python imshow, set certain value to defined color

问题

I have a RGB Image, which I plot with matplotlib.pyplot.imshow and it works fine. But now I want to change the plot, that where the value of the picture is e.g 1, the color of the plot should change to white at all this positions.

Is there a way to do this?

回答1:

Assuming that your image is a single-channel image rather than a three-channel image, the required task can be performed by defining a palette that maps indices (e.g. gray level intensities or picture values) into colors:

import numpy as np
palette = np.array([[  0,   0,   0],   # black
                    [255,   0,   0],   # red
                    [  0, 255,   0],   # green
                    [  0,   0, 255],   # blue
                    [255, 255, 255]])  # white
I = np.array([[ 0, 1, 2, 0],    # 2 rows, 4 columns, 1 channel
              [ 0, 3, 4, 0]])

Image conversion is efficiently accomplished through NumPy's broadcasting:

RGB = palette[I]

And this is how the transformed image looks like:

>>> RGB
array([[[  0,  0,    0],  # 2 rows, 4 columns, 3 channels
        [255,  0,    0],
        [  0, 255,   0],
        [  0,   0,   0]],

       [[  0,   0,   0],
        [  0,   0, 255],
        [255, 255, 255],
        [  0,   0,   0]]])

回答2:

I will answer the general question of how to set a particular value to a particular color regardless of the color map.

In the code below for illustration purposes I supposed that is the value -1 that you want to map white. You will be wanting to do something different for your code.

This technique uses a masked array to set the parts where your data is equal to -1 (the value you wish to map) and then uses cmap.set_bad() to assign the color white to this value.

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

value = -1
data = np.arange(100).reshape((10, 10))
data[5, :] = -1  # Values to set -1

masked_array = np.ma.masked_where(data == value, data)

cmap = matplotlib.cm.spring  # Can be any colormap that you want after the cm
cmap.set_bad(color='white')

plt.imshow(masked_array, cmap=cmap)
plt.show()

Hope it helps.

来源：https://stackoverflow.com/questions/37719304/python-imshow-set-certain-value-to-defined-color

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python

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colors

imshow