F-Number axes scale for matplotlib

f_number_axes_scale.py

import numpy as np
import matplotlib.scale as mscale
import matplotlib.transforms as mtransforms
from matplotlib.ticker import Formatter, FixedLocator


class FNumberScale(mscale.ScaleBase):
    name = 'fnumber'

    def __init__(self, axis, **kwargs):
        mscale.ScaleBase.__init__(self)
        self.thresh = None  # thresh

    def get_transform(self):
        return self.FNumberTransform(self.thresh)

    def set_default_locators_and_formatters(self, axis):
        class FNumberFormatter(Formatter):

            def __call__(self, x, pos=None):
                return "%.1f" % x

        # Set major tick at each f-stop
        axis.set_major_locator(FixedLocator(np.sqrt(2**np.arange(-2, 16))))
        # Set minor tick at each half f-stop
        axis.set_minor_locator(FixedLocator(np.sqrt(2**np.arange(-0.5, 14.5))))
        axis.set_major_formatter(FNumberFormatter())

    class FNumberTransform(mtransforms.Transform):
        input_dims = 1
        output_dims = 1
        is_separable = True

        def __init__(self, thresh):
            mtransforms.Transform.__init__(self)
            self.thresh = thresh

        def transform_non_affine(self, a):
            # Inverse of sqrt(2**a)
            return np.log(a**2) / np.log(2)

        def inverted(self):
            return FNumberScale.InvertedFNumberTransform(self.thresh)

    class InvertedFNumberTransform(mtransforms.Transform):
        input_dims = 1
        output_dims = 1
        is_separable = True

        def __init__(self, thresh):
            mtransforms.Transform.__init__(self)
            self.thresh = thresh

        def transform_non_affine(self, a):
            return np.sqrt(2**a)

        def inverted(self):
            return FNumberScale.FNumberTransform(self.thresh)


mscale.register_scale(FNumberScale)