import cv2
import numpy as np

img = cv2.imread('captcha1.jpg')
phase = -0.8 * np.pi
omega = 2.0 * np.pi / img.shape[1]
amp = 15.0

# NB: Mat_<Vec2f> -> 2 channels
proj = np.zeros((img.shape[0], img.shape[1], 2), np.float32)

for y in range(img.shape[0]):
    for x in range(img.shape[1]):
        u = 0.0
        v = np.sin(phase + float(x) * omega) * amp
        proj[y, x] = (float(x) + u, float(y) + v)

corr = cv2.remap(img, proj, None, cv2.INTER_LINEAR)
cv2.imshow('in', img)
cv2.imshow('out', corr)
cv2.waitKey()