaoxin_ling/article/details/3980642

http://blog.sina.com.cn/s/blog_6de8b2a10100p3pq.html

http://www.cnblogs.com/hnrainll/archive/2011/06/08/2075629.html

http://blog.csdn.net/gengshenghong/article/details/8648577