[Openexr-devel] ImathBoxAlgo.h intersects() problem

[John Haddon]

Hi all,

I'm encountering some possibly strange behaviour with the intersects() 
function in ImathBoxAlgo.h, in that it seems to be returning 
intersections outside of the box. Running an interactive test with the 
code in a Maya plugin reveals intersections that /are/ on the plane of 
the front face of the box, but aren't actually on the box face.

Comparing the code to the Graphics Gems original, i noticed this section 
near the bottom of the function is different :

IMath :

        if ((quadrant[i] == right && result[i] < minB[i]) ||
                (quadrant[i] == left && result[i] > maxB[i]))
            {
                return false;   /* outside box */
            }


GG :

        if (coord[i] < minB[i] || coord[i] > maxB[i])
                return (FALSE);

(result and coord serve equivalent purposes for each case)

This section does seem to be attempting to throw away the cases that are 
on the right plane but not within the face, and taking out the 
additional quadrant[i]==XX conditional from Imath (making it equivalent 
to the GG form) seems to give the desired result i.e. no more unwanted 
intersections.

I first noticed this when using the box intersections in a tree 
acceleration structure for a mesh intersection test, and the modified 
function seems to produce valid results still, but far fewer tests are 
performed due to throwing away a great deal more bounding box nodes a 
lot earlier.

Could it be that the intersects() code is broken? Or have i missed 
something?

Cheers...

John