Re: [Matplotlib-users] Wind barb bug

Hi Tom,

Thanks for the reply. I will open it as an issue on github.

Best regards,
Jesper

2015-04-25 22:20 GMT+02:00 Thomas Caswell <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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