parallel lines

If a vector cross product zero , the vectors are parallel right?
But what's a good value for messuring that the product is zero?

If (cx>0.00001) And (cy>0.00001) And (cz>0.00001) ?
Short and long vectors will not have the same 'parallellity tolerance' :)

Should I calculate the magnitude first and do something like:

If (cx>0.00001*mag) And (cy>0.00001*mag) And (cz>0.00001*mag) ?

But the magnitude would be close to or zero also, so I'd have to check that against another threshold first. What threshold?

you'd need abs(cx) wouldnt you? to capture small -tive values

I haven't really tested this extensively, but if I were trying to find parallel lines I'd do something like this:

thx, but I'm looking for cross product test.
I got another suggestion on IRC on normalising the cross vectors first. That seems like a good idea, to get an even compare.

Meanwhile I coded myself out of needing to test for parallel lines :)

A ratio test is faster and simpler. A cross product test has a cool, clever sounding name. Both do the same job. Take your pick.