Gray-level image histogram

graylevel_histogram.py

## Original code from the series on image processing in python
## Read the tutorial at https://thesourcecodenotes.blogspot.com/2021/07/image-processing-in-python-4-grey-level.html

from PIL import Image, ImageTk 
import tkinter as tk



def histogram(input_image):
    if input_image.mode != 'L' and input_image.mode != 'P':
        return None
    else:
        IHIST = [0 for i in range(256)]
        HIST = [0 for i in range(256)]
        SUM = 0        
        for x in range(input_image.width):
            for y in range(input_image.height):
                pix = input_image.getpixel((x, y))
                IHIST[pix] = IHIST[pix]+1
                SUM += 1
        for i in range(256):
            HIST[i] = float(IHIST[i]/SUM)
            
    return HIST                      

def draw_histogram(canvas, IHIST, hist_w, hist_h):
        ## A bin is a bar of the histogram
        bin_w = round(float(hist_w/512))
        offset = hist_w + 20 ## where we draw the first bin
        for i in range(256):                                                                                                                                                   
            canvas.create_line(bin_w*i+offset, hist_h,
                               bin_w*i+offset, hist_h-IHIST[i])


if __name__=="__main__":
    root = tk.Tk()
    root.title("IMAGE GREY-LEVEL HISTOGRAM")
    img = Image.open("retriver_gray.png")

    width = img.width*2 
    height = img.height
    root.geometry(f'{width}x{height}')

    IHIST = histogram(img)
    if IHIST != None:
        hist_w = img.width
        hist_h = img.height

        hist_max = max(IHIST) ## get the max value
        ## Normalize between 0 and hist_h
        for i in range(256):
            IHIST[i] = float(IHIST[i]/hist_max) * hist_h
    
        input_im = ImageTk.PhotoImage(img)
     
        canvas = tk.Canvas(root, width=width, height=height, bg="#ffffff")
        canvas.create_image(width/4-1, height/2-1, image=input_im, state="normal")

        draw_histogram(canvas, IHIST, hist_w, hist_h)
             
        canvas.place(x=0, y=0)
        canvas.pack()
        
        root.mainloop()
    else:
        print("Input image's mode must be 'L' or 'P'"\
              "Check https://pillow.readthedocs.io/en/"\
              "latest/handbook/concepts.html#concept-modes")