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View Full Version : Fastest way to determin which side of a triangle



NicholasFrancis
01-02-2002, 01:52 PM
I have a real dump question... Here goes:

What is the fastest way to determine if a point lies on one side or the other of a triangle (for shadow volume backface culling)?

Any help would be appreciated...

Nich.

ScottManDeath
01-02-2002, 02:08 PM
Hi

I would use following:

prerequisite:
triangles vertices
t[0]
t[1]
t[2]
point
p
build two vector from the triangles

a=t[1]-t[0]
b=t[2]-t[0]

cross product a x b = n
(be aware of order)

build vector from p to one triangle vertex

c=p-t[0]

dot product c with n

switch(c dot n)
case 0:
p lies on plane containing triangle
case < 0:
p lies on the side the normal is not pointg to
case >0:
p lies on the side normal is pointing to

i hope its correct and useful

bye
ScottManDeath

NicholasFrancis
01-02-2002, 02:19 PM
Cool. Exactly what I was looking for. I've already got the surface normal, so I don't know why I didn't see this one.

THNX.