Test for curve/straight line

I'm trying to test if a segment of a polyline is a curve or a straight section using the following code. Could someone explain to me why this sometimes returns nil on a straight section? When evaluated the difference is small (e.g. 2.84217e-014) but it doesn't evaluate to zero.

(zerop (- (abs (distance (vlax-curve-getPointAtParam obj (1- idx))(vlax-curve-getPointAtParam obj idx)))
           (abs (- (vlax-curve-getDistAtParam obj (1- idx))(vlax-curve-getDistAtParam obj idx)))
           )
        )

Although someone may say differently I would use the vlax-curve-getSecondDeriv for this exercise

A return value of 0 indicates a straight segment, anything else is a curve

(angle
 (vlax-curve-getSecondDeriv obj idx)
 (list 0 0 0))

Its been a while since I've used calculus thou.. For a bit more info see

http://en.wikipedia.org/wiki/Second_derivative

Nice one Jammie

With regard to general Floating-Point Calculations 2.84217e-014 is below the tolerance level so should be considered zero. See the link attached to Post # 7 here:

http://www.cadtutor.net/forum/showthread.php?t=43500

Using vlax-curve-getSecondDeriv does sound like a sensible course of action, but that result, too, may need to be compared to a tolerance if the return is not an explicit 0.

Perhaps use:

(equal
 (angle '(0 0 0)
   (vlax-curve-getSecondDeriv obj idx)) 0.0 1e-

Nice one Jammie

Thanks Lee

Perhaps use:

 

(equal
 (angle '(0 0 0)
   (vlax-curve-getSecondDeriv obj idx)) 0.0 1e-

That looks pretty good.

Thanks guys...

I guess instead of (zerop I could use (equal with a fuzz factor to compare the two equations in my example.

But I'll try the SecondDeriv.