purple_gradient.cpp

#include <cv.h>
#include <highgui.h>

int main() {
    cv::Mat image(200, 200, CV_8UC3);
    for (int i = 0; i < 200; ++i)
        for (int j = 0; j < 200; ++j) {
            image.ptr<unsigned char>(i)[3*j] = j * 255 / 200;
            image.ptr<unsigned char>(i)[3*j + 2] = j * 255 / 200;
        }   
    cv::imwrite("circle.jpg", image);
    return 0;
}