In [5]:
# Visualize the input image histogram
histogram(img_float, bins=2**8, interval=[0, 1])
Histogram equalization concept and algorithm applied to digital image color processing.
%matplotlib inline
import matplotlib as mpl
import matplotlib.pyplot as plt
import numpy as np
import imageio
from _utils import *
Histogram equalization of a grayscale images $x$ (single channel) considers the probability density of the gray levels $i$, defined by:
$$ \large p_x(i)=p(x=i)=\frac{n_i}{n}, \quad 0 \leq i \leq L $$where:
Considering that we have the ordered probability density $p_x(i)$, the equalization is given by the cumulative distribution function, defined by:
$$ \large cdf_x(i)=\sum_{j=0}^{i}p_x(j) $$The motivation comes from the continuous space concept of CDF, which could be understanded by:
$$ CDF_X(x)=\int_{-\infty}^{x}p_x(t)dt $$# Read grayscale image
img_float = imageio.imread('../_data/ship.png')
summary(img_float)
min: 0.0000
1st Quar: 113.0000
median: 143.0000
mean: 129.7080
3rd Quar: 157.0000
max: 255.0000
sigma: 46.6772
# Change range from [0-255] to [0.0-1.0]
img_float = img_float/(2**8 - 1)
summary(img_float)
min: 0.0000
1st Quar: 0.4431
median: 0.5608
mean: 0.5087
3rd Quar: 0.6157
max: 1.0000
sigma: 0.1830
# Visualize the input image histogram
histogram(img_float, bins=2**8, interval=[0, 1])
One easier and more performatic way to do the process of histogram equalization is by ordering the gray values and finding its sorted indices, instead of finding the probability distribution and do the cummulative sum of those values. At the end of this process, it is necessary to normalize the output value by the total number of pixels. The result is exactly the same.
def eqHist1(matrix_in):
# Read input grayscale image
matrix_in = matrix_in.copy()
N1, N2 = matrix_in.shape
# Flat image to be sorted
flat = matrix_in.ravel()
# Sort the pixels
sort = np.sort(flat)
# Find the sorted index for each gray value
search = sort.searchsorted(matrix_in)
# Normalize the output
norm = search/(N1*N2 - 1)
return norm
img_float_eq = eqHist1(img_float)
# Visualized the histogram of the equalized image
histogram(img_float_eq, bins=2**8, interval=[0, 1])
img_rgb = imageio.imread('../_data/aerial03.png')/(2**8 - 1)
summary(img_rgb)
R G B
min: 0.0000 0.0000 0.0000
1st Quar: 0.4784 0.4510 0.4314
median: 0.5961 0.6235 0.5765
mean: 0.5893 0.5948 0.5625
3rd Quar: 0.7176 0.7373 0.6941
max: 1.0000 0.9529 0.9608
sigma: 0.1530 0.1813 0.1715
histogram(img_rgb, bins=2**8, interval=[0, 1])
Applies histogram equalization, stretching each channel separately. This process does not preserve the hue distribution what it means that the colors might be changed.
def eqHist3(img_in):
img_in = img_in.copy()
# Split channels
R = img_in[:,:,0]
G = img_in[:,:,1]
B = img_in[:,:,2]
N1, N2, _ = img_in.shape
# Sort channels
Rs = np.sort(R.ravel())
Gs = np.sort(G.ravel())
Bs = np.sort(B.ravel())
# Find sorted indices
R[:,:] = Rs.searchsorted(R)
G[:,:] = Gs.searchsorted(G)
B[:,:] = Bs.searchsorted(B)
# Return normalized result
return img_in/(N1*N2 - 1)
img_rgb_eq = eqHist3(img_rgb)
summary(img_rgb_eq)
R G B
min: 0.0000 0.0000 0.0000
1st Quar: 0.2479 0.2468 0.2486
median: 0.4950 0.4963 0.4984
mean: 0.4963 0.4969 0.4969
3rd Quar: 0.7495 0.7435 0.7485
max: 0.9999 1.0000 1.0000
sigma: 0.2882 0.2883 0.2885
%%time
histogram(img_rgb_eq, bins=2**8, interval=[0, 1])
Wall time: 1.24 s
Applies histogram equalization preserving the hue distribution. This process can affect the saturation value or not.
def eqHist3hsv(img_in, saturation=False):
img_rgb = img_in.copy()
# Convert from RGB to HSV
img_hsv = mpl.colors.rgb_to_hsv(img_rgb)
N1, N2, _ = img_hsv.shape
if saturation:
# If saturation is true
# stretches the saturation component
S = img_hsv[:,:,1]
Ss = np.sort(S.ravel())
S[:,:] = Ss.searchsorted(S)/(N1*N2 - 1)
# Stretches the value component
V = img_hsv[:,:,2]
Vs = np.sort(V.ravel())
V[:,:] = Vs.searchsorted(V)/(N1*N2 - 1)
return mpl.colors.hsv_to_rgb(img_hsv)
img_hsv_eq = eqHist3hsv(img_rgb)
summary(img_hsv_eq)
R G B
min: 0.0000 0.0000 0.0000
1st Quar: 0.2166 0.2020 0.1797
median: 0.4374 0.4604 0.4194
mean: 0.4540 0.4691 0.4386
3rd Quar: 0.6933 0.7225 0.6710
max: 0.9999 0.9998 0.9998
sigma: 0.2753 0.2953 0.2821
%%time
histogram(img_hsv_eq, bins=2**8, interval=[0, 1])