How to decode color mapping in matplotlib’s Colormap?

by

Inverting the colormapping is possible, if
(a) you know the data range it is mapping and
(b) if you know the colormap that has been used, and
(c) if the colormap is unambiguous.

The following function would return the value given a color, a colormap and the range over which the colormap has been used.

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

def get_value_from_cm(color, cmap, colrange=[0.,1.]):
    color=matplotlib.colors.to_rgb(color)
    r = np.linspace(colrange[0],colrange[1], 256)
    norm = matplotlib.colors.Normalize(colrange[0],colrange[1])
    mapvals = cmap(norm(r))[:,:3]
    distance = np.sum((mapvals - color)**2, axis=1)
    return r[np.argmin(distance)]


b = get_value_from_cm(plt.cm.coolwarm(0.5), plt.cm.coolwarm, [0.,1.])
c = get_value_from_cm(np.array([1,0,0]), plt.cm.coolwarm)

print b                   # 0.501960784314
print plt.cm.coolwarm(b)
# (0.86742763508627452, 0.86437659977254899, 0.86260246201960789, 1.0)
print plt.cm.coolwarm(0.5)
#(0.86742763508627452, 0.86437659977254899, 0.86260246201960789, 1.0)

Note that this method involves an error, so you only get the closest value from the colormap and not the value that has initially been used to create the color from the map.

In the updated code from the question, you have the color defined as integers between 0 and 255 for each channel. You therefore need to first map those to the range 0 to 1.

from PIL import Image
import numpy as np
import matplotlib
import matplotlib.cm as cm
values = [670, 894, 582, 103, 786, 348, 972, 718, 356, 692]
minima = 103
maxima = 972
norm = matplotlib.colors.Normalize(vmin=minima, vmax=maxima, clip=True)
mapper = cm.ScalarMappable(norm=norm, cmap=cm.gist_rainbow_r)
c = []
for i in range(10):
    c.append(mapper.to_rgba(values[i], bytes=True))
print(c) # [(75, 255, 0, 255), (255, 77, 0, 255), (0, 255, 64, 255), (255, 0, 191, 255), (255, 250, 0, 255), (0, 72, 255, 255), (255, 0, 40, 255), (151, 255, 0, 255), (0, 83, 255, 255), (108, 255, 0, 255)]

def get_value_from_cm(color, cmap, colrange):
    color = np.array(color)/255. 
    r = np.linspace(colrange[0], colrange[1], 256) 
    norm = matplotlib.colors.Normalize(colrange[0], colrange[1])
    mapvals = cmap(norm(r))[:, :4] # there are 4 channels: r,g,b,a
    distance = np.sum((mapvals - color) ** 2, axis=1)
    return r[np.argmin(distance)]

decoded_colors = []
for i in range(10):
    decoded_colors.append(get_value_from_cm(c[i], cm.gist_rainbow_r, colrange=[minima, maxima]))
print(decoded_colors)

More Related Contents:

  1. Set Colorbar Range in matplotlib
  2. Defining the midpoint of a colormap in matplotlib
  3. Add alpha to an existing matplotlib colormap
  4. Matplotlib Plot Lines with Colors Through Colormap
  5. Change the facecolor of boxplot in pandas
  6. How to pick a new color for each plotted line within a figure
  7. How do you change the size of figures drawn with Matplotlib?
  8. Matplotlib scatter plot with different text at each data point
  9. Scatter plots in Pandas/Pyplot: How to plot by category [duplicate]
  10. Editing the date formatting of x-axis tick labels in matplotlib
  11. Pandas timeseries plot setting x-axis major and minor ticks and labels
  12. Matplotlib fill between multiple lines
  13. Improve subplot size/spacing with many subplots in matplotlib
  14. How to draw intersecting planes?
  15. Horizontal stacked bar plot and add labels to each section
  16. How to change the datetime tick label frequency for matplotlib plots
  17. Wrapping long y labels in matplotlib tight layout using setp
  18. Add margin when plots run against the edge of the graph
  19. ubuntu 14.04, pip cannot upgrade matplotllib
  20. Resampling irregularly spaced data to a regular grid in Python
  21. how to annotate heatmap with text in matplotlib
  22. How to move the y axis scale factor to the position next to the y axis label?
  23. Animation with pcolormesh routine in matplotlib, how do I initialize the data?
  24. 2D and 3D Scatter Histograms from arrays in Python
  25. Seaborn Catplot set values over the bars
  26. Matplotlib figure facecolor (background color)
  27. Is there a matplotlib flowable for ReportLab?
  28. No outlines on bins of Matplotlib histograms or Seaborn distplots
  29. Placing Custom Images in a Plot Window–as custom data markers or to annotate those markers
  30. pylab/networkx; no node labels displayed after update

Leave a Comment