24位色BMP转换8位色BMP的算法，从24位真彩色转换为8位彩色的算法，效果可以，失真误差比较均衡，算法效率也比较高。(24 bit color BMP conversion 8 bit color BMP algorithm, from 24 bit true color to 8 bit color algorithm, the effect is good, the distortion error is more balanced, the efficiency of the algo