2019-11-24

OpenCV trace VideoCapture 流程

本篇紀錄一下 OpenCV 的 VideoCapture 的啟動流程。

Trace 的 OpenCV 版本為：3.4.8 / 4.1.2
以下範例為 Python 的開啟 VideoCapture 範例，C++版本就先不列出來了，
以下紀錄為 VideoCapture 建構流程，以及每幀frame讀取出來的流程。

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import cv2

cap = cv2.VideoCapture(0)
while(True):
    ret, frame = cap.read()
    cv2.imshow('frame', frame)

VideoCapture 建構流程

VideoCapture::VideoCapture()建構子
從 VideoCapture::VideoCapture() 開始吧！
https://github.com/opencv/opencv/blob/master/modules/videoio/src/cap.cpp

modules/videoio/src/cap.cpplink

VideoCapture::VideoCapture(const String& filename, int apiPreference) : throwOnFail(false)
{
    open(filename, apiPreference); // 1-1
}

VideoCapture::VideoCapture(int index, int apiPreference) : throwOnFail(false)
{
    open(index, apiPreference); // 2-1
}

VideoCapture::open()
VideoCapture::open() 有兩種，一種是帶字串的，另一種是帶整數的。
帶字串的會呼叫 getAvailableBackends_CaptureByFilename
帶整數的會呼叫 getAvailableBackends_CaptureByIndex

modules/videoio/src/cap.cpplink

bool VideoCapture::open(const String& filename, int apiPreference) // 1-1
{
    if (isOpened()) release();

    const std::vector<VideoBackendInfo> backends =
        cv::videoio_registry::getAvailableBackends_CaptureByFilename(); // 1-2

    for (size_t i = 0; i < backends.size(); i++)
    {
        const VideoBackendInfo& info = backends[i];
        if (apiPreference == CAP_ANY || apiPreference == info.id)
        {
            const Ptr<IBackend> backend = info.backendFactory->getBackend(); // 1-9 backendFactory
            if (!backend.empty())
            {
                try
                {
                    icap = backend->createCapture(filename); // 1-10
                    if (!icap.empty())
                    {
                        if (icap->isOpened())
                            return true;
                        icap.release();
                    }
                } catch(...) {
                    // ...
                }
            }
        }
    }

    return false;
}

bool  VideoCapture::open(int cameraNum, int apiPreference) // 2-1
{
    if (isOpened()) release();

    if (apiPreference == CAP_ANY)
    {
        // interpret preferred interface (0 = autodetect)
        int backendID = (cameraNum / 100) * 100;
        if (backendID)
        {
            cameraNum %= 100;
            apiPreference = backendID;
        }
    }

    const std::vector<VideoBackendInfo> backends =
        cv::videoio_registry::getAvailableBackends_CaptureByIndex(); // 2-2
    for (size_t i = 0; i < backends.size(); i++)
    {
        const VideoBackendInfo& info = backends[i];
        if (apiPreference == CAP_ANY || apiPreference == info.id)
        {
            const Ptr<IBackend> backend = info.backendFactory->getBackend();
            if (!backend.empty())
            {
                try
                {
                    icap = backend->createCapture(cameraNum);
                    if (!icap.empty())
                    {
                        if (icap->isOpened())
                            return true;
                        icap.release();
                    }
                } catch(...) {
                    // ...
                }
            }
        }
    }

    return false;
}

IBackend 與 IBackendFactory 的關係，IBackendFactory 就是專門生產 IBackend 的物件。
常見的 Singleton 設計模型 static VideoBackendRegistry& getInstance

https://github.com/opencv/opencv/blob/master/modules/videoio/src/videoio_registry.cpp

modules/videoio/src/videoio_registry.cpplink

#define DECLARE_DYNAMIC_BACKEND(cap, name, mode) \
{ \
    cap, (BackendMode)(mode), 1000, name, createPluginBackendFactory(cap, name) \
}

#define DECLARE_STATIC_BACKEND(cap, name, mode, createCaptureFile, createCaptureCamera, createWriter) \
{ \
    cap, (BackendMode)(mode), 1000, name, createBackendFactory(createCaptureFile, createCaptureCamera, createWriter) \
}

// 1-5
static const struct VideoBackendInfo builtin_backends[] = 
{
    // ...
    // Windows
#ifdef HAVE_DSHOW
    DECLARE_STATIC_BACKEND(CAP_DSHOW, "DSHOW", MODE_CAPTURE_BY_INDEX, 0, create_DShow_capture, 0),
#endif

    // Linux, some Unix
#if defined HAVE_CAMV4L2 // 1-6
    DECLARE_STATIC_BACKEND(CAP_V4L2, "V4L2", MODE_CAPTURE_ALL, create_V4L_capture_file, create_V4L_capture_cam, 0),
#elif defined HAVE_VIDEOIO
    DECLARE_STATIC_BACKEND(CAP_V4L, "V4L_BSD", MODE_CAPTURE_ALL, create_V4L_capture_file, create_V4L_capture_cam, 0),
#endif
    // ...
};

class VideoBackendRegistry
{
protected:
    std::vector<VideoBackendInfo> enabledBackends;
    VideoBackendRegistry() // 1-4
    {
        const int N = sizeof(builtin_backends)/sizeof(builtin_backends[0]);
        enabledBackends.assign(builtin_backends, builtin_backends + N); // 1-5 builtin_backends
        for (int i = 0; i < N; i++)
        {
            VideoBackendInfo& info = enabledBackends[i];
            info.priority = 1000 - i * 10;
        }

        // ...
        std::sort(enabledBackends.begin(), enabledBackends.end(), sortByPriority); // 排序
    }

    // ...
    static VideoBackendRegistry& getInstance() // 1-3, 2-3 常見的 instance 技巧！
    {
        static VideoBackendRegistry g_instance; // 1-4 VideoBackendRegistry()
        return g_instance;
    }
    // ...
    inline std::vector<VideoBackendInfo> getAvailableBackends_CaptureByIndex() const // 2-4
    {
        std::vector<VideoBackendInfo> result;
        for (size_t i = 0; i < enabledBackends.size(); i++)
        {
            const VideoBackendInfo& info = enabledBackends[i];
            if (info.mode & MODE_CAPTURE_BY_INDEX)
                result.push_back(info);
        }
        return result;
    }
    inline std::vector<VideoBackendInfo> getAvailableBackends_CaptureByFilename() const // 1-8
    {
        std::vector<VideoBackendInfo> result;
        for (size_t i = 0; i < enabledBackends.size(); i++)
        {
            const VideoBackendInfo& info = enabledBackends[i];
            if (info.mode & MODE_CAPTURE_BY_FILENAME)
                result.push_back(info);
        }
        return result;
    }

};

namespace videoio_registry {

std::vector<VideoBackendInfo> getAvailableBackends_CaptureByIndex() // 2-2
{
    // 2-3 getInstance, 2-4 getAvailableBackends_CaptureByIndex
    const std::vector<VideoBackendInfo> result = VideoBackendRegistry::getInstance().getAvailableBackends_CaptureByIndex();
    return result;
}
std::vector<VideoBackendInfo> getAvailableBackends_CaptureByFilename() // 1-2
{
    // 1-3 getInstance, 1-8 getAvailableBackends_CaptureByFilename
    const std::vector<VideoBackendInfo> result = VideoBackendRegistry::getInstance().getAvailableBackends_CaptureByFilename();
    return result;
}
}

https://github.com/opencv/opencv/blob/master/modules/videoio/src/videoio_registry.hpp

modules/videoio/src/videoio_registry.hpplink

struct VideoBackendInfo {
    VideoCaptureAPIs id;
    BackendMode mode;
    int priority;     // 1000-<index*10> - default builtin priority
                      // 0 - disabled (OPENCV_VIDEOIO_PRIORITY_<name> = 0)
                      // >10000 - prioritized list (OPENCV_VIDEOIO_PRIORITY_LIST)
    const char* name;
    Ptr<IBackendFactory> backendFactory; // 1-9
};

https://github.com/opencv/opencv/blob/master/modules/videoio/src/backend.hpp

modules/videoio/src/backend.hpplink

class IBackend
{
public:
    virtual ~IBackend() {}
    virtual Ptr<IVideoCapture> createCapture(int camera) const = 0;
    virtual Ptr<IVideoCapture> createCapture(const std::string &filename) const = 0; // 1-10
    virtual Ptr<IVideoWriter>  createWriter(const std::string &filename, int fourcc, double fps, const cv::Size &sz, bool isColor) const = 0;
};

class IBackendFactory
{
public:
    virtual ~IBackendFactory() {}
    virtual Ptr<IBackend> getBackend() const = 0; // 要看是實作哪種 IBackendFactory
};

https://github.com/opencv/opencv/blob/master/modules/videoio/src/cap_interface.hpp

modules/videoio/src/cap_interface.hpplink

Ptr<IVideoCapture> create_DShow_capture(int index);
// ...
Ptr<IVideoCapture> create_V4L_capture_cam(int index); // 1-6, 1-7
// ...
Ptr<IVideoCapture> create_Images_capture(const std::string &filename);

https://github.com/opencv/opencv/blob/master/modules/videoio/src/cap_v4l.cpp

modules/videoio/src/cap_v4l.cpplink

Ptr<IVideoCapture> create_V4L_capture_cam(int index) // 1-7
{
    cv::CvCaptureCAM_V4L* capture = new cv::CvCaptureCAM_V4L(); // 下一篇再介紹

    if (capture->open(index))
        return makePtr<LegacyCapture>(capture);

    delete capture;
    return NULL;
}