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31

answers:

1

Does anyone know what the ddx and ddy values do in the AABB ray intersect algorithm? Taken from the following site http://www.blitzbasic.com/codearcs/codearcs.php?code=1029 (show below).

Local txmin#,txmax#,tymin#,tymax#

// rox, rdx are the ray origin on the x axis, and ray delta on the x axis ... y-axis is roy and rdy

Local ddx# =1.0/(rox-rdx)
Local ddy# =1.0/(roy-rdy)

If ddx >= 0
    txmin = (bminx - rox) * ddx
    txmax = (bmaxx - rox) * ddx
Else
    txmin = (bmaxx - rox) * ddx
    txmax = (bminx - rox) * ddx
EndIf

If ddy >= 0
    tymin = (bminy - roy) * ddy
    tymax = (bmaxy - roy) * ddy
Else
    tymin = (bmaxy - roy) * ddy
    tymax = (bminy - roy) * ddy
EndIf

If ( (txmin > tymax) Or (tymin > txmax) ) Return 0

If (tymin > txmin) txmin = tymin
If (tymax < txmax) txmax = tymax

Local tzmin#,tzmax#
Local ddz# =1.0/(roz-rdz)

If ddz >= 0
    tzmin = (bminz - roz) * ddz
    tzmax = (bmaxz - roz) * ddz
Else
    tzmin = (bmaxz - roz) * ddz
    tzmax = (bminz - roz) * ddz
EndIf

If (txmin > tzmax) Or (tzmin > txmax) Return 0

Return 1
A: 

(rox-rdx, roy-rdy) is the vector from the destination point to the origin point of the ray. ddx and ddy are the inverse of those 2 coordinates.

The inversion was used as a precomputation in order to only have to use a multiplication (by those inverses) instead of a division in the rest of the function. Computers compute multiplications faster than divisions.

Vincent