cmaps.py

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import numpy as np
import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap

"""
Creating a colormap from a list of colors
-----------------------------------------
Creating a colormap from a list of colors can be done with the `from_list`
method of `LinearSegmentedColormap`. You must pass a list of RGB tuples that
define the mixture of colors from 0 to 1.


Creating custom colormaps
-------------------------
It is also possible to create a custom mapping for a colormap. This is
accomplished by creating dictionary that specifies how the RGB channels
change from one end of the cmap to the other.

Example: suppose you want red to increase from 0 to 1 over the bottom
half, green to do the same over the middle half, and blue over the top
half.  Then you would use:

cdict = {'red':   ((0.0,  0.0, 0.0),
                   (0.5,  1.0, 1.0),
                   (1.0,  1.0, 1.0)),

         'green': ((0.0,  0.0, 0.0),
                   (0.25, 0.0, 0.0),
                   (0.75, 1.0, 1.0),
                   (1.0,  1.0, 1.0)),

         'blue':  ((0.0,  0.0, 0.0),
                   (0.5,  0.0, 0.0),
                   (1.0,  1.0, 1.0))}

If, as in this example, there are no discontinuities in the r, g, and b
components, then it is quite simple: the second and third element of
each tuple, above, is the same--call it "y".  The first element ("x")
defines interpolation intervals over the full range of 0 to 1, and it
must span that whole range.  In other words, the values of x divide the
0-to-1 range into a set of segments, and y gives the end-point color
values for each segment.

Now consider the green. cdict['green'] is saying that for
0 <= x <= 0.25, y is zero; no green.
0.25 < x <= 0.75, y varies linearly from 0 to 1.
x > 0.75, y remains at 1, full green.

If there are discontinuities, then it is a little more complicated.
Label the 3 elements in each row in the cdict entry for a given color as
(x, y0, y1).  Then for values of x between x[i] and x[i+1] the color
value is interpolated between y1[i] and y0[i+1].

Going back to the cookbook example, look at cdict['red']; because y0 !=
y1, it is saying that for x from 0 to 0.5, red increases from 0 to 1,
but then it jumps down, so that for x from 0.5 to 1, red increases from
0.7 to 1.  Green ramps from 0 to 1 as x goes from 0 to 0.5, then jumps
back to 0, and ramps back to 1 as x goes from 0.5 to 1.

row i:   x  y0  y1
                /
               /
row i+1: x  y0  y1

Above is an attempt to show that for x in the range x[i] to x[i+1], the
interpolation is between y1[i] and y0[i+1].  So, y0[0] and y1[-1] are
never used.

"""
# Make some illustrative fake data:

x = np.arange(0, np.pi, 0.1)
y = np.arange(0, 2*np.pi, 0.1)
X, Y = np.meshgrid(x, y)
Z = np.cos(X) * np.sin(Y) * 10


# --- Colormaps from a list ---

colors = [(1, 0, 0), (0, 1, 0), (0, 0, 1)]  # R -> G -> B
n_bins = [3, 6, 10, 100]  # Discretizes the interpolation into bins
cmap_name = 'my_list'
fig, axs = plt.subplots(2, 2, figsize=(6, 9))
fig.subplots_adjust(left=0.02, bottom=0.06, right=0.95, top=0.94, wspace=0.05)
for n_bin, ax in zip(n_bins, axs.ravel()):
    # Create the colormap
    cm = LinearSegmentedColormap.from_list(
        cmap_name, colors, N=n_bin)
    # Fewer bins will result in "coarser" colomap interpolation
    im = ax.imshow(Z, interpolation='nearest', origin='lower', cmap=cm)
    ax.set_title("N bins: %s" % n_bin)
    fig.colorbar(im, ax=ax)


# --- Custom colormaps ---

cdict1 = {'red':   ((0.0, 0.0, 0.0),
                   (0.5, 0.0, 0.1),
                   (1.0, 1.0, 1.0)),

         'green': ((0.0, 0.0, 0.0),
                   (1.0, 0.0, 0.0)),

         'blue':  ((0.0, 0.0, 1.0),
                   (0.5, 0.1, 0.0),
                   (1.0, 0.0, 0.0))
        }

cdict2 = {'red':   ((0.0, 0.0, 0.0),
                   (0.5, 0.0, 1.0),
                   (1.0, 0.1, 1.0)),

         'green': ((0.0, 0.0, 0.0),
                   (1.0, 0.0, 0.0)),

         'blue':  ((0.0, 0.0, 0.1),
                   (0.5, 1.0, 0.0),
                   (1.0, 0.0, 0.0))
        }

cdict3 = {'red':  ((0.0, 0.0, 0.0),
                   (0.25, 0.0, 0.0),
                   (0.5, 0.8, 1.0),
                   (0.75, 1.0, 1.0),
                   (1.0, 0.4, 1.0)),

         'green': ((0.0, 0.0, 0.0),
                   (0.25, 0.0, 0.0),
                   (0.5, 0.9, 0.9),
                   (0.75, 0.0, 0.0),
                   (1.0, 0.0, 0.0)),

         'blue':  ((0.0, 0.0, 0.4),
                   (0.25, 1.0, 1.0),
                   (0.5, 1.0, 0.8),
                   (0.75, 0.0, 0.0),
                   (1.0, 0.0, 0.0))
        }

# Make a modified version of cdict3 with some transparency
# in the middle of the range.
cdict4 = cdict3.copy()
cdict4['alpha'] = ((0.0, 1.0, 1.0),
                #   (0.25,1.0, 1.0),
                   (0.5, 0.3, 0.3),
                #   (0.75,1.0, 1.0),
                   (1.0, 1.0, 1.0))


# Now we will use this example to illustrate 3 ways of
# handling custom colormaps.
# First, the most direct and explicit:

blue_red1 = LinearSegmentedColormap('BlueRed1', cdict1)

# Second, create the map explicitly and register it.
# Like the first method, this method works with any kind
# of Colormap, not just
# a LinearSegmentedColormap:

blue_red2 = LinearSegmentedColormap('BlueRed2', cdict2)
plt.register_cmap(cmap=blue_red2)

# Third, for LinearSegmentedColormap only,
# leave everything to register_cmap:

plt.register_cmap(name='BlueRed3', data=cdict3)  # optional lut kwarg
plt.register_cmap(name='BlueRedAlpha', data=cdict4)