Draw a circle in a paper, if the line of sight perpendular to the paper , we see a circle ,but if the line of sight is not perpendular to the paper, must we see an ellipse ? how to prove it ? how to find out the major axis or minor axis ? It seem that when we observe the circle ,it should be wide in near place and narrow in far place and should look like a chicken egg ,but it is not true ,how to explain the contradiction. When we view by eyes,we make an affine transformation or a projective transformation ? Whether the relation is true below : parallel projection = affine transformation, central projection = Perspective projection?