How to display out of range values on image histogram?

2
votes

I want to plot the RGB histograms of an image using numpy.histogram.

(See my function draw_histogram below)

It works well for a regular range of [0, 255] :

import numpy as np
import matplotlib.pyplot as plt

im = plt.imread('Bulbasaur.jpeg')
draw_histogram(im, minimum=0., maximum=255.)

Bulbasaur.jpegHistogram_Bulbasaur

What I want to do :

I expect the images I use to have out of range values. Sometimes they will be out of range, sometimes not. I want to use the RGB histogram to analyse how bad the values are out of range.

Let's say I expect the values to be at worst in the interval [-512, 512]. I still want the histogram to display the in-range intensities at the right spot, and leave blank the unpopulated range sections. For example, if I draw the histogram of Bulbasaur.jpeg again but with range [-512, 512], I expect to see the same histogram but contracted along the "x" axis (between the two dashed lines in the histogram below).

The problem :

When I try to draw the histogram for an unregular range, something goes wrong :

import numpy as np
import matplotlib.pyplot as plt

im = plt.imread('Bulbasaur.jpeg')
draw_histogram(im, minimum=-512., maximum=512.)

enter image description here

My code for draw_histogram() :

def draw_histogram(im, minimum, maximum):

    fig = plt.figure()
    color = ('r','g','b')

    for i, col in enumerate(color):
        hist, bins = np.histogram(im[:, :, i], int(maximum-minimum), (minimum, maximum))
        plt.plot(hist, color=col)
        plt.xlim([int(minimum), int(maximum)])

    # Draw vertical lines to easily locate the 'regular range'
    plt.axvline(x=0, color='k', linestyle='dashed')
    plt.axvline(x=255, color='k', linestyle='dashed')

    plt.savefig('Histogram_Bulbasaur.png')
    plt.close(fig)

    return 0

Question

Does anyone know a way of properly drawing RGB histogram with unregular ranges?

1
pythonnumpyimage-processinghistogramrgb
Deleted comment, realized what the problem was. – c. leather

1 Answers

1
votes

You should pass x values to 'plt.plot'

I changed:

plt.plot(hist, color=col)

to this:

plt.plot(np.arange(minimum,maximum),hist, color=col)

With this change, the graph began to appear normally. Essentially, plt.plot was trying to start plotting the y-values you gave it from np.hist starting at 0. This works when your expected range starts at 0, but when you want to include negative numbers, plt.plot shouldn't start at 0, rather, it should start at minimum, so using np.range to manually assign x values fixes the problem.

enter image description here