Re: [Plib-users] R: culling problems w/ off-axis projection

[Fay, J. F Dr C. U. A. 46 S. <joh...@eg...>]

Paolo,

	Congratulations and very good on having solved the problem.  Is there a code change that you want me to put into Subversion?

Dr. John F. Fay
Technical Fellow
Jacobs Technology TEAS Group
850-883-1294


-----Original Message-----
From: Paolo Leoncini [mailto:p.l...@ci...] 
Sent: Monday, April 12, 2010 3:21 AM
To: 'PLIB Users'
Subject: [Plib-users] R: culling problems w/ off-axis projection

Dear friends,

I think to have solved the problem.

> -----Messaggio originale-----
> Da: p.l...@ci... [mailto:p.l...@ci...] 
> Inviato: martedì 6 aprile 2010 11.19
> A: pli...@li...
> Oggetto: [Plib-users] culling problems w/ off-axis projection
> 
> Dear all,
> 
> I'm doing off-axis projection (the most general form of 
> projection) by translating the four corners of the screen 
> into a projection matrix, for which I added a new method to 
> the sgFrustum class and to the Context too. The only 
> difference with giving the six frustum extrema values is that 
> no calculation occurs. Culling seems to rely only on the 
> projection matrix values, so, apparently, no difference w/ 
> the on-axis standard case.
> 
> As far as the projection is not far from the on-axis case the 
> culling gives no problem. Instead when screen position 
> definition results in a large rotation in the projection 
> matrix objects suddenly disappear and reappear by apparently no rule.
> 
> Are culling calculations based on a particular form of the 
> projection matrix?
> should culling planes be extracted in a different way to 
> support the more general case?

In sgFrustum::contains( const sgSphere *s ) (sg.cxx), the for the non-ortho
case which is presently done this way:

    sp1 = plane[ SG_LEFT_PLANE  ][0] * center[0] + plane[ SG_LEFT_PLANE
][2] * center[2] ;
    sp2 = plane[ SG_RIGHT_PLANE ][0] * center[0] + plane[ SG_RIGHT_PLANE
][2] * center[2] ;
    sp3 = plane[ SG_BOT_PLANE   ][1] * center[1] + plane[ SG_BOT_PLANE
][2] * center[2] ;
    sp4 = plane[ SG_TOP_PLANE   ][1] * center[1] + plane[ SG_TOP_PLANE
][2] * center[2] ;

should be substituted by the following general form:

    sp1 = sgScalarProductVec3 ( plane[ SG_LEFT_PLANE  ], center ) + plane[
SG_LEFT_PLANE  ][3] ;
    sp2 = sgScalarProductVec3 ( plane[ SG_RIGHT_PLANE ], center ) + plane[
SG_RIGHT_PLANE ][3] ;
    sp3 = sgScalarProductVec3 ( plane[ SG_BOT_PLANE   ], center ) + plane[
SG_BOT_PLANE   ][3] ;
    sp4 = sgScalarProductVec3 ( plane[ SG_TOP_PLANE   ], center ) + plane[
SG_TOP_PLANE   ][3] ;

The hint were just below there (see comment "Note: in the general case, we
would have to do:").
I propose to patch it since this general form also manages the default
on-axis case, but Steve should definitely give the go-ahead.
Should someone be concerned about performances I could supply a bit larger
patch for introducing the ability to supply the Context a general
perspective matrix, so to have the general case only calculated in this
eventuality.

> Thanks a lot,
> 
> Paolo Leoncini

Thanks in advance to a good will updater (John, it's you as usual?).

Paolo Leoncini


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