Obtaining list of unique pixel values in OpenCV Mat

No, there is not! You can code your own, though:

std::vector<float> unique(const cv::Mat& input, bool sort = false)

Find the unique elements of a single channel cv::Mat.

Parameters:

input: It will be treated as if it was 1-D.

sort: Sorts the unique values (optional).

The implementation of such function is pretty straight forward, however, the following only works with single channel CV_32F:

#include <algorithm>#include <vector>std::vector<float> unique(const cv::Mat& input, bool sort = false){    if (input.channels() > 1 || input.type() != CV_32F)     {        std::cerr << "unique !!! Only works with CV_32F 1-channel Mat" << std::endl;        return std::vector<float>();    }    std::vector<float> out;    for (int y = 0; y < input.rows; ++y)    {        const float* row_ptr = input.ptr<float>(y);        for (int x = 0; x < input.cols; ++x)        {            float value = row_ptr[x];            if ( std::find(out.begin(), out.end(), value) == out.end() )                out.push_back(value);        }    }    if (sort)        std::sort(out.begin(), out.end());    return out;}

Example:

float data[][3] = {  {  9.0,   3.0,  7.0 },  {  3.0,   9.0,  3.0 },  {  1.0,   3.0,  5.0 },  { 90.0, 30.0,  70.0 },  { 30.0, 90.0,  50.0 }};cv::Mat mat(3, 5, CV_32F, &data);std::vector<float> unik = unique(mat, true);for (unsigned int i = 0; i < unik.size(); i++)    std::cout << unik[i] << " ";std::cout << std::endl;

Outputs:

1 3 5 7 9 30 50 70 90 

You could try to build a histogram with number of bins equal to number of possible pixel values.

Here is another suggestion for an implementation using the standard library.

opencv-unique.cpp

#include <opencv2/opencv.hpp>#include <vector>#include <iostream>#include <algorithm>#include <cstdint>/** * @brief Find unique elements of an OpenCV image * * @tparam type is the C++ type to access the image elements. * @param in is the OpenCV single-channel image to find the unique values. Note: This * modifies the image. Make a copy with .clone(), if you need the image afterwards. * * @returns vector of unique elements */template<typename type>std::vector<type> unique(cv::Mat in) {    assert(in.channels() == 1 && "This implementation is only for single-channel images");    auto begin = in.begin<type>(), end = in.end<type>();    auto last = std::unique(begin, end);    // remove adjacent duplicates to reduce size    std::sort(begin, last);                 // sort remaining elements    last = std::unique(begin, last);        // remove duplicates    return std::vector<type>(begin, last);}int main() {    cv::Mat img = (cv::Mat_<uint16_t>(3, 4) << 1, 5, 3, 4, 3, 1, 5, 5, 1, 3, 4, 3);    std::cout << "unique values: ";    auto u = unique<uint16_t>(img);    for (auto v : u)        std::cout << v << " ";    std::cout << std::endl;    return 0;}

Compile and execute yields:

$ g++ -Wall opencv-unique.cpp -o unique -lopencv_core -I /usr/include/opencv4 && ./uniqueunique values: 1 3 4 5

The version above is for single-channel images. You can extend this to multi-channel images (to get unique colors), like this.