Sudoku Solver – with source code – fun project – easiest way – 2024

So guys, in today’s blog we will see that how we can implement Sudoku Solver using Computer Vision and Image Processing techniques. So without any further due, Let’s do it…

Code for Sudoku Solver…

import cv2
import imutils
from imutils.perspective import four_point_transform
from skimage.segmentation import clear_border
from tensorflow.keras.models import load_model
import numpy as np
from Sudoku import solveSudoku
from tensorflow.keras.preprocessing.image import img_to_array
import matplotlib.pyplot as plt

# Sudoku Solver
model = load_model('model/model_mnist/')
img_path = 'sudoku images/6.png'
img_shape = [28,28]

def find_puzzle(img):
    real = img.copy()
    gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
    blur = cv2.GaussianBlur(gray,(7,7),1)
    thresh = cv2.adaptiveThreshold(blur,255,cv2.ADAPTIVE_THRESH_GAUSSIAN_C,cv2.THRESH_BINARY,11,2)
    thresh = cv2.bitwise_not(thresh)

    cnts = cv2.findContours(thresh.copy(),cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)
    cnts = imutils.grab_contours(cnts)
    cnts = sorted(cnts,key=cv2.contourArea,reverse=True)

    puzzle_cnt = None

    for c in cnts:
        peri = cv2.arcLength(c,True)
        approx = cv2.approxPolyDP(c,0.02*peri,True)
        if len(approx)==4:

    if puzzle_cnt is None:
        raise Exception(("Could not find Sudoku puzzle outline.Try debugging your thresholding and contour steps."))

    cv2.drawContours(real, [puzzle_cnt], -1, (0, 255, 0), 2)

    puzzle = four_point_transform(img, puzzle_cnt.reshape(4, 2))
    warped = four_point_transform(gray, puzzle_cnt.reshape(4, 2))

    return puzzle,warped

def extract_digit(cell):
    thresh = cv2.threshold(cell,0,255,cv2.THRESH_BINARY_INV|cv2.THRESH_OTSU)[1]
    thresh = clear_border(thresh) #just clear the extra white pixels along the border

    cnts = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)
    cnts = imutils.grab_contours(cnts)

    # if no contours were found than this is an empty cell
    if len(cnts) == 0:return None

    # otherwise, find the largest contour in the cell and create a mask for the contour
    c = max(cnts, key=cv2.contourArea)
    mask = np.zeros(thresh.shape, dtype="uint8")
    cv2.drawContours(mask, [c], -1, 255, -1)

    (h, w) = thresh.shape
    percentFilled = cv2.countNonZero(mask) / float(w * h)

    # if less than 3% of the mask is filled then we are looking at noise and can safely ignore the contour
    if percentFilled < 0.03:return None

    # apply the mask to the thresholded cell
    digit = cv2.bitwise_and(thresh, thresh, mask=mask)
    kernel = np.ones((1,1),np.uint8)
    digit = cv2.dilate(digit,kernel,iterations=1)

    # return the digit to the calling function
    return digit

def display_numbers_on_board(board,puzzle):
    x = puzzle.copy()
    k = 0
    for i in range(9):
        for j in range(9):
            startX,startY,endX,endY = cell_locs[k]
            testX = int((endX - startX) * 0.33)
            testY = int((endY - startY) * -0.2)
            testX += startX
            testY += endY
    return x

img = cv2.imread(img_path)
img = imutils.resize(img,width=600)

puzzle,warped = find_puzzle(img)
puzzle = imutils.resize(puzzle,width=600)
warped = imutils.resize(warped,width=600)

step_x = warped.shape[1]//9
step_y = warped.shape[0]//9

board = np.zeros(shape=(9,9),dtype='int')
cell_locs = []

for i in range(9):
    for j in range(9):
        topleftx = j*step_x
        toplefty = i*step_y
        rightendx= (j+1)*step_x
        rightendy = (i+1)*step_y
        cell = warped[toplefty:rightendy, topleftx:rightendx]
        digit = extract_digit(cell)
        if digit is not None:
            roi = cv2.resize(digit,tuple(img_shape))
            roi = roi.astype('float')/255.0
            roi = img_to_array(roi)
            roi = np.expand_dims(roi,axis=0)
            pred = model.predict(roi).argmax(axis=1)[0]
            board[i,j] = pred



while 1:
    res = input('Are all numbers predicted correctly? (y/n)')
    if res=='n':
        cx,cy,ele = input('Input row no, col no, correct element of cell For eg. --> 1 2 1:  ').split()
            board[int(cx),int(cy)] = int(ele)
            print('out of range...')
        _ = display_numbers_on_board(board,puzzle)
    elif res=='y':
        print('Wrong choice!!!')
solved = solveSudoku(board)

x = display_numbers_on_board(board,puzzle)

  • Line 1-10 – Importing libraries required for Sudoku Solver.
  • Line 12-14 – Load the mnist model and the sudoku image. Also declare a constant img_shape which will be the size of our sudoku image after resizing.
Sudoku Solver
  • Line 16-45 – This function will find the puzzle in the image through Image Processing techniques.
    • Read the image, convert it to grayscale, and then apply Gaussian Blur to remove Gaussian Noise.
    • The threshold the image and inverse it. Means convert white to black and black to white.
    • Then find the contours and extract the biggest contour in the image. This is probably our sudoku.
    • Apply four point transformation to get the straight view of the puzzle and return it.
  • Line 49-77 – This function will extract the digit cell from the puzzle and return it so that the mnist model could identify the number in it.
  • Line 80-97 – This function will simply put the numbers on the image/board.
  • Line 99-100 – Simply read and resize the image.
  • Line 102-104 – Extract the puzzle and resize it.
  • Line 106-107 – This will tell us the step size we have to take in the x and y-direction. This step is basically the cell size in the x and y directions respectively.
  • Line 109 – Create the board with all values initialized with 0s.
  • Line 110 – Create array cell_locs which will contain the cell locations/coordinates.
  • Line 112-127 – Traverse in the 9*9 loop where the first 9 represents the 9 rows of the board and the second 9 represents the 9 columns of the board. On traversing it will extract the cell images, predict the number using mnist and put that number in the board array we created in Line 109. Also, we will keep on storing the cell locations in the cell_loc array.
  • Line 133-145 – If by chance any of the numbers is wrongly predicted by the mnist, correct it.
Sudoku Solver

NOTE – The numbers in blue are predicted numbers for that cell. The text in red is the address of that cell. If any number is wrongly classified by mnist, correct that. Like the first wrong classified number is at (4,7). The original number there is 1 but the mnist said it is 7 so we will correct it first.

Sudoku Solver
  • Line 147 – Solve the sudoku using the solveSudoku function that we imported from This is also written by me and I have used Backtracking for solving sudoku.
  • Line 149-150 – Display the solved sudoku numbers on the final image and display it.
  • Line 152-153 – Destroy all windows.

Final result of Sudoku Solver…

Sudoku Solver

Download Source Code for Sudoku Solver…

Do let me know if there’s any query regarding the Sudoku Solver by contacting me on email or LinkedIn. You can also comment down below for any queries.

So this is all for this blog folks, thanks for reading it and I hope you are taking something with you after reading this and till the next time ?…


Check out my other machine learning projectsdeep learning projectscomputer vision projectsNLP projectsFlask projects at

Abhishek Sharma
Abhishek Sharma

Started my Data Science journey in my 2nd year of college and since then continuously into it because of the magical powers of ML and continuously doing projects in almost every domain of AI like ML, DL, CV, NLP.

Articles: 520

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