OpenCV putText does not work after flipping the image

Question

I’d like to grab images from the camera and flip them left/right so that the view performs like a mirror. However, I also like to add some text to the view, but it turns out that after flipping the image using np.fliplr(frame), cv.putText does no longer work.

Here is my minimal example using python 3.5.2:

import numpy as np
import cv2
import platform

if __name__ == "__main__":
    print("python version:", platform.python_version())
    cap = cv2.VideoCapture(0)
    while(True):
        # Capture frame-by-frame
        ret, frame = cap.read()

        cv2.putText(frame,'Hello World : Before flip',(100, 100), cv2.FONT_HERSHEY_SIMPLEX, 1,(255,255,255),2,cv2.LINE_AA)
        frame = np.fliplr(frame)
        cv2.putText(frame,'Hello World : After flip',(100, 200), cv2.FONT_HERSHEY_SIMPLEX, 1,(255,255,255),2,cv2.LINE_AA)

        # Process the keys
        key = cv2.waitKey(1) & 0xFF
        if key == ord('q'):
            print("quit")
            break
        # show the images
        cv2.imshow('frame',frame)

    cap.release()
    cv2.destroyAllWindows()

Resulting frame w/ flip:

Resulting frame w/o flip:

Asked By: Tik0

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Source

Answer 1

I suspect it’s due to cv2.putText is not compatible with np.array which is the return value of np.fliplr(frame). I suggest that you use frame = cv2.flip(frame, 1) instead.

import numpy as np
import cv2
import platform

if __name__ == "__main__":
    print("python version:", platform.python_version())
    cap = cv2.VideoCapture(0)
    while(True):
        # Capture frame-by-frame
        ret, frame = cap.read()

        cv2.putText(frame,'Hello World : Before flip',(100, 100), cv2.FONT_HERSHEY_SIMPLEX, 1,(255,255,255),2,cv2.LINE_AA)
        frame = cv2.flip(frame, 1)
        cv2.putText(frame,'Hello World : After flip',(100, 200), cv2.FONT_HERSHEY_SIMPLEX, 1,(255,255,255),2,cv2.LINE_AA)

        # Process the keys
        key = cv2.waitKey(1) & 0xFF
        if key == ord('q'):
            print("quit")
            break
        # show the images
        cv2.imshow('frame',frame)

    cap.release()
    cv2.destroyAllWindows()

Answered By: jackieyang

Answer 2

Just flip the frame before putting the text:

frame = cv2.flip(frame, 1)
cv2.putText(...)

Answered By: Broteen Das

OpenCV putText does not work after flipping the image

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