What does it mean when vanishing points of a box is not on the horizon level?

Question

My understanding has been that for drawing in three or two point perspective, the vanishing points should be on the horizon.

But in the image below, we see that the marked vanishing points on the page are not on equal level, meaning one of the vanishing point is above/below the horizon.

Here's the finished box in the video:

So what does it mean for the the two vanishing points at the top of the page to be not on the same height? Does it mean that the top plane of the box is tilted in some weird way?

Thank you.

Here is the video: https://www.youtube.com/watch?v=86g7QL7gOWg

Answer 1

Does it mean that the top plane of the box is tilted in some weird way?

Not in some weird way, just that the box is indeed tilted relative to an imaginary horizon (or the horizon is tilted relative to the picture plane).

Linear perspective is a wholly rational approach to depicting what we see, and often constructed in a way that pushes objects into an invisible grid. But most objects (with straight edges) in our surroundings are not parallel or perpendicular to other apparent straight lines, among which the (imaginary) horizon line.

Besides, the author of the video seems to be sketching, to demonstrate how 3-point perspective works, and is not using a horizon at all. Rather, they demonstrate how, based on one constructed box in an undefined space, other boxes can be constructed.