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I have the assignment to scale/ resize an image without the use functions from external libraries and need to create the algorithm myself, have searched, but the code that I have found on different forums have not worked, but I have come up with a code that I would believe would work, but I get the index error on a side note I also need to make forward and backward mapping in relation to scaling the image.

My code can be seen below

import cv2 as cv

img = cv.imread('Scale.jpg', 1)
cv.imshow('unscaled', img)
h,w = img.shape[:2]
print(h)
print(w)
def resizePixels(pixels,w1,h1,w2,h2) :

 retval = [w2,h2]
 # EDIT: added +1 to remedy an early rounding problem
 x_ratio = (int)((w1<<16)/w2) +1
 y_ratio = (int)((h1<<16)/h2) +1
 #int x_ratio = (int)((w1<<16)/w2)
 #int y_ratio = (int)((h1<<16)/h2)
 #two = int(x2,y2)
 for i in range (h2):
 i += 1
 for j in range(w2):
 j += 1
 x2 = ((j*x_ratio)>>16)
 y2 = ((i*y_ratio)>>16)
 retval[(i*w2)+j] = pixels[(y2*w1)+x2]

 return retval;

dst = resizePixels(img,h,w,300,300)

#cv.imshow('Resize',dst)
cv.waitKey(0)

EDIT: This is the Traceback I receive

Traceback (most recent call last):
File "C:/Users/Asus/PycharmProjects/Scaling/Scaling.py", line 27, in 
<module>
dst = resizePixels(img,h,w,300,300)

File "C:/Users/Asus/PycharmProjects/Scaling/Scaling.py", line 23, in 
resizePixels

retval[(i*w2)+j] = pixels[(y2*w1)+x2]
IndexError: list assignment index out of range

The picture I use to scale

I make some little changes in your code and it works. Read my comment in the code for more detail.

import cv2 
import numpy as np

img = cv2.imread('1.png', 0)
cv2.imshow('unscaled', img)

h,w = img.shape[:2]
print(h)
print(w)
def resizePixels(pixels,w1,h1,w2,h2):
 retval = 
 # EDIT: added +1 to remedy an early rounding problem
 x_ratio = (int)((w1<<16)/w2) +1
 print(x_ratio)
 y_ratio = (int)((h1<<16)/h2) +1
 print(y_ratio)
 #int x_ratio = (int)((w1<<16)/w2)
 #int y_ratio = (int)((h1<<16)/h2)
 #two = int(x2,y2)
 for i in range(h2):
 # no need to have this: i += 1
 for j in range(w2):
 # no need to have this too: j += 1
 x2 = ((j*x_ratio)>>16)
 y2 = ((i*y_ratio)>>16)
 #add pixel values from original image to an array
 retval.append(pixels[y2,x2])

 return retval;

ret = resizePixels(img,w,h,300,300)
# reshape the array to get the resize image
dst = np.reshape(ret,(300,300))

cv2.imshow('Resize',dst)
cv2.waitKey(0)
cv2.destroyAllWindows()