The next section will be about the c plane, that is the projective plane generated for a photon observer.
I read a news about ultrasound planes a few days ago and thought this was connected to photon projection. Things are still unclear now but since all objects approach the observer at the speed of light, it is comparable to an object moving at the speed of sound. Here all sound is accumulated at one point and moves alongside the object. In the photon example, the spatial axis accumulates at one one and there is no distinction between z=0 and z=1.