using OpenCV to capture images, not video

Question

I'm using OpenCV4 to read from a camera. Similar to a webcam. Works great, code is somewhat like this:

cv::VideoCapture cap(0);
cap.set(cv::CAP_PROP_FRAME_WIDTH , 1600);
cap.set(cv::CAP_PROP_FRAME_HEIGHT, 1200);
while (true)
{
    cv::Mat mat;

    // wait for some external event here so I know it is time to take a picture...

    cap >> mat;
    process_image(mat);
}

Problem is, this gives many video frames, not a single image. This is important because in my case I don't want nor need to be processing 30 FPS. I actually have specific physical events that trigger reading the image from the camera at certain times. Because OpenCV is expecting the caller to want video -- not surprising considering the class is called cv::VideoCapture -- it has buffered many seconds of frames.

What I see in the image is always from several seconds ago.

So my questions:

• Is there a way to flush the OpenCV buffer?
• Or to tell OpenCV to discard the input until I tell it to take another image?
• Or to get the most recent image instead of the oldest one?

The other option I'm thinking of investigating is using V4L2 directly instead of OpenCV. Will that let me take individual pictures or only stream video like OpenCV?