说明：  Floyd-Steinberg算法的源代码.要注意的是，误差传播有时会引起流水效应，即误差不断向下，向右累加传播。解决的办法是：奇数行从左到右传播，偶数行从右到左传播。
(Floyd-Steinberg algorithm source code. It should be noted, Error Propagation sometimes causes water effects, that is, constant downward error, right cumulative spread. The solution is : OK whatever odd communication, even from right-to-left line transmission.)