Custom Source and Sink

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Original author	Amir Hassan (kallaballa) amir@.nosp@m.viel.nosp@m.-zu.o.nosp@m.rg
Compatibility	OpenCV >= 4.7

Reading and writing to V4D using custom sources and sinks

In the previous tutorial we used a default video source and a video sink to stream a video through V4D which can be manipulated using OpenGL, NanoVG or OpenCV. In this example we are creating a custom source that generates rainbow frames. For each time the source is invoked the frame is colored a slightly different color. Additionally the custom sink saves individual images instead of a video (only in native builds).

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#include <opencv2/v4d/v4d.hpp>
#ifndef __EMSCRIPTEN__
#  include <opencv2/imgcodecs.hpp>
#endif
 
using namespace cv;
using namespace cv::v4d;
 
class CustomSourceAndSinkPlan : public Plan {
    string hr_ = "Hello Rainbow!";
public:
    void infer(cv::Ptr<V4D> win) override {
        win->capture();
 
        //Render "Hello Rainbow!" over the video
        win->nvg([](const Size& sz, const string& str) {
            using namespace cv::v4d::nvg;
 
            fontSize(40.0f);
            fontFace("sans-bold");
            fillColor(Scalar(255, 0, 0, 255));
            textAlign(NVG_ALIGN_CENTER | NVG_ALIGN_TOP);
            text(sz.width / 2.0, sz.height / 2.0, str.c_str(), str.c_str() + str.size());
        }, win->fbSize(), hr_);
 
        win->write();
    }
};
 
int main() {
    Ptr<V4D> window = V4D::make(960, 960, "Custom Source/Sink");
 
    //Make a source that generates rainbow frames.
    cv::Ptr<Source> src = new Source([](cv::UMat& frame){
        static long cnt = 0;
        //The source is responsible for initializing the frame..
        if(frame.empty())
            frame.create(Size(960, 960), CV_8UC3);
        frame = colorConvert(Scalar(++cnt % 180, 128, 128, 255), COLOR_HLS2BGR);
        return true;
    }, 60.0f);
 
    //Make a sink the saves each frame to a PNG file (does nothing in case of WebAssembly).
    cv::Ptr<Sink> sink = new Sink([](const uint64_t& seq, const cv::UMat& frame){
        try {
#ifndef __EMSCRIPTEN__
            imwrite(std::to_string(seq) + ".png", frame);
#else
            CV_UNUSED(frame);
#endif
        } catch(std::exception& ex) {
            cerr << "Unable to write frame: " << ex.what() << endl;
            return false;
        }
        return true;
    });
 
    //Attach source and sink
    window->setSource(src);
    window->setSink(sink);
 
    window->run<CustomSourceAndSinkPlan>(0);
}