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You can use this function to change your desired brightness or contrast using C++ just like the same way you do it on photoshop or other similar photo editing software. You can check details of the python implementation More details about python implementation

def apply_brightness_contrast(input_img, brightness = 255, contrast = 127):
        brightness = map(brightness, 0, 510, -255, 255)
        contrast = map(contrast, 0, 254, -127, 127)

        if brightness != 0:
            if brightness > 0:
                shadow = brightness
                highlight = 255
            else:
                shadow = 0
                highlight = 255 + brightness
            alpha_b = (highlight - shadow)/255
            gamma_b = shadow

            buf = cv2.addWeighted(input_img, alpha_b, input_img, 0, gamma_b)
        else:
            buf = input_img.copy()

        if contrast != 0:
            f = float(131 * (contrast + 127)) / (127 * (131 - contrast))
            alpha_c = f
            gamma_c = 127*(1-f)

            buf = cv2.addWeighted(buf, alpha_c, buf, 0, gamma_c)

        cv2.putText(buf,'B:{},C:{}'.format(brightness,contrast),(10, 30), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 0, 255), 2)
        return buf