Automatic detection of dominant intensity area in an image - c++

Question

I need some help in detecting the dominant intensity area of an image. Suppose I have the following images and I like to automatically detect the dominant intensity area and find the mean/average intensity value of that dominant area.
Here, in Image-1, the dominant intensity area is the area with light gray color and in Image-2, the dominant area is with the dark gray color. How can I detect the dominant areas in those images and find the mean intensity value of the dominant area.
Image-1:
Image-2:
Any suggestion will be helpful!
Update: I used the following codes to get the histogram for Image-2. Figure-3 shows the histogram. Now, I need to find out which bin holds the most of the values i.e. the mode of the histogram. But, couldn't figure out how to calculate the bin with most of the values.
Figure-3:
int main(int, char**)
{
Mat gray=imread("Depth_frames_27/Image23.png",0);
namedWindow( "Gray", 1 ); imshow( "Gray", gray );
// Initialize parameters
int histSize = 256; // bin size
float range[] = { 0, 255 };
const float *ranges[] = { range };
// Calculate histogram
MatND hist;
calcHist( &gray, 1, 0, Mat(), hist, 1, &histSize, ranges, true, false );
double minVal=0, maxVal=0;
minMaxLoc(hist, &minVal, &maxVal, 0, 0);
// cout<<"Max:"<<maxVal<<endl;
// cout<<"Min:"<<minVal<<endl;
// Show the calculated histogram in command window
double total;
total = gray.rows * gray.cols;
for( int h = 0; h < histSize; h++ )
{
float binVal = hist.at<float>(h);
cout<<" "<<binVal;
}
// Plot the histogram
int hist_w = 512; int hist_h = 400;
int bin_w = cvRound( (double) hist_w/histSize );
Mat histImage( hist_h, hist_w, CV_8UC1, Scalar( 0,0,0) );
normalize(hist, hist, 0, histImage.rows, NORM_MINMAX, -1, Mat() );
for( int i = 1; i < histSize; i++ )
{
line( histImage, Point( bin_w*(i-1), hist_h - cvRound(hist.at<float>(i-1)) ) ,
Point( bin_w*(i), hist_h - cvRound(hist.at<float>(i)) ),
Scalar( 255, 0, 0), 2, 8, 0 );
}
namedWindow( "Result", 1 ); imshow( "Result", histImage );
waitKey();
return 0;
}
Update-2: Worked out finally! I did the following to get the location and value of the maximum bin of the histogram.
double minVal=0, maxVal=0; int minIdx, maxIdx;
minMaxIdx(hist,&minVal,&maxVal, &minIdx, &maxIdx);
cout<<"Max:"<<maxVal<<endl;
cout<<"MaxIdx:"<<maxIdx<<endl;
The MaxIdx gives the location of the highest bin of the histogram and that's the dominant intensity value for the image!

Answer 1

What you are after is the mode of the histogram of intensities (the bin with the highest frequency). It directly tells you the average intensity.
For the given images, the histogram is made of two perfectly sharp peaks.
In some bad cases, the main peak can be spread over several secondary peaks. In such cases, you need to apply smoothing to the histogram before taking the mode.
It can be interesting to look at the relative heights of the first and second maxima, to check how dominant the color is.