Re: [Matplotlib-users] Wind barb bug

[Thomas C. <tca...@gm...>]

Jesper,

Can you open an issue on this on github.  If you are feeling ambitious a
pull request fixing the bug (as you seem to have a good idea of where the
problem is) would also be great!

Tom

On Fri, Apr 24, 2015 at 8:38 AM Jesper Larsen <jes...@gm...>
wrote:

> Hi Matplotlib Users,
>
> When I make wind barbs with rounding enabled and custom barb increments I
> noticed that there were no wind barbs with half barbs above 2 full barbs.
> The reason seems to be a bug in the _find_tails method. The bug is
> illustrated by this small script (_find_tails is a copy of the one in
> matplotlib):
>
> import numpy as np
>
> def _find_tails(self, mag, rounding=True, half=5, full=10, flag=50):
>     '''
>     Find how many of each of the tail pieces is necessary.  Flag
>     specifies the increment for a flag, barb for a full barb, and half for
>     half a barb. Mag should be the magnitude of a vector (ie. >= 0).
>
>     This returns a tuple of:
>
>         (*number of flags*, *number of barbs*, *half_flag*, *empty_flag*)
>
>     *half_flag* is a boolean whether half of a barb is needed,
>     since there should only ever be one half on a given
>     barb. *empty_flag* flag is an array of flags to easily tell if
>     a barb is empty (too low to plot any barbs/flags.
>     '''
>
>     #If rounding, round to the nearest multiple of half, the smallest
>     #increment
>     if rounding:
>         mag = half * (mag / half + 0.5).astype(np.int)
>
>     num_flags = np.floor(mag / flag).astype(np.int)
>     mag = np.mod(mag, flag)
>
>     num_barb = np.floor(mag / full).astype(np.int)
>     mag = np.mod(mag, full)
>
>     half_flag = mag >= half
>     empty_flag = ~(half_flag | (num_flags > 0) | (num_barb > 0))
>
>     return num_flags, num_barb, half_flag, empty_flag
>
> def main():
>     mag = np.arange(0,21,1)
>     barb_incs = {'half': 2.57222,
>                  'full': 5.14444,
>                  'flag': 25.7222}
>     print 'With rounding'
>     num_flags, num_barb, half_flag, empty_flag =  _find_tails(None, mag,
> rounding=True, **barb_incs)
>     for i in range(len(mag)):
>         print mag[i], num_flags[i], num_barb[i], half_flag[i],
> empty_flag[i]
>     print 'Without rounding'
>     num_flags, num_barb, half_flag, empty_flag =  _find_tails(None, mag,
> rounding=False, **barb_incs)
>     for i in range(len(mag)):
>         print mag[i], num_flags[i], num_barb[i], half_flag[i],
> empty_flag[i]
>
> if __name__ == '__main__':
>     exit(main())
>
> It seems like the error is not present when the barb increments are not
> set. I believe the reason for the bug is the float comparison (half_flag =
> mag >= half) where the value is rounded to a value very close to/identical
> to the 'half' increment. And it seems like python does the right thing when
> the "half" increment is a whole number but not always when it is not.
>
> But in any case the code should probably not depend two floats being equal.
>
> Best regards,
> Jesper
>
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