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Everyone posts code, none of it was really helpful. Here's how you can do it:

Also posted here on PasteBin: https://pastebin.com/tA1R8Qtm

// -----------------------------------------------------
// ---------- FIND AVG LUMINENCE OF FRAME --------------
// -----------------------------------------------------

// Assuming you have a cv::Mat to start with called "input_mat"
cv::Mat grayMat; 
cv::cvtColor(input_mat, grayMat, CV_BGR2GRAY);

int Totalintensity = 0;
for (int i=0; i < grayMat.rows; ++i){
    for (int j=0; j < grayMat.cols; ++j){
        Totalintensity += (int)grayMat.at<uchar>(i, j);
    }
}

// Find avg lum of frame
float avgLum = 0;
avgLum = Totalintensity/(grayMat.rows * grayMat.cols);