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This code will work merely OpenCV3.x. But I haven't tested on OpenCV4.0.1. Code:

def interpolate_frames(frame, coords, flow, n_frames):
    frames = [frame]
    for f in range(1, n_frames):
        pixel_map = coords + (f/n_frames) * flow
        inter_frame = cv2.remap(frame, pixel_map, None, cv2.INTER_LINEAR)
        frames.append(inter_frame)
    return frames

flow = cv2.calcOpticalFlowFarneback(curr, prev, None, *optflow_params)
inter_frames = interpolate_frames(prev_frame, coords, flow, 4)

This code will work merely OpenCV3.x. But I haven't tested on OpenCV4.0.1. OpenCV4.0.1. Btw, it is essential to used cv2.remap. Code:

def interpolate_frames(frame, coords, flow, n_frames):
    frames = [frame]
    for f in range(1, n_frames):
        pixel_map = coords + (f/n_frames) * flow
        inter_frame = cv2.remap(frame, pixel_map, None, cv2.INTER_LINEAR)
        frames.append(inter_frame)
    return frames

flow = cv2.calcOpticalFlowFarneback(curr, prev, None, *optflow_params)
inter_frames = interpolate_frames(prev_frame, coords, flow, 4)