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Clean up formatting of custom cmap example #23393

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Jul 7, 2022
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Clean up formatting of custom cmap example #23393

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226 changes: 135 additions & 91 deletions 226 examples/color/custom_cmap.py
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@


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.
Expand All @@ -21,51 +21,78 @@
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))}
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")
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
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.::
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.
- 0.75 < ``x`` <= 1, ``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 a cookbook example::

cdict = {
'red': (
(0.0, 0.0, 0.0),
(0.5, 1.0, 0.7),
(1.0, 1.0, 1.0),
),
'green': (
(0.0, 0.0, 0.0),
(0.5, 1.0, 0.0),
(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),
)
}

and look at ``cdict['red'][1]``; 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.
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.

"""
import numpy as np
Expand All @@ -82,7 +109,8 @@


###############################################################################
# --- Colormaps from a list ---
# 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
Expand All @@ -99,60 +127,79 @@


###############################################################################
# --- 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))
}
# 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,
'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)),
}
cdict4 = {
**cdict3,
'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),
),
}


###############################################################################
Expand All @@ -173,13 +220,11 @@
mpl.colormaps.register(LinearSegmentedColormap('BlueRedAlpha', cdict4))

###############################################################################
# Make the figure:
# Make the figure, with 4 subplots:

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)

# Make 4 subplots:

im1 = axs[0, 0].imshow(Z, cmap=blue_red1)
fig.colorbar(im1, ax=axs[0, 0])

Expand Down Expand Up @@ -213,7 +258,6 @@
# colorbar after they have been plotted.
im4.set_cmap('BlueRedAlpha')
axs[1, 1].set_title("Varying alpha")
#

fig.suptitle('Custom Blue-Red colormaps', fontsize=16)
fig.subplots_adjust(top=0.9)
Expand Down
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