How to Do Histogram Matching Using OpenCV

In this tutorial, you will learn how to do histogram matching using OpenCV. Histogram matching (also known as histogram specification), is the transformation of an image so that its histogram matches the histogram of an image of your choice (we’ll call this image of your choice the “reference image”).

For example, consider this image below.

aspens_in_fall

We want the image above to match the histogram of the reference image below.

forest-resized

After performing histogram matching, the output image needs to look like this:

aspens_in_fall_forest_output

Then, to make things interesting, we want to use this mask to mask the output image.

mask
Mask
aspens_in_fall_forest_output_masked
Masked output image

You Will Need

Directions

Below is the source code for the program that makes everything happen. Make sure you copy and paste this code into a single Python file (mine is named histogram_matching.py). Then put that file, as well as your source, reference, and mask images all in the same directory (or folder) in your computer. Once you have done that, run the code using the following command (note: mask image is optional):

python histogram_matching.py <source_image> <ref_image> [<mask_image>]

For example (put this command all on one line):

python histogram_matching.py aspens_in_fall.jpg forest_resized.jpg mask.jpg

Source Code

#!/usr/bin/env python

'''
Welcome to the Histogram Matching Program!

Given a source image and a reference image, this program
returns a modified version of the source image that matches
the histogram of the reference image.

Image Requirements:
  - Source image must be color.
  - Reference image must be color.
  - The sizes of the source image and reference image do not
    have to be the same.
  - The program supports an optional third image (mask) as
    an argument.
  - When the mask image is provided, it will be rescaled to
    be the same size as the source image, and the resulting
    matched image will be masked by the mask image.

Usage:
  python histogram_matching.py <source_image> <ref_image> [<mask_image>]
'''

# Python 2/3 compatibility
from __future__ import print_function

import cv2 # Import the OpenCV library
import numpy as np # Import Numpy library
import matplotlib.pyplot as plt # Import matplotlib functionality
import sys # Enables the passing of arguments

# Project: Histogram Matching Using OpenCV
# Author: Addison Sears-Collins
# Date created: 9/27/2019
# Python version: 3.7

# Define the file name of the images
SOURCE_IMAGE = "aspens_in_fall.jpg"
REFERENCE_IMAGE = "forest_resized.jpg"
MASK_IMAGE = "mask.jpg"
OUTPUT_IMAGE = "aspens_in_fall_forest_output"
OUTPUT_MASKED_IMAGE = "aspens_in_fall_forest_output_masked.jpg"

def calculate_cdf(histogram):
    """
    This method calculates the cumulative distribution function
    :param array histogram: The values of the histogram
    :return: normalized_cdf: The normalized cumulative distribution function
    :rtype: array
    """
    # Get the cumulative sum of the elements
    cdf = histogram.cumsum()

    # Normalize the cdf
    normalized_cdf = cdf / float(cdf.max())

    return normalized_cdf

def calculate_lookup(src_cdf, ref_cdf):
    """
    This method creates the lookup table
    :param array src_cdf: The cdf for the source image
    :param array ref_cdf: The cdf for the reference image
    :return: lookup_table: The lookup table
    :rtype: array
    """
    lookup_table = np.zeros(256)
    lookup_val = 0
    for src_pixel_val in range(len(src_cdf)):
        lookup_val
        for ref_pixel_val in range(len(ref_cdf)):
            if ref_cdf[ref_pixel_val] >= src_cdf[src_pixel_val]:
                lookup_val = ref_pixel_val
                break
        lookup_table[src_pixel_val] = lookup_val
    return lookup_table

def match_histograms(src_image, ref_image):
    """
    This method matches the source image histogram to the
    reference signal
    :param image src_image: The original source image
    :param image  ref_image: The reference image
    :return: image_after_matching
    :rtype: image (array)
    """
    # Split the images into the different color channels
    # b means blue, g means green and r means red
    src_b, src_g, src_r = cv2.split(src_image)
    ref_b, ref_g, ref_r = cv2.split(ref_image)

    # Compute the b, g, and r histograms separately
    # The flatten() Numpy method returns a copy of the array c
    # collapsed into one dimension.
    src_hist_blue, bin_0 = np.histogram(src_b.flatten(), 256, [0,256])
    src_hist_green, bin_1 = np.histogram(src_g.flatten(), 256, [0,256])
    src_hist_red, bin_2 = np.histogram(src_r.flatten(), 256, [0,256])    
    ref_hist_blue, bin_3 = np.histogram(ref_b.flatten(), 256, [0,256])    
    ref_hist_green, bin_4 = np.histogram(ref_g.flatten(), 256, [0,256])
    ref_hist_red, bin_5 = np.histogram(ref_r.flatten(), 256, [0,256])

    # Compute the normalized cdf for the source and reference image
    src_cdf_blue = calculate_cdf(src_hist_blue)
    src_cdf_green = calculate_cdf(src_hist_green)
    src_cdf_red = calculate_cdf(src_hist_red)
    ref_cdf_blue = calculate_cdf(ref_hist_blue)
    ref_cdf_green = calculate_cdf(ref_hist_green)
    ref_cdf_red = calculate_cdf(ref_hist_red)

    # Make a separate lookup table for each color
    blue_lookup_table = calculate_lookup(src_cdf_blue, ref_cdf_blue)
    green_lookup_table = calculate_lookup(src_cdf_green, ref_cdf_green)
    red_lookup_table = calculate_lookup(src_cdf_red, ref_cdf_red)

    # Use the lookup function to transform the colors of the original
    # source image
    blue_after_transform = cv2.LUT(src_b, blue_lookup_table)
    green_after_transform = cv2.LUT(src_g, green_lookup_table)
    red_after_transform = cv2.LUT(src_r, red_lookup_table)

    # Put the image back together
    image_after_matching = cv2.merge([
        blue_after_transform, green_after_transform, red_after_transform])
    image_after_matching = cv2.convertScaleAbs(image_after_matching)

    return image_after_matching

def mask_image(image, mask):
    """
    This method overlays a mask on top of an image
    :param image image: The color image that you want to mask
    :param image mask: The mask
    :return: masked_image
    :rtype: image (array)
    """

    # Split the colors into the different color channels
    blue_color, green_color, red_color = cv2.split(image)

    # Resize the mask to be the same size as the source image
    resized_mask = cv2.resize(
        mask, (image.shape[1], image.shape[0]), cv2.INTER_NEAREST)

    # Normalize the mask
    normalized_resized_mask = resized_mask / float(255)

    # Scale the color values
    blue_color = blue_color * normalized_resized_mask
    blue_color = blue_color.astype(int)
    green_color = green_color * normalized_resized_mask
    green_color = green_color.astype(int)
    red_color = red_color * normalized_resized_mask
    red_color = red_color.astype(int)

    # Put the image back together again
    merged_image = cv2.merge([blue_color, green_color, red_color])
    masked_image = cv2.convertScaleAbs(merged_image)
    return masked_image

def main():
    """
    Main method of the program.
    """
    start_the_program = input("Press ENTER to perform histogram matching...") 

    # A flag to indicate if the mask image was provided or not by the user
    mask_provided = False

    # Pull system arguments
    try:
        image_src_name = sys.argv[1]
        image_ref_name = sys.argv[2]
    except:
        image_src_name = SOURCE_IMAGE
        image_ref_name = REFERENCE_IMAGE

    try:
        image_mask_name = sys.argv[3]
        mask_provided = True
    except:
        print("\nNote: A mask was not provided.\n")

    # Load the images and store them into a variable
    image_src = cv2.imread(cv2.samples.findFile(image_src_name))
    image_ref = cv2.imread(cv2.samples.findFile(image_ref_name))

    image_mask = None
    if mask_provided:
        image_mask = cv2.imread(cv2.samples.findFile(image_mask_name))

    # Check if the images loaded properly
    if image_src is None:
        print('Failed to load source image file:', image_src_name)
        sys.exit(1)
    elif image_ref is None:
        print('Failed to load reference image file:', image_ref_name)
        sys.exit(1)
    else:
        # Do nothing
        pass 

    # Convert the image mask to grayscale
    if mask_provided:
        image_mask = cv2.cvtColor(image_mask, cv2.COLOR_BGR2GRAY)
       
    # Calculate the matched image
    output_image = match_histograms(image_src, image_ref)

    # Mask the matched image
    if mask_provided:
        output_masked = mask_image(output_image, image_mask)

    # Save the output images
    cv2.imwrite(OUTPUT_IMAGE, output_image)
    if mask_provided:
        cv2.imwrite(OUTPUT_MASKED_IMAGE, output_masked)
  
    ## Display images, used for debugging
    cv2.imshow('Source Image', image_src)
    cv2.imshow('Reference Image', image_ref)
    cv2.imshow('Output Image', output_image)
    if mask_provided:
        cv2.imshow('Mask', image_mask)
        cv2.imshow('Output Image (Masked)', output_masked)

    cv2.waitKey(0) # Wait for a keyboard event

if __name__ == '__main__':
    print(__doc__)
    main()
    cv2.destroyAllWindows()

Sample Output

histogram-matching-1

What is the Difference Between Mathematical Morphology Filters and Convolution Filters?

What is the Difference Between Mathematical Morphology Filters and Convolution Filters?

Answer: Linearity

Convolution filters generate output images in which the brightness value at a particular pixel depends on the weighted sum (i.e. linear combination) of the brightness of the neighboring pixels.

Mathematical morphology filters on the other hand perform nonlinear processing on images. These filters depend only on the relative ordering of pixel values as opposed to their numerical values. This property of mathematical morphology filters makes them really good when applied to binary images (a digital image that can only have two possible values for each pixel).

Types of Convolution and Mathematical Morphology Filters

This page at Harrisgeospatial.com has a good overview of the different convolution filters and morphology filters.

The standard convolution filters are:

  • High Pass
  • Low Pass
  • Laplacian
  • Directional
  • Gaussian Low Pass
  • Gaussian High Pass
  • Median 
  • Sobel
  • Roberts 
  • User-Defined Convolution

The standard mathematical morphology filters are:

  • Dilation
  • Erosion
  • Opening
  • Closing

Noise Reduction Using Mathematical Morphology vs. Convolution Filters

Someone asked me this question the other day: What are the benefits and limitations of applying an image processing application such as noise reduction using mathematical morphology vs. convolution?

Before we get into the pros and cons of mathematical morphology and convolutions filters applied to noise reduction in images, let us take a look at the definitions of these terms.

Mathematical morphology is an image processing technique based on two operations: erosion and dilation. Erosion enlarges objects in an image, while dilation shrinks objects in an image.

Convolution filtering involves taking an image as input and generating an output image where each new pixel value is determined by the weighted values of itself and its neighboring pixels.

Noise reduction involves “cleaning up” an image. The goal is to take an image as input and get rid of all the unnecessary elements in that image so that it looks better.

Below are the pros and cons of doing noise reduction using mathematical morphology vs. convolution filters.

Mathematical Morphology

Pros

  • Simplicity from a theoretical perspective (it is based on basic set theory) 
  • Simplicity from an operational perspective (can be implemented with a few lines of code: https://docs.opencv.org/2.4/doc/tutorials/imgproc/erosion_dilatation/erosion_dilatation.html)
  • Computationally efficient
  • Useful for removing noise in grayscale images
  • Useful for detaching two objects that are connected together (erosion)
  • Useful for connecting an object that is broken apart in an image (dilation)
  • Can remove noise without substantially altering the underlying shape of an object

Cons

Convolution Filters

Pros

Cons

  • More complicated from an operational perspective (so many techniques and kernels to choose from…how does one decide which one is best?)
  • Can remove important image gradients because filter output is proportional to the contrast of a given section of an image
  • Shape of an object can become altered or distorted