# Experiments with images #3 – Pixel frequency plotter, greyscale conversion & thresholding Image : Output of a poor thresholding algorithm for color to binary image conversion

Date (s) : Multiple days [Unaccounted]

Time : Unaccounted

In my previous post, I had written about my experiments with pseudo-random images with integers & prime integers b/w 0-255. As I had mentioned in my previous post, I would be covering pixel frequency generator in this post. Along with it, I would also be reviewing other normal topics, I have covered such as color to greyscale image conversion, greyscale to binary conversion & pixel to integer composition & integer to pixel decomposition. I will also be posting the output of above programs :

1. Pixel frequency generator : It generates frequency plots for the image using a library called as matplotlib. It’s quite simple to make, though it takes lots of time for processing large images (e.g. An image w/ a resolution of 480*640 would contain 307,200 pixels. Bigger the resolution, more time it will take.). Here is it’s program :
``````#Plotting frequency of pixel color in a given image
#Created by asxyzp (Aashish Panigrahi)
import matplotlib.pyplot as plt
import cv2
import numpy
RGBArr = []                     #Will Store RGB pixel list for conversion into hex value
RGBHex = []                     #Will Store RGB hex values for plotting
PixFrq =[]                      #Will store frequency for each pixel
tempArr = []                    #Temporary array for storing pixel values in a 1-D list
'''
Name : RGBToHex(RGBList)
Utility : Conversion of list [R,G,B] into a hexadecimal value
Parameter(s) : RGBList - A list storing pixel values [R,G,B]
Return value : Hex - Hexadecimal value corresponding to [R,G,B] pixel
'''
def RGBToHex(RGBList):
R = hex(RGBList)
G = hex(RGBList)
B = hex(RGBList)
return R+G[2:4]+B[2:4]
'''
Name : RGBArrToRGBHex()
Utility : Convets & stores values of RGBArr to RGBHex
Parameter(s) : None
Return value : None
'''
def RGBArrToRGBHex():
for i in range(len(RGBArr)):
RGBHex.append(RGBToHex(RGBArr[i]))
'''
Name : FreqPlot()
Utility : Generates frquency plot for each pixel
Parameters(s) : None
Return value : None
'''
def FreqPlot():
countArr = []
for i in range(len(RGBHex)):
countArr.append(i)
'''
#Plotting points
plt.plot(countArr,PixFrq,'k.')
plt.xlabel("Pixel values")
plt.ylabel("Frequency")
plt.grid()
plt.show()
'''
#Plotting bar graph
plt.bar(countArr,PixFrq)
plt.xlabel("Pixel values")
plt.ylabel("Frequency")
plt.grid()
plt.show()
'''
Name : PixFreq(ImgFile)
Utility : Computes the frequency of [R,G,B] lists from an image & stores it in PixFreq
Parameter(s) : ImgFile - Name of the image file for which stats have to be formed
Return value : None
'''
def PixFreq(ImgFile):
ImgFrame = cv2.imread(ImgFile,cv2.IMREAD_COLOR)     #Reading Image
ImgRowLen = len(ImgFrame)                           #Stores the length of row of pixels
ImgColLen = len(ImgFrame)                        #Stores the length of col of pixels

#For storing pixel values in tempArr
#2D list to 1D list conversion of pixels
for i in range(ImgRowLen):
for j in range(ImgColLen):
tempArr.append(numpy.ndarray.tolist(ImgFrame[i][j]))
#numpy.ndarray.tolist() converts numpy array to list

for i in range(len(tempArr)):
if tempArr[i] not in RGBArr:                    #Stores the value of unique value in the list
RGBArr.append(tempArr[i])

#Stores freq. of each pixel
for i in range(len(RGBArr)):
count = 0
for j in range(len(tempArr)):
if RGBArr[i] == tempArr[j]:
count+=1
PixFrq.append(count)

RGBArrToRGBHex()                                   #Conversion of [R,G,B] to hex
FreqPlot()                                         #Frequency plot
ImgName =  input("Name of image file/path (w/ file type. e.g. PNG/JPEG)?\t")
PixFreq(ImgName)``````

There are two sets of output for the above program : One from an image file asxyzp.jpeg & another from a pseudo-random image, as described in the previous post.

1. Output from pseudo-random image : As you can see below, the frequency represented in both the plots is a straight line, which means that the frequency of all pixels in the image is roughly the same. Line plot of frequency of Pseudo-random image

2. Output from asxyzp.jpeg : Here, you can find, something really fascinating : While most pixels have a frequency in b/w 0-500, there are pixels which have a frequency above 2500. Line plot for frequencies of asxyzp.jpeg

2. Pixel to integer composition & integer to pixel decomposition : Being more curious, I wondered whether an image which is composed of thousands of pixel lists/arrays can be converted into a list/array of integers, because once we have an integer array/list, then we can do other kinds of experiments with it, such as matrix operations. And finally, today I made the program which does the composition of pixel to integers & vice versa, but initially I was a bit confused & I had to take help from mathematics stack exchange. The program for this is below :

``````#Program to compose & decompose pixel to integer & integer to pixel, respectively
#Created by asxyzp (Aashish Panigrahi)
import cv2
import numpy
PhotoFrame = []                 #Stores photo frame
PhotoNum   = []                 #Stores integer values after composition of pixel to an integer
NumDecomp  = []                 #Stores decomposed pixel lists
'''
Name : PixToInt(Pix)
Utility : Conversion of a Pixel to an integer
Parameter(s) : Pix - A numpy array/list which contains pixel [R,G,B] list
Return value : An integer value after composition
'''
def PixToInt(Pix):
return Pix*pow(2,16) + Pix*pow(2,8) + Pix*pow(2,0)     #Composition [REFER: MATH STACKEX]
'''
Name : IntToPix(Int)
Utility : Conversion of an integer
Parameter(s) : Int - An integer for pixel decomposition
Return value : A list/numpy array containing pixel images
'''
def IntToPix(Int):
R = Int//pow(2,16)                                             #R Decomposition
Int = Int - R*pow(2,16)
G = Int//pow(2,8)                                              #G Decomposition
B = Int - G*pow(2,8)                                           #B Decomposition
return list([R,G,B])
'''
Name : RowPixToInt(ImgRow)
Utility : Converting a row of [R,G,B] pixels into a row of integers by composition
Parameter : ImgRow : A Row of pixels of an image
Return value : None
'''
def RowPixToInt(ImgRow):
Row = []
for pixel in ImgRow:
Int = PixToInt(pixel)   #Converts Pixels into Integers
Row.append(Int)
PhotoNum.append(list(Row))
'''
Name : RowIntToPix(NumRow)
Utility : Converting a row of integers into a row of Pixels by decomposition
Parameter : NumRow : A row of integers for decomposition
Return value : None
'''
def RowIntToPix(NumRow):
Row = []
for num in NumRow:         #Converts Numbers into Pixels
Pix = IntToPix(num)
Row.append(list(Pix))
NumDecomp.append(list(Row))
'''
Name : ImgArr(FileName)
Utility : Converting an image into a list
Parameter : FileName : Name of file of the image
Return value : None
'''
def ImgArr(FileName):
PhotoFrame = cv2.imread(FileName,cv2.IMREAD_COLOR)
for PixRow in PhotoFrame:
RowPixToInt(PixRow)
'''
Name : ArrImg(ArrName)
Utility : Converting an array into an image
Parameter : ArrName : Name of the list/numpy array storing the photo frame
Return value : None
'''
def ArrImg(ArrName):
for NumRow in ArrName:
RowIntToPix(NumRow)
cv2.imwrite("DEC"+FileName,numpy.asarray(NumDecomp))
FileName = input("Name of image file/path (w/ file type. e.g. PNG/JPEG)?\t")
ImgArr(FileName)
ArrImg(PhotoNum)``````

NOTE : I really wanted to move ahead with programs upon matrix operations, but then I am already a month late, for completing CS50.

Here there is one input & one output & they are exactly same. Input image for image composition into integer list/array

3. Color to greyscale image conversion : Okay, this one is one of the most common digital image processing techniques. Here, I learnt about two new things :

a. Greyscale images. Earlier, I used to think that greyscale images = b/w images, but it’s not true. B/W images or Bitonal/Binary images only have two pixel values, either [0,0,0] or [255,255,255], unlike greyscale images which represents the intensity of a pixel, which varies from 0 to 255.

b. Human eyes perceive color intensity differently for different colors & this has led to development of Luma method :

``Grey = Red * 0.3 + Green * 0.59 + Blue * 0.11``

Here’s the program for color to greyscale conversion :

``````#Description: Converting RGB image to Greyscale/Grayscale image
#Author(s)  : asxyzp
import cv2
import sys
import os.path
'''
Name : RGBToGrey(FileName)
Utility : RGB to Greyscale converter program
Parameter(s) : FileName is the name of the image file for which the conversion has to be done
Return value : None
'''
def RGBToGrey(FileName):
if os.path.isfile(FileName):                            #Checking whether the image exists or not
Frame = cv2.imread(FileName,cv2.IMREAD_COLOR)       #Stores image into a photo frame
RowSize = len(Frame)
ColSize = len(Frame)
for i in range(RowSize):
for j in range(ColSize):
Grey = 0.3*Frame[i][j] + 0.59*Frame[i][j] + 0.11*Frame[i][j] #Formula for RGB to Greyscale conversion
Frame[i][j] = Grey
Frame[i][j] = Grey
Frame[i][j] = Grey
FileName="Grey_"+FileName
cv2.imwrite(FileName,Frame)
else:
print("Image file doesn't exists.\nEnter proper file name.")
sys.exit(1)
FileName = input("Name of image file for greyscale conversion (w/ file type)?\t")
RGBToGrey(FileName)``````

Input & output for the above program : Input image

4. Greyscale to Binary image conversion : Obviously, after color to greyscale conversion, the next step would be greyscale to binary conversion & I tried to do it using two algorithms & both failed :

a. Algorithm 1:

``````if grey <= 127:
red = 0
green = 0
blue = 0
else:
red = 255
green = 255
blue = 255``````

b. Algorithm 2: Similar to algorithm 1, but replacing 127 by an average value of all pixels.

Here’s the program for algorithm 1:

``````#Description: Converting greyscale image to binary image
#Author(s)  : asxyzp
import os.path
import sys
import cv2
'''
Name : GreyToBinary(FileName)
Utility : Converts GreyScale image to Binary image
Parameter(s) : FileName, which is the name of the image file
Return value : None
'''
def GreyToBinary(FileName):
if os.path.isfile(FileName):                            #Checking whether the image exists or not
Frame = cv2.imread(FileName,cv2.IMREAD_COLOR)       #Stores image into a photo frame
RowSize = len(Frame)
ColSize = len(Frame)
for i in range(RowSize):
for j in range(ColSize):
if (Frame[i][j]<=127):                    #Converting all pixels w/ intensity below 127 to Black
Frame[i][j] = 0
Frame[i][j] = 0
Frame[i][j] = 0
elif (Frame[i][j]>127):                  #Converting all pixels w/ intensity above 127 to Black
Frame[i][j] = 255
Frame[i][j] = 255
Frame[i][j] = 255
FileName="Bin_"+FileName
cv2.imwrite(FileName,Frame)
else:
print("Image file doesn't exists.\nEnter proper file name.")
sys.exit(1)
FileName = input("Name of image file for greyscale conversion (w/ file type)?\t")
GreyToBinary(FileName)``````

Output of the above mentioned algorithms :

1. For algorithm 1: Input Image

2. For algorithm 2: Input image

After these two mistakes, I started searching for, two different ways to make this conversion happen & then I came across thresholding. I would be implementing thresholding later wards, but this is the last blog of this small series of blogs on playing around with images. Thank you. Bye.