The "obvious" answer looks wrong. Is it?

@peterdrake shooting from the hip— if you take all the guiding lines and vanishing points and reflect them about the vertical line through your central vanishing point, you get a third point on the horizon (off-page) and a new set of guiding lines. Objects to the right of the central vanishing would seem to need to be aligned with the guides that go towards the right hand off page point.

Does that make any sense?

@bfjvii Yes. I think that would be approaching 5-point perspective, but with the top and bottom VPs infinitely far away.

https://www.creativebloq.com/how-to/draw-5-point-perspective

@peterdrake

This is on the right track.

The problem is that your vertical lines (that is, representing vertical edges in 3D space) are all parallel on your 2D image plane.

This means that your image plane is itself vertical. But your viewpoint is above the horizon, and the horizon is above the midpoint of the page. This implies that your image plane is oblique relative to the direction of observation (red line on my diagram) instead of normal to it (green line). That's the difference between a two- and three-point perspective, and the cause of the distortion you see.

The solution is to also draw the vertical dimension with perspective. Because your direction of observation is slightly downward, the third vanishing point should be directly below the midpoint of the image, and probably at quite a distance (it would be infinitely far away only if you were looking perfectly horizontally, which is when your verticals should come out parallel).

I think that with some trig you could work out the proper distance as a function of the coordinates, relative to the centre of the page, of the two existing vanishing points.

@khird What if the horizon was at the (vertical) midpoint of the page?