Counting black & white pixels with a threshold

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I do not know enough about your approach, but you can use threshold two times: one for almost white, and one for almost black, then use countNonZero to get the number of white/black pixels. Be careful to put on white all the pixels that you want (make white the almost black pixels, so you can count non zeros). Sorry, but I do not have enough time to create a nice answer

Any advice is helpful for me. Thanks for the links, i will keep an eye them right now.

For now did it with

  threshold( partROI1, partROI1, 80, 255, 0 );

and then just counted black & white's with previous func

If you have just 2 types of pixels, then you can do numOfWhitePixels = contNonZero and then numOfBlackPixels = imgROI.cols * imgROI.rows - numOfWhitePixels

I try'ed to use your way as

count_white = countNonZero(partROI1);
count_black = partROI1.cols * partROI1.rows - count_white;

but i got terminal crash with this error:

terminate called after throwing an instance of 'cv::Exception'
what():  /home/iurt/rpmbuild/BUILD/opencv-2.4.6.1/modules/core/src/stat.cpp:496: error: (-215) src.channels() ==      1 && func != 0 in function countNonZero

Your image partROI1 is not binary (0 and 1). Have you threshold it? How? You should use cv::THRESH_BINARY or cv::THRESH_BINARY_INV. That is why I have said you shall threshold it 2 times: one for almost white pixels (with a threshold of 240 and cv::THRESH_BINARY) and one for almost black pixels (with a threshold of 20 and cv::THRESH_BINARY_INV) and then countNonZero for each case. You can adapt your thresholds to your approach

did it for now with this code.

threshold( partROI1, partROI1, 20, 255, THRESH_BINARY );
count_black = countNonZero(partROI1);
threshold( partROI1, partROI1, 240, 255, THRESH_BINARY_INV );
count_white= countNonZero(partROI1);

since i were using a ROI of an image

cvtColor(partROI1, partROI1, CV_BGR2GRAY);

helped

The error happens in the first or the second call of the countNonZero? You have some problems there:

1. threshold( partROI1, partROI1, 20, 255, THRESH_BINARY ); is changing the initial image if you have done something like cv::Mat partROI1 = initImage(roiRect); (use initImage(roiRect).clone if that is the case)
2. you are counting the wrong: count_black is counting the pixel that are not almost black and count_white the ones that are not almost white. (you can do some imshow s to see exactly what you are doing; the white pixels are counted)
3. you are trying to threshold an already thresholded image here: threshold( partROI1, partROI1, 240, 255, THRESH_BINARY_INV ); and inverse it.

So, as I have said, do an imshow after each step to see exactly what are you doing

no, error doesn't show up anymore cvtColor(partROI, partROI, CV_BGR2GRAY); helped me to fix that. And yes, i were first counting the white pixels with THRESH_BINARY and then inverting them with THRESH_BINARY_INV and counting again the black ones. But you are right and this way will be more efficient

            threshold( partROI, partROI, 240, 255, THRESH_BINARY );
            count_white = countNonZero(partROI);
            count_black = partROI.cols * partROI.rows - count_white2;

That approach is ok, if you have just white and black pixels in the image and no gray values (that means you so not want only the white and black pixels from an image that may contain gray values too). So the problem was that you were trying to apply threshold on a RGB image...