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Ectara

OpenGL Projection Matrices

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Using a projection matrix to project a point from 3D space to screen coordinates, I get bizarre and invalid values when attempting to project a point outside the viewing frustum. Is this normal behavior, and if so, how do APIs like OpenGL draw lines and polygons that are partially outside the viewing frustum?

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On a 640x480 viewport, I'd get things like -674, -842 and -24235334, 5234523. All seemingly random, yet consistent if I send the same vertices through. Everything in the frustum comes up as expected.

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I have a bit similar question:
I use gluProject, and if the the projected point is behind the camera (behind the plane of the screen), the depth values > 1.0
I exploit that thing, so I don't have to check again, if the point is behind the camera.
Can I trust this behavior, or it's not API/lib/whatever independent?

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Without knowing what the actual vertices and matrices are, it's difficult to tell what's going wrong, if anything. Keep in mind that by the time you get to screen coordinates you've already performed a homogeneous divide by 'w', so if that 'w' happened to very large in clip-space you could see very large 'x' and 'y' values before the divide. But 1 million into 50 million is still just 50, so it all depends on what steps you've taken and what data you're looking at.

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I'll take a closer look and see what's going on at each step under the hood, and check during various circumstances. I'll post code and vertices with the current matrices if I can't find it today; no sense bothering you further if I find the bug myself.

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@szecs, my mind is a bit hazy on my installed OpenGL implementation, but unless you see code for each version, I wouldn't rely on it, seeing as one implementation could change it slightly and throw it all off. Try caching the 6 frustum planes(or even just the near plane), and test if it is behind the near plane.

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I did find one thing out. Projecting a point slightly out of the frustum acts accordingly; for instance, a point might be projected to -60, 20 on a 800x600 display, so the projection is working correctly, per se. But I did notice, that when a vertex went around +/-20000, it changed sign, causing the usual havoc. I swapped some of the float variables with integers where possible, in hopes that the float variables had over/underflowed(I have little way of telling what the next value after +/- 20000 was supposed to be).

Wait, wait, wait. When it goes around +/- 20000, something else also happens: the vertices begin to pass the 90 degree rotation mark. Meaning the signs have flipped because z is now past my clamped range of [0,1], being that the vertices are now behind me. A couple quick lines of code, and all is well, except for now if my focal point is behind me, everything goes haywire. But this isn't really an issue, seeing as my focal point should never be behind me in the first place. Thanks to those who offered advice.

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