Re: converting to screen coords
Angus Dorbie (dorbie++at++sgi.com)
Tue, 09 Jun 1998 14:27:17 -0700
Angus Dorbie wrote:
>
> leif.stubbfalt++at++enator.se wrote:
> >
> > Hello!
> >
> > I want to transform a point from world coordinates to screen
> > coordinates. I know that the first step is to convert the point to
> > viewing coordinates. I did this by multiply the point by the inverse of
> > the viewing matrix (I was told that it had to be the inverse). I got the
> > viewing matrix by pfGetChanViewMat. Is this OK?
> >
> > After this I multiplied the converted point by the projection matrix. I
> > got the matrix by pfGetFrustGLProjMat. Is this OK? I guess not, because
> > the resulting coordinates where not normalized. When transforming I used
> > pfFullXformPt3. The last step, converting from normalized to screen
> > coordinates, won't be a problem.
>
> You have the right idea, note if a point is outside the frustum it will
> not
> lie between -1 and 1 and should be clipped.
> Also before projection you should eliminate points in eye space which
> are
> beyond a y threshold (they are behind eye or near clip).
>
> There may be a 90 degree rotation about x to bring performer y into
> alignment with gl projection -z, this is probably what you are missing
> but that depends on exactly what's in the GLFrustumMatrix projection
> returned by the pfFrustum.
>
> You are definitely on the right track.
>
> The other thing you could do ofcourse is to just divide eye x & y by
> screen z and then scale yourself. You have to do the scale anyway for
Ysing Performer modelview you'd wnat to divide x & z by eye y to give
x & z corresponding to screen x & y after mapping.
> your viewport so you may as well build in the FOV scale and use a
> normalized projection for speed with the added bonus that you know
> you are in Performer channel eye space. It's your choice.
>
> Cheers,Angus.
>
> --
> "Only the mediocre are always at their best." -- Jean Giraudoux
>
> For advanced 3D graphics Performer + OpenGL based examples and tutors:
> http://www.dorbie.com/
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--
"Only the mediocre are always at their best." -- Jean Giraudoux
For advanced 3D graphics Performer + OpenGL based examples and tutors:
http://www.dorbie.com/
=======================================================================
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on Mon Aug 10 1998 - 17:57:31 PDT