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How to get those unique lable in a mask with efficient

If I have a such label image

Then I have a such mask

I want to get those unique integer lables in the given mask(same size with the label image). The result is {0,1,2,3,6} in my this example. This is my current method

#include<string>
using namespace std;
using namespace cv;

int main() {
    uchar a[6][7] = { { 0,0,0,0,0,0,0 },{ 0,1,2,2,3,3,0 },{ 0,0,0,0,0,0,0 },{ 0,5,5,6,6,0,0 },{ 0,5,5,6,6,0,0 },{ 0,0,0,0,0,0,0 } };
    Mat label = Mat(6, 7, CV_8UC1, a);
    uchar b[6][7]= { { 0,0,0,0,0,0,0 },{ 0,255,255,255,255,0,0 },{ 255,255,255,0,0,0,0 },{ 0,0,0,255,255,0,0 },{ 0,0,0,255,255,0,0 },{ 0,0,0,0,0,0,0 } };
    Mat mask = Mat(6, 7, CV_8UC1, b);
    vector<int> list;
    for (int i = 0; i < mask.rows; i++) {
        uchar* mask_p = mask.ptr<uchar>(i);
        for (int j = 0; j < mask.cols; j++)
            if (mask_p[j] == 255)
                list.push_back(int(label.at<uchar>(i,j)));
    }
    sort(list.begin(), list.end());
    list.erase(unique(list.begin(), list.end()), list.end());

    return 0;
}

But in my real case, I have just a small area in a huge mask image. I think the current method which iterate every pixel in the mask waste my time too much in my case. Can anyone give me some advice?

How to get those unique lable in a mask with efficient

If I have a such label image

Then I have a such mask

I want to get those unique integer lables in the given mask(same size with the label image). The result is {0,1,2,3,6} in my this example. This is my current method

#include<string>
using namespace std;
using namespace cv;

int main() {
    uchar a[6][7] = { { 0,0,0,0,0,0,0 },{ 0,1,2,2,3,3,0 },{ 0,0,0,0,0,0,0 },{ 0,5,5,6,6,0,0 },{ 0,5,5,6,6,0,0 },{ 0,0,0,0,0,0,0 } };
    Mat label = Mat(6, 7, CV_8UC1, a);
    uchar b[6][7]= { { 0,0,0,0,0,0,0 },{ 0,255,255,255,255,0,0 },{ 255,255,255,0,0,0,0 },{ 0,0,0,255,255,0,0 },{ 0,0,0,255,255,0,0 },{ 0,0,0,0,0,0,0 } };
    Mat mask = Mat(6, 7, CV_8UC1, b);
    vector<int> list;
    for (int i = 0; i < mask.rows; i++) {
        uchar* mask_p = mask.ptr<uchar>(i);
        for (int j = 0; j < mask.cols; j++)
            if (mask_p[j] == 255)
                list.push_back(int(label.at<uchar>(i,j)));
    }
    sort(list.begin(), list.end());
    list.erase(unique(list.begin(), list.end()), list.end());

    return 0;
}

But in my real case, I have just a small ~~area~~ ROI in a huge mask image. I think the current method which iterate every pixel in the mask waste my time too much in my case. Can anyone give me some advice?

How to get those unique lable in a mask with efficient

If I have a such label image

Then I have a such mask

I want to get those unique integer lables in the given mask(same size with the label image). The result is {0,1,2,3,6} in my this example. This is my current method

#include<opencv.hpp>
#include<string>
using namespace std;
using namespace cv;

int main() {
    uchar a[6][7] = { { 0,0,0,0,0,0,0 },{ 0,1,2,2,3,3,0 },{ 0,0,0,0,0,0,0 },{ 0,5,5,6,6,0,0 },{ 0,5,5,6,6,0,0 },{ 0,0,0,0,0,0,0 } };
    Mat label = Mat(6, 7, CV_8UC1, a);
    uchar b[6][7]= { { 0,0,0,0,0,0,0 },{ 0,255,255,255,255,0,0 },{ 255,255,255,0,0,0,0 },{ 0,0,0,255,255,0,0 },{ 0,0,0,255,255,0,0 },{ 0,0,0,0,0,0,0 } };
    Mat mask = Mat(6, 7, CV_8UC1, b);
    vector<int> list;
    for (int i = 0; i < mask.rows; i++) {
        uchar* mask_p = mask.ptr<uchar>(i);
        for (int j = 0; j < mask.cols; j++)
            if (mask_p[j] == 255)
                list.push_back(int(label.at<uchar>(i,j)));
    }
    sort(list.begin(), list.end());
    list.erase(unique(list.begin(), list.end()), list.end());

    return 0;
}

But in my real case, I have just a small ROI in a huge mask image. I think the current method which iterate every pixel in the mask waste my time too much in my case. Can anyone give me some advice?