Convert a single color with cvtColor

Question

I have a color that I want to convert to a different color space. Is it possible to use cvtColor on a cv::Vec3f directly without creating a 1x1 cv::Mat and populating it with that pixel, using cvtColor on the cv::Mat, then getting the only pixel out of the output? I have tried the following, but it doesn't seem to like getting passed a vector.

Any suggestions?

#include <iostream>

#include <opencv2/opencv.hpp>

int main(int, char*[])
{
    cv::Vec3f hsv;
    hsv[0] = .9;
    hsv[1] = .8;
    hsv[2] = .7;

    std::cout << "HSV: " << hsv << std::endl;

    cv::Vec3b bgr;
    cvtColor(hsv, bgr, CV_HSV2BGR); // OpenCV Error: Assertion failed (scn == 3 && (dcn == 3 || dcn == 4) && (depth == CV_8U || depth == CV_32F)) in cvtColor

    std::cout << "BGR: " << bgr << std::endl; 

    return EXIT_SUCCESS;
}

I also tried this, but get a different error:

#include <iostream>

#include <opencv2/opencv.hpp>

int main(int, char*[])
{
    cv::Mat_<cv::Vec3f> hsv(cv::Vec3f(0.7, 0.7, 0.8));

    std::cout << "HSV: " << hsv << std::endl;

    cv::Mat_<cv::Vec3b> bgr;

    cvtColor(hsv, bgr, CV_HSV2BGR); // OpenCV Error: Assertion failed (!fixedType() || ((Mat*)obj)->type() == mtype) in create

    std::cout << "BGR: " << bgr << std::endl;

    return EXIT_SUCCESS;
}

Answer 1

Your second approach is correct, but you have source and destination of different types in cvtColor, and that causes the error.

Be sure to have both hsv and bgr of the same type, CV_32F here:

#include <opencv2/opencv.hpp>
#include <iostream>

int main()
{
    cv::Mat3f hsv(cv::Vec3f(0.7, 0.7, 0.8));

    std::cout << "HSV: " << hsv << std::endl;

    cv::Mat3f bgr;
    cvtColor(hsv, bgr, CV_HSV2BGR); 

    std::cout << "BGR: " << bgr << std::endl;

    return 0;
}

---

You can use Mat3f for brevity. It's just a typedef:

typedef Mat_<Vec3f> Mat3f;