how do I increase exposure before taking photo in opencv?

Question

I wrote a program in C++, using wiring pi for the LED, trigger, ad openCV3.2 for the camera. It works, taking a photo, when the button is pressed, but the photo quality is low, even in a well lit room. I think this is because the exposure time is too short.

How can I increase the exposure time, to produce higher quality images?

here is my source, from the function that needs help:

...
void imgGet(){

    cv::VideoCapture cap(0);
    cap.set(CAP_PROP_EXPOSURE,1); 
    Mat frame;
    cap >> frame;
    string fileName = getFName(); //returns a string for file naming
    imwrite(fileName, frame);
    }
 ...

The VideoCapture documentation is here: but the program returns

VIDIOC_S_CTRL: Invalid argument

it takes the photo, but the exposure time never changes.

Here is an example of what I'm experiencing (both are taken without flash): This is the image quality after running my program:

This image is from another OpenCV program, that streams Video. The image becomes saturated as I expect, because the camera is on for a longer period of time. This is a side effect of the video streaming continuously. What I want to do is control it.

Answer 1

You can also configure some captures parameters through v4l2-ctl. My camera don't have exposure setting, but the one you use may have it.

Run in the console :

v4l2-ctl -l

to retrieve the parameters you can change, then run

v4l2-ctl --set-ctrl=contrast=36

to define new values.

I use this approach to correct a too dark camera, successfully :)