射影几何研究图形在射影变换下的不变性,射影变换可以直观地看成是由连续施行若干次中心投影所得到的变换,为了使中心投影成为两平面的点之间的一一对应,我们必须把通常的欧氏平面加以拓广,添加无穷远点和无穷远直线,即对平面上的一族平行线添加一个无穷远点,且规定平面上所有无穷远点的集合为一条无穷远直线,这和经过拓广以后的平面,若对 The invariability of the projective geometry study graph under the projective transformation, the projective transformation can be seen intuitively as a transformation resulting from the continuous implementation of a number of central projections. In order for the central projection to be a one-to-one correspondence between the points of the two planes, we must To expand the usual Euclidean plane, add infinity points and infinity straight lines, that is, add an infinity point to a family of parallel lines on the plane, and the set of all infinity points on the specified plane is an infinite straight line. After the extension of the plane, if