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OpenGL projection matrix with more than one vanishing point

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kreaThor    122
Hello! I saw some very interesting pictures of a cube drawn by projecting it with a perspective projection with more than one vanishing point. Can this be done in OpenGL? If yes, how can the projection matrix be calculated?

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haegarr    7372
It is not fully clear: The "standard" perspective projection has 1, 2, or 3 vanishing points visible, dependend on how the viewed object is oriented and positioned w.r.t. the cam (to be precise: the projection plane).

However, dozens of kinds of (also perspective) projections exist. Unfortunately due to the fact that it could not be modeled using homogeneous matrix math most of them could not be simulated by OpenGL. Obviously no projections w/ curved projectors could be done. Also no "inverse" projection could be done (as far as I know). EDIT: Projections w/ curved projection planes could be done but only approximately.

Whether the projections you have in mind could be done isn't answerable as long as you don't name them. But I'm not confident.

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Sneftel    1788
Just do a Google image search for "two-point perspective" and you'll turn up plenty of examples.

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haegarr    7372
Original post by kreaThor
ok U guys thanks. I will delve some more into the topic, though I understood from haegarr that it can't be done in opengl.

Notice please that I said that I'm not _confident_ that it will be possible, but due to lack of knowledge of what you've actually found I could not definitely say "yes" or "no".

The standard 1/2/3 point perspective projections work simply by unaligning the projection plane, as the site following Sneftel's link above shows. As a real projection mode, the unalignment is done by the projection matrix. However, the _effect_ is the same as could be seen if the objects are unaligned. E.g. take a wire-cube, and place the camera in front of it. What could be seen is a single vanishing point. Changing the heading of the cube by 45 degrees, and you see 2 vanishing points.

I somewhere have seen projections with more than 3 vanishing points. As far as I remember, these projections cannot be simulated by OpenGL.

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