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Subject: Re: Basic perspective/normal q



"Hans-Bernhard Broeker" wrote in message
news:4bl0akF12979iU1@news.dfncis.de...
> [jongware] wrote:
>
>> What I've done sofar is: transform points with a matrix. Transform normal
>> vector with same. Test if it is visible;
>
> You can't test visibility of a plane from nothing but its normal
> vector. You *need* the 4th parameter, 'd', or a reference point.
> E.g. there could be two parallel planes, orthogonal to the camera's
> "z" axis, with the camera in between. One would thus be fully
> visible, the other not at all.
>
> Your impression that the perspective transformation has anything to do
> with this is wrong. Perspective doesn't affect visibility of a plane.

That's my observation exactly -- when looking to the inside of a box, my
current code displays either one or the other side but never both. (The same
thing obviously happens when looking to the *outside* of a box, visible in
wireframe mode.)
Which leads me back to my original question. Despite clearing up the
terminology, I'm still missing the aha-erlebnis.
"He doesn't understand. Explain ... as you would a child." (Galaxy Quest)

[jw]



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