this is actually quite arbitrary, this object merely determines the scale, not the geometry, of the model
Because this algorithm doesn't make any assumptions about the relative angles of the rectangle planes, often scenes that have very strong right angles show up very badly in 3d-models. My method does not take advantage of the data redundancy that other single-view metrology algorithms use
I use parrallel lines on rectangles to compute the camera depth, but I have no method to extract the center of projection. I currently assume its at the center of the image. Some of the models I attempted (most notably, Albrecht Durer's "St Jerome in his study") had a center of projection other then the center of the image.