opencv/modules/gapi/samples/pipeline_modeling_tool/dummy_source.hpp

#ifndef OPENCV_GAPI_PIPELINE_MODELING_TOOL_DUMMY_SOURCE_HPP
#define OPENCV_GAPI_PIPELINE_MODELING_TOOL_DUMMY_SOURCE_HPP

#include <thread>
#include <memory>
#include <chrono>

#include <opencv2/gapi.hpp>
#include <opencv2/gapi/streaming/cap.hpp> // cv::gapi::wip::IStreamSource

#include "utils.hpp"

class DummySource final: public cv::gapi::wip::IStreamSource {
public:
    using WaitStrategy = std::function<void(std::chrono::microseconds)>;
    using Ptr = std::shared_ptr<DummySource>;
    using ts_t = std::chrono::microseconds;

    template <typename DurationT>
    DummySource(const DurationT    latency,
                const OutputDescr& output,
                const bool         drop_frames,
                WaitStrategy&&     wait);

    bool pull(cv::gapi::wip::Data& data) override;
    cv::GMetaArg descr_of() const override;

private:
    int64_t       m_latency;
    cv::Mat       m_mat;
    bool          m_drop_frames;
    int64_t       m_next_tick_ts = -1;
    int64_t       m_curr_seq_id  = 0;
    WaitStrategy  m_wait;
};

template <typename DurationT>
DummySource::DummySource(const DurationT    latency,
                         const OutputDescr& output,
                         const bool         drop_frames,
                         WaitStrategy&&     wait)
    : m_latency(std::chrono::duration_cast<ts_t>(latency).count()),
      m_drop_frames(drop_frames),
      m_wait(std::move(wait)) {
    utils::createNDMat(m_mat, output.dims, output.precision);
    utils::generateRandom(m_mat);
}

bool DummySource::pull(cv::gapi::wip::Data& data) {
    using namespace std::chrono;
    using namespace cv::gapi::streaming;

    // NB: Wait m_latency before return the first frame.
    if (m_next_tick_ts == -1) {
        m_next_tick_ts = utils::timestamp<ts_t>() + m_latency;
    }

    int64_t curr_ts = utils::timestamp<ts_t>();
    if (curr_ts < m_next_tick_ts) {
        /*
         *            curr_ts
         *               |
         *    ------|----*-----|------->
         *                     ^
         *               m_next_tick_ts
         *
         *
         * NB: New frame will be produced at the m_next_tick_ts point.
         */
        m_wait(ts_t{m_next_tick_ts - curr_ts});
    } else if (m_latency != 0) {
        /*
         *                                       curr_ts
         *                         +1         +2    |
         *    |----------|----------|----------|----*-----|------->
         *               ^                     ^
         *         m_next_tick_ts ------------->
         *
         */

        // NB: Count how many frames have been produced since last pull (m_next_tick_ts).
        int64_t num_frames =
            static_cast<int64_t>((curr_ts - m_next_tick_ts) / m_latency);
        // NB: Shift m_next_tick_ts to the nearest tick before curr_ts.
        m_next_tick_ts += num_frames * m_latency;
        // NB: if drop_frames is enabled, update current seq_id and wait for the next tick, otherwise
        // return last written frame (+2 at the picture above) immediately.
        if (m_drop_frames) {
            // NB: Shift tick to the next frame.
            m_next_tick_ts += m_latency;
            // NB: Wait for the next frame.
            m_wait(ts_t{m_next_tick_ts - curr_ts});
            // NB: Drop already produced frames + update seq_id for the current.
            m_curr_seq_id += num_frames + 1;
        }
    }
    // NB: Just increase reference counter not to release mat memory
    // after assigning it to the data.
    cv::Mat mat = m_mat;
    data.meta[meta_tag::timestamp] = utils::timestamp<ts_t>();
    data.meta[meta_tag::seq_id] = m_curr_seq_id++;
    data = mat;
    m_next_tick_ts += m_latency;

    return true;
}

cv::GMetaArg DummySource::descr_of() const {
    return cv::GMetaArg{cv::descr_of(m_mat)};
}

#endif // OPENCV_GAPI_PIPELINE_MODELING_TOOL_DUMMY_SOURCE_HPP