I asked this question to understand difference between projection plane and projective plane. But below the @bubba's answer, the user @Acccumulation comments this text which given below:
The projection plane is where you're projecting onto a plane. The projective plane is where you're projecting from a plane, and filling in the points at infinity (those points exist in the projection despite not being in the original plane).
My first question is what does mean second sentence of this comments? I mean when we do projection from 3D space to 2D onto projection plane, but what he mean, he means 3D space exists in projective plane?
When see the answer of @Lee Mosher he said homogenous coordinates are the points of $\mathbb R^3.$ But we see in many other answers homogenous coordinates are the points of projective plane $P^2.$ My second question is what is right? Homogenous coordinates belongs to $\mathbb R^3 or P^2?$