# Computing max values and channel indices pixel-wise

Given a cv::Mat input with n-channels, I want to recieve the max channel value of each pixel in one cv::mat out_val and in another cv::Mat out_idx the channel index of the maximum value, again of each pixel. So at the end input.size() = out.idx.size() = out_val.size(). Furthermore, if various channels have the same max value, a random channel index should be saved in the out_idx. So, you could do this with several for-loops, looping over each pixel and then comparing each channel value and so on. But I wanted to do this with kind of matrix operations to be more efficient. So I came up with something like the attached code. However, there is still missing the random choice for several maximum values and for sure there is a nicer coding instead of all these help matrices. So the question is: How to implement my task in a more efficient way (1) and adding the random idx choice (2)

std::vector<cv::Mat> tmp_ch;
cv::split(input, tmp_ch);

out_idx = cv::Mat::zeros(input.size().height, input.size().width, CV_32SC1);
out_val = cv::Mat::zeros(input.size().height, input.size().width, CV_64FC1);

for(int i = 0; i < tmp_ch.size(); i++) {

cv::Mat bin = out_val < tmp_ch.at(i); // 0 -> old value stays; 255 -> update value in out_*
cv::Mat buff_ch, buff_max, buff_idx, curr_idx;

out_val.copyTo(buff_max, 255-bin);
tmp_ch.at(i).copyTo(buff_ch, bin);

curr_idx = cv::Mat(input.size().height, input.size().width, CV_32SC1, cv::Scalar(i));
curr_idx.copyTo(curr_idx, bin);
out_idx.copyTo(buff_idx, 255-bin);

}

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so you want the maximum channel of each pixel ? like:

[123,22,77] -> 0 [19,20,23] -> 2 ?

( 2017-02-23 03:00:34 -0600 )edit

Exactly! And if [123,22,77,123] -> random(0,3). I have also to mention, that input mat has more then 3 channels.

( 2017-02-23 03:48:35 -0600 )edit

just curious - what is your use-case ?

( 2017-02-23 04:07:58 -0600 )edit

my input mat is a proba bility distribution for each pixel...

( 2017-02-23 04:16:00 -0600 )edit

@berak so do you have any idea how to treat this problem?

( 2017-02-26 10:01:40 -0600 )edit

not really, unfortunately ;(

( 2017-02-26 10:15:55 -0600 )edit