Simple zoom via warpAffine.

import cv2
import numpy as np

img = cv2.imread('lena3.png', cv2.IMREAD_COLOR)
h, w = img.shape[:2]

scale = 1.0

while(True):
    scale *= 1.001

    # Prepare transformation matrices
    # Translate to have origin at the center of the image
    M_translate_0 = np.array([[1,0,-w/2],[0,1,-h/2],[0,0,1]], dtype=np.float32)
    # Scale around the origin
    M_scale = np.array([[scale,0,0],[0,scale,0],[0,0,1]], dtype=np.float32)
    # Translate back to have origin at the top-left corner again
    M_translate_1 = np.array([[1,0,w/2],[0,1,h/2],[0,0,1]], dtype=np.float32)

    # Calculate the composite transformation matrix
    M = np.matmul(M_translate_1, np.matmul(M_scale, M_translate_0))

    # Apply transformation (dropping the last row of the matrix, since it's not needed here)
    result = cv2.warpAffine(img, M[:2,], (w, h))

    cv2.imshow('', result)
    cv2.waitKey(20)