opencv: performing night vision - c++

Question

First I am not talking about real night vision. I am talking about the technique used to improve picture brightness/light when light condition is poor. You can see this technique perfectly in smart phones, superb in phablets. I know the technique used in here, get the existing light and used it to make the pic clear. But how to do this in opencv? Any method or step by step process?

Answer 1

There are essentially 2 ways to brighten your image:
Get more photons in the camera
Give each photon more 'weight'
For approach 1, supposing that you can't control the lighting, then the only way to get more photons is to expose your sensor for a longer period of time. That assumes that you can change your camera's integration time. The drawback of this approach is that you can get more motion blur.
For approach 2, this amounts to applying a multiplicative gain to the input image, which makes each photon contribute more DN's to the resulting image. Applying such a gain though supposes that you have a priori information about the input image's brightness. If your gain value is not good you'll have an image that's either saturated or too dark.
To improve your image automatically, the best approach would be to use OpenCV's equalizeHist function, as described here. The operation isn't exactly a multiplicative gain but the effect is similar.
The last step would be, as previously suggested in comments, to apply a gamma correction as described here. Gamma correction tends to reduce the contrast in an image, but since you improved the contrast using histogram equalization you should get good results.

Answer 2

As Michel points out try equalizeHist.
Here's a minimal example:
#include <opencv2/opencv.hpp>
using namespace cv;
int main(int argc, char *argv[])
{
namedWindow("input");
namedWindow("output");
Mat in = imread("yourDarkImage.jpg");;
Mat out;
if(in.empty())exit(1);
//equalize histograms per channel
vector<Mat> colors;
split(in, colors);
equalizeHist(colors[0], colors[0]);
equalizeHist(colors[1], colors[1]);
equalizeHist(colors[2], colors[2]);
merge(colors, out);
imshow("input", in);
imshow("output", out);
waitKey(0);
return 0;
}