Draw a siemens star in opencv and c++

Question

I'm new in opencv. I should implement a siemens star (http://upload.wikimedia.org/wikipedia/commons/5/50/Siemens_star.svg) in c++ and opencv. After days of searching the internet I'm very frustrated and I hope you can help me. I don't know how to start. My first idea to realize it, is to draw filled triangles from center and crop them with a circle. But I think there are no functions in opencv to draw a triangle. My 2nd approach is to plot all points between two vectors, but my mathematical knowledge is very poor (like my english, sorry ;)). Pleaseeee help me..

Thanks!

Answer 1

this is how you can do it with cv::ellipse :

cv::Mat siemensStar(unsigned int radius, unsigned int number_of_pieces)
{
    // create output image
    cv::Mat star(2*radius, 2*radius, CV_8UC1, 255);

    // center in the middle of the image
    cv::Point2f center(radius,radius);

    // angle of a single arc (mind that there is always one black and one white arc
    float angle = 360.0f/(2*(float)number_of_pieces);

    // draw all arcs
    for(float i=0; i<360; i+=2*angle)
    {
        // draw the outlines anti-aliased or change the line type if you want
        cv::ellipse(star, center, cv::Size(radius,radius), 0, i, i+angle, cv::Scalar(0),-1, CV_AA);
    }

    return star;
}

this works and gives me with the call cv::imshow("siemens star", siemensStar(256,16)); this result:

beware that this might not be what you want (since the optical effect in the center isnt really visible), but thats the problem of rastering (during drawing) I think. We don't work with vector images here. Maybe better if the number of elements is reduced...

Answer 2

You can use ellipse() to draw a group of filled ellipse sectors.

Answer 3

Your assumption is wrong, there is a function to draw a triangle: fillPoly. You could have found that by simply googling "OpenCV draw".