[Ether]

Originally Posted by compwiztobe

I'm pretty sure LabVIEW has two different atan commands in its Math/Trig library (which I believe is supported on the cRio). The first takes a float which should be your y/x value, and then spits out theta on the interval (+pi/2, -pi/2) relative to +X (note the inability to deal with infinite slope, one of the problems avoided by the other command). The other takes an x and a y separately, then does all the logic for you to return theta on the interval [0,2pi) from +X.

Two things:

1) The question was asked in the context of the handling of atan2(0,0) for FRC. The answer, for LabVIEW, is that it returns 0, at least according to the simple test I ran on a desktop PC (not in a cRIO). Not being a LabVIEW guru, I don't know if it sets any error flags which might have side effects elsewhere. Does anyone know what the FRC versions of C++ and Java do when running on cRIO?

2) LabVIEW atan2 does not return [0,2pi). It returns an answer in the range (-pi..pi] (not surprisingly).

Originally Posted by compwiztobe

Take the arc cosine of x,

You are given arbitrary (x,y). So you must take the arc cosine of x/sqrt(x²+y²). Not that it matters for purposes of FRC, but numerically that is inferior to a properly implemented atan2(y,x) (and also suffers from the 0,0 problem). Also, since the sign of y is lost in the squaring operation, additional conditional logic is necessary to get the desired range.