Subject:
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Re: 3D matrix math question
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Newsgroups:
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lugnet.cad.dev
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Date:
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Sun, 14 Dec 2008 08:24:40 GMT
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Viewed:
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11721 times
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In lugnet.cad.dev, Travis Cobbs wrote:
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In lugnet.cad.dev, Timothy Gould wrote:
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If I understand correctly you shouldn’t be checking on the projection, only
on the full 3D transformed points. That means you should check after stage
2.5.
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Actually, the projected points are all that I am checking, and that’s how
it’s supposed to work.
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You’ll need the Z value to work out which side things lie on.
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No. The way that conditional lines work is that if the two control points
lie on the same side of the conditional line on the screen (in window
coordinates), then the conditional line should be drawn. If they lie on
opposite sides, the conditional line shouldn’t be drawn. For the purposes of
determining this, the actual conditional line is considered to be infinitely
long. The Z values are irrelevant for making this determination.
Unfortunately, as far as I can tell, the projection matrix doesn’t produce
useful results for points that lie outside the view frustum. But I can’t
skip it entirely, because that would leave out the effects of perspective,
which are necessary in order to get the right results.
--Travis
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Hmmm... Though I can’t quite pin it down I have a suspicion that this has
something to do with the spatial reduction cause by projection. What projection
matrix are you using?
Would a simple solution just be to move the conditional points right in close to
the original points at parse time? If the length of the conditional vectors is
unimportant then you’d be decreasing the number of conditional points that lie
outside the viewing frustum. With a distance of, say, 1/10th LDU from the line
you’d rarely encounter any problems.
Tim
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Message has 1 Reply:
 | | Re: 3D matrix math question
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| (...) OK... I think I understand what I was getting at here. When you introduce perspective then the distance of the conditional point from the line becomes important. Eg. if you have a point that is directly behind its line in a perspective free (...) (16 years ago, 14-Dec-08, to lugnet.cad.dev, FTX)
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Message is in Reply To:
| | Re: 3D matrix math question
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| (...) Actually, the projected points are all that I am checking, and that's how it's supposed to work. (...) No. The way that conditional lines work is that if the two control points lie on the same side of the conditional line on the screen (in (...) (16 years ago, 14-Dec-08, to lugnet.cad.dev, FTX)
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