How to set camera to auto-exposure with OpenCV 3.4.2?

Question

I am working with a PS-Eye-3 camera, libusb, PSEye driver, OpenCV 3.4.2 and Visual Studio 2015 / C++ on Windows 10.

I can set the exposure of the camera to any value by using this code:

cv::VideoCapture *cap;  
...
cap = new cv::VideoCapture(0);
cap->set(CV_CAP_PROP_EXPOSURE, exposure); // exposure = [0, 255]

Now I would like to switch to auto-exposure too. How can I set the camera to auto-exposure mode?

I tried the following:

cap->set(CV_CAP_PROP_EXPOSURE, 0);       // not working
cap->set(CV_CAP_PROP_EXPOSURE, -1);      // not working
cap->set(CV_CAP_PROP_AUTO_EXPOSURE, 1);  // not working, exposure stays fixed
cap->set(CV_CAP_PROP_AUTO_EXPOSURE, 0);  // not working, exposure stays fixed
cap->set(CV_CAP_PROP_AUTO_EXPOSURE, -1); // not working, exposure stays fixed

Some idea?

Answer 1

It depends on the capture api you are using. If you are using CAP_V4L2, then automatic exposure is set to 'on' with value 3 and 'off' with value 1.

All settable values can be found by typing v4l2-ctl -l in terminal.

I think for OpenCV < 4.0 default api is CAP_GSTREAMER and automatic exposure is set to 'on' with value 0.75 and 'off' with value 0.25.

Answer 2

i think finally i found a solution, at least for my problem,

capture = cv2.VideoCapture(id)
capture.set(cv2.CAP_PROP_AUTO_EXPOSURE, 3) # auto mode
capture.set(cv2.CAP_PROP_AUTO_EXPOSURE, 1) # manual mode
capture.set(cv2.CAP_PROP_EXPOSURE, desired_exposure_value)

i have to first set the auto_exposure to 3 (auto mode)
then i have to set it to 1 (manual mode)
and then i can set the manual exposure

you can set the settings with shell too
list available options

video_id=1
v4l2-ctl --device /dev/video$video_id -l

set them with python

def set_manual_exposure(dev_video_id, exposure_time):
    commands = [
        ("v4l2-ctl --device /dev/video"+str(id)+" -c exposure_auto=3"),
        ("v4l2-ctl --device /dev/video"+str(id)+" -c exposure_auto=1"),
        ("v4l2-ctl --device /dev/video"+str(id)+" -c exposure_absolute="+str(exposure_time))
    ]
    for c in commands: 
        os.system(c)
# usage 
set_manual_exposure(1, 18)

Answer 3

Try cap->set(CV_CAP_PROP_AUTO_EXPOSURE, X); where X is a camera-dependent value such as 0.25 or 0.75. For a similar issue see the discussion: https://github.com/opencv/opencv/issues/9738