This is a question that's bugged me for a while.
Let's say you're doing a simple two-point perspective, with parallel vertical lines. You draw your basic box, whose horizontal sides each line up with one of the two vanishing points.
Now you add a new box, stacked on top of it, but it's rotated with respect to the first one. Where do the new vanishing points go?
Let's say you're looking north, and you have two vanishing points, northwest, and northeast. They're equally spaced to either side of the center of the picture. As we know, NW and NE are 90 degrees apart on the compass, so in a sense the two vanishing points are "90 degrees apar