以R、G、B为轴建立空间直角坐标系，则RGB图的每个象素的颜色可以用该三维空间的一个点来表示，

而Gray图的每个象素的颜色可以用直线R=G=B上的一个点来表示。于是rgb转gray图的本质就是寻找

一个三维空间到一维空间的映射，最容易想到的就是射影（即过rgb空间的一个点向直线R=G=B做垂线）-With R, G, B axis to build the space right angle coordinate system, then each pixel of RGB graph colors can be used for a point in the three-dimensional space to represent,

Each pixel of Gray graph colors can be used for a linear R=G=B to indicate. The essence and RGB transgenic gray map is looking for

The mapping of one dimensional space into a three-dimensional space, the most easy to think of is projective (a point that RGB space to do a line perpendicular to the line R=G=B)