In page 220 of Multiple View Geometry in Computer Vision is the following figure:
So in this figure, $\mathbf{l}$ is the vanishing line of the scene plane $\pi$ projected onto the image plane. What I do not understand is the text about diagram b. For example how can any scene point projecting onto the vanishing line have the same distance from the plane $\pi$ as the camera centre? A simple contradiction would be to project point $C_a$ from diagram a onto the image plane in diagram b. The distance between $C_a$ and $\pi$ is clearly much smaller than the distance between $C_a$ and $C_b$. But in the figure description it states that this distance should be equal. Have I misunderstood something?