With floating point numbers you usually don't want to check for zero.
Usually you want to check if fabs( number ) < epsilon (where epsilon is some
really small number.) This is more stable since inside the computer, a
floating point number is just an approximation ... 1/10 for instance becomes
a repeating decimal in it's IEEE binary floating point representation and
thus get's clipped off. So you can't actually represent 0.1 exactly as a
floating point, but you do get something that's *very* close.
Also notice that you need to use fabs() to return the absolute value of a
floating point number. abs() is an integer function and returns and integer
result ... common mistake ... and has caused me more than a little head
scratching on a couple occasions.
Regards,
Curt.
On Wed, Apr 22, 2009 at 6:57 PM, Patrice Poly <p.pol...@gmail.com> wrote:
> Le Wednesday 22 April 2009 19:20:54 Martin Spott, vous avez écrit :
> > Curtis Olson wrote:
> > > I'm seeing a ton of these nan's when I start at KHAF ... it's just one
> or
> > > two or three per frame, but that ends up spewing an awful lot of extra
> > > text to my console.
> >
> > That's weird, at least using the ASK 21 I don't see a single one - on
> > Linux/AMD64 with GCC-4.3.2,
> >
> > Martin.
>
> Here on OpenSuse 11.0, 64 bits , core2quad, and GCC 4.3.1, I don't have a
> single one, with any plane, anywhere.
>
> I suspect the Nans to have the following probable origins:
>
> **get_elevation_m doesn't hit the ground at all, ( but then its said to
> return
> the sea level )
>
> **a division by zero occurs somewhere here:
>
> for (int i = 0; i <= 4; i++)
> {
> probe_lat_rad[i] =
> asin(sin(deg2rad*user_latitude_deg)*cos(dist_probe_m[i]/earth_rad_m)
>
>
> +cos(deg2rad*user_latitude_deg)*sin(dist_probe_m[i]/earth_rad_m)*cos(ground_wind_from_rad));
>
> if (probe_lat_rad[i] == 0.0) {
> probe_lon_rad[i] = (deg2rad*user_latitude_deg);
> // probe on a pole
> }
> else {
> probe_lon_rad[i] =
> fmod((deg2rad*user_longitude_deg+asin(sin(ground_wind_from_rad)
>
> *sin(dist_probe_m[i]/earth_rad_m)/cos(probe_lon_rad[i]))+PI)
> ,(2.0*PI))-PI;
> }
> probe_lat_deg[i]= rad2deg*probe_lat_rad[i];
> probe_lon_deg[i]= rad2deg*probe_lon_rad[i];
> }
>
> but I thought checking for poles would prevent this ?
>
> ** divisions by zero here ?
>
> slope[0] = (probe_elev_m[0] - probe_elev_m[1]) / dist_probe_m[1];
> slope[1] = (probe_elev_m[1] - probe_elev_m[2]) / dist_probe_m[2];
> slope[2] = (probe_elev_m[2] - probe_elev_m[3]) / dist_probe_m[3];
> slope[3] = (probe_elev_m[4] - probe_elev_m[0]) / -dist_probe_m[4];
>
> unlikely, as these are initialized with good, positive
> values
>
> ** division by zero again ?
>
> else
> {
> agl_factor = exp(-(2 + 2 * probe_elev_m[0] / 4000) *
> (user_altitude_agl_m - BOUNDARY2_m) /
> max(probe_elev_m[0],200.0));
> }
>
> I'm a bit at lost for now, as I really don't see any NaN anymore here, and
> don't have another platform to test on.
>
> I would be interested to see on which platforms / configuration this
> happens,
> maybe when more feedback comes in ??
>
>
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--
Curtis Olson: http://baron.flightgear.org/~curt/
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