... I made this study, with floor at an angle to the picture plane. I haven't pinned down the vanishing points yet, for the moment I just want to draw what I see, and maybe follow the lines later to see where they meet.
Sat on the floor, in amongst the shelves, I'm low down; my eye level is only about 20" above the floor. But I observed that, with the picture plane skew to the floor, the horizontals running left to right are always parallel, whereas the verticals veer upwards and outwards. One vanishing point is out of sight, high above me and outside of my vision.
Are you following me still, or have I lost you already? I'm very excited. I don't know and I don't care if all of this has been worked out before by someone else. I have the need to discover the rules for myself.