Rectilinear planes that are parallel to the ground plane will have vanishing points that fall on the horizon line.
If one axis of a receding plane is not parallel to the ground plane, its vanishing point will not fall on the horizon line. Instead, it will fall on a line perpendicular to the horizon line that runs through the original vanishing point. This line is called the vertical vanishing line (VVL). Vanishing points that fall on this line are called vertical vanishing points (VVP).
The steeper the angle of a plane's ascent or descent from the ground plane, the further up or down on the vertical vanishing line the points will fall.
It is important to realize that the vertical vanishing line operates just like the horizon line, except that it is perpendicular to the horizon.
Turn the page on end and note that the image becomes a three-point-perspective setup.
Note also how the sloping planes diminish toward vanishing points below the horizon line after they have passed an angle 90 degrees to the ground plane.
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