videoio: update plugin API

modules/core/include/opencv2/core.hpp

@@ -92,6 +92,7 @@
         @{
             @defgroup core_hal_intrin_impl Private implementation helpers
         @}
+        @defgroup core_lowlevel_api Low-level API for external libraries / plugins
     @}
 @}
  */

modules/core/include/opencv2/core/llapi/llapi.h

+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+
+
+#ifndef OPENCV_CORE_LLAPI_LLAPI_H
+#define OPENCV_CORE_LLAPI_LLAPI_H
+/**
+@addtogroup core_lowlevel_api
+
+API for OpenCV external plugins:
+- HAL accelerators
+- VideoIO camera backends / decoders / encoders
+- Imgcodecs encoders / decoders
+
+Plugins are usually built separately or before OpenCV (OpenCV can depend on them - like HAL libraries).
+
+Using this approach OpenCV provides some basic low level functionality for external plugins.
+
+@note Preview only (no backward compatibility)
+
+@{
+*/
+
+#ifndef CV_API_CALL
+//! calling convention (including callbacks)
+#define CV_API_CALL
+#endif
+
+typedef enum cvResult
+{
+    CV_ERROR_FAIL = -1,                          //!< Some error occured (TODO Require to fill exception information)
+    CV_ERROR_OK = 0                              //!< No error
+} CvResult;
+
+typedef struct OpenCV_API_Header_t
+{
+    /** @brief valid size of this structure
+     @details `assert(api.header.valid_size >= sizeof(OpenCV_<Name>_API_v<N>));`
+     */
+    size_t valid_size;
+    unsigned min_api_version;                    //!< backward compatible API version
+    unsigned api_version;                        //!< provided API version (features)
+    unsigned opencv_version_major;               //!< compiled OpenCV version
+    unsigned opencv_version_minor;               //!< compiled OpenCV version
+    unsigned opencv_version_patch;               //!< compiled OpenCV version
+    const char* opencv_version_status;           //!< compiled OpenCV version
+    const char* api_description;                 //!< API description (debug purposes only)
+} OpenCV_API_Header;
+
+
+
+#if 0
+
+typedef int (CV_API_CALL *cv_example_callback1_cb_t)(unsigned const char* cb_result, void* cb_context);
+
+struct OpenCV_Example_API_v1
+{
+    OpenCV_API_Header header;
+
+    /** @brief Some API call
+
+    @param param1 description1
+    @param param2 description2
+
+    @note API-CALL 1, API-Version >=1
+     */
+    CvResult (CV_API_CALL *Request1)(int param1, const char* param2);
+
+    /** @brief Register callback
+
+    @param callback function to handle callback
+    @param cb_context context data passed to callback function
+    @param[out] cb_handle callback id (used to unregister callback)
+
+    @note API-CALL 2, API-Version >=1
+     */
+    CvResult (CV_API_CALL *RegisterCallback)(cv_example_callback1_cb_t callback, void* cb_context, CV_OUT unsigned* cb_handle);
+
+    /** @brief Unregister callback
+
+    @param cb_handle callback handle
+
+    @note API-CALL 3, API-Version >=1
+     */
+    CvResult (CV_API_CALL *UnegisterCallback)(unsigned cb_handle);
+
+    ...
+};
+#endif // 0
+
+//! @}
+
+#endif // OPENCV_CORE_LLAPI_LLAPI_H

modules/videoio/CMakeLists.txt

@@ -14,7 +14,8 @@ set(videoio_srcs
   "${CMAKE_CURRENT_LIST_DIR}/src/cap_images.cpp"
   "${CMAKE_CURRENT_LIST_DIR}/src/cap_mjpeg_encoder.cpp"
   "${CMAKE_CURRENT_LIST_DIR}/src/cap_mjpeg_decoder.cpp"
-  "${CMAKE_CURRENT_LIST_DIR}/src/backend.cpp"
+  "${CMAKE_CURRENT_LIST_DIR}/src/backend_plugin.cpp"
+  "${CMAKE_CURRENT_LIST_DIR}/src/backend_static.cpp"
   "${CMAKE_CURRENT_LIST_DIR}/src/container_avi.cpp")
 
 file(GLOB videoio_ext_hdrs

modules/videoio/cmake/plugin.cmake

@@ -12,7 +12,7 @@ function(ocv_create_builtin_videoio_plugin name target videoio_src_file)
 
   add_library(${name} MODULE
     "${CMAKE_CURRENT_LIST_DIR}/src/${videoio_src_file}"
-    "${CMAKE_CURRENT_LIST_DIR}/src/plugin_api.cpp")
+  )
   target_include_directories(${name} PRIVATE "${CMAKE_CURRENT_BINARY_DIR}")
   target_compile_definitions(${name} PRIVATE BUILD_PLUGIN)
   target_link_libraries(${name} PRIVATE ${target})
@@ -60,7 +60,7 @@ function(ocv_create_videoio_plugin default_name target target_desc videoio_src_f
   set(imgproc_ROOT "${modules_ROOT}/imgproc")
   set(imgcodecs_ROOT "${modules_ROOT}/imgcodecs")
 
-  add_library(${OPENCV_PLUGIN_NAME} MODULE "${videoio_ROOT}/src/${videoio_src_file}" "${videoio_ROOT}/src/plugin_api.cpp")
+  add_library(${OPENCV_PLUGIN_NAME} MODULE "${videoio_ROOT}/src/${videoio_src_file}")
   target_include_directories(${OPENCV_PLUGIN_NAME} PRIVATE
     "${CMAKE_CURRENT_BINARY_DIR}"
     "${videoio_ROOT}/src"

modules/videoio/src/backend.hpp

@@ -15,71 +15,30 @@ namespace cv {
 class IBackend
 {
 public:
-    Ptr<IVideoCapture> tryOpenCapture(const std::string & backendName, const std::string & filename, int cameraNum) const;
-    Ptr<IVideoWriter> tryOpenWriter(const std::string & backendName, const std::string& filename, int _fourcc, double fps, const Size &frameSize, bool isColor) const;
-protected:
-    virtual Ptr<IVideoCapture> createCapture(const std::string &filename, int camera) const = 0;
-    virtual Ptr<IVideoWriter> createWriter(const std::string &filename, int fourcc, double fps, const cv::Size &sz, bool isColor) const = 0;
     virtual ~IBackend() {}
+    virtual Ptr<IVideoCapture> createCapture(int camera) const = 0;
+    virtual Ptr<IVideoCapture> createCapture(const std::string &filename) const = 0;
+    virtual Ptr<IVideoWriter>  createWriter(const std::string &filename, int fourcc, double fps, const cv::Size &sz, bool isColor) const = 0;
 };
 
-//==================================================================================================
-
-class StaticBackend : public IBackend
+class IBackendFactory
 {
-    typedef Ptr<IVideoCapture> (*OpenFileFun)(const std::string &);
-    typedef Ptr<IVideoCapture> (*OpenCamFun)(int);
-    typedef Ptr<IVideoWriter> (*OpenWriterFun)(const std::string&, int, double, const Size&, bool);
-private:
-    OpenFileFun FUN_FILE;
-    OpenCamFun FUN_CAM;
-    OpenWriterFun FUN_WRITE;
 public:
-    StaticBackend(OpenFileFun f1, OpenCamFun f2, OpenWriterFun f3)
-        : FUN_FILE(f1), FUN_CAM(f2), FUN_WRITE(f3)
-    {
-    }
-protected:
-    Ptr<IVideoCapture> createCapture(const std::string &filename, int camera) const CV_OVERRIDE
-    {
-        if (filename.empty() && FUN_CAM)
-            return FUN_CAM(camera);
-        if (FUN_FILE)
-            return FUN_FILE(filename);
-        return 0;
-    }
-    Ptr<IVideoWriter> createWriter(const std::string &filename, int fourcc, double fps, const Size &sz, bool isColor) const CV_OVERRIDE
-    {
-        if (FUN_WRITE)
-            return FUN_WRITE(filename, fourcc, fps, sz, isColor);
-        return 0;
-    }
+    virtual ~IBackendFactory() {}
+    virtual Ptr<IBackend> getBackend() const = 0;
 };
 
-//==================================================================================================
+//=============================================================================
 
-class DynamicBackend : public IBackend
-{
-public:
-    class CaptureTable;
-    class WriterTable;
-    class DynamicLib;
-private:
-    DynamicLib * lib;
-    CaptureTable const * cap_tbl;
-    WriterTable const * wri_tbl;
-public:
-    DynamicBackend(const std::string &filename);
-    ~DynamicBackend();
-    static Ptr<DynamicBackend> load(VideoCaptureAPIs api, int mode);
-protected:
-    bool canCreateCapture(cv::VideoCaptureAPIs api) const;
-    bool canCreateWriter(VideoCaptureAPIs api) const;
-    Ptr<IVideoCapture> createCapture(const std::string &filename, int camera) const CV_OVERRIDE;
-    Ptr<IVideoWriter> createWriter(const std::string &filename, int fourcc, double fps, const cv::Size &sz, bool isColor) const CV_OVERRIDE;
-};
+typedef Ptr<IVideoCapture> (*FN_createCaptureFile)(const std::string & filename);
+typedef Ptr<IVideoCapture> (*FN_createCaptureCamera)(int camera);
+typedef Ptr<IVideoWriter>  (*FN_createWriter)(const std::string& filename, int fourcc, double fps, const Size& sz, bool isColor);
+Ptr<IBackendFactory> createBackendFactory(FN_createCaptureFile createCaptureFile,
+                                          FN_createCaptureCamera createCaptureCamera,
+                                          FN_createWriter createWriter);
 
-} // cv::
+Ptr<IBackendFactory> createPluginBackendFactory(VideoCaptureAPIs id, const char* baseName);
 
+} // namespace cv::
 
 #endif // BACKEND_HPP_DEFINED

modules/videoio/src/backend_static.cpp

+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+
+#include "precomp.hpp"
+
+#include "backend.hpp"
+
+namespace cv {
+
+class StaticBackend: public IBackend
+{
+public:
+    FN_createCaptureFile fn_createCaptureFile_;
+    FN_createCaptureCamera fn_createCaptureCamera_;
+    FN_createWriter fn_createWriter_;
+
+    StaticBackend(FN_createCaptureFile fn_createCaptureFile, FN_createCaptureCamera fn_createCaptureCamera, FN_createWriter fn_createWriter)
+        : fn_createCaptureFile_(fn_createCaptureFile), fn_createCaptureCamera_(fn_createCaptureCamera), fn_createWriter_(fn_createWriter)
+    {
+        // nothing
+    }
+    ~StaticBackend() CV_OVERRIDE {}
+
+    Ptr<IVideoCapture> createCapture(int camera) const CV_OVERRIDE
+    {
+        if (fn_createCaptureCamera_)
+            return fn_createCaptureCamera_(camera);
+        return Ptr<IVideoCapture>();
+    }
+    Ptr<IVideoCapture> createCapture(const std::string &filename) const CV_OVERRIDE
+    {
+        if (fn_createCaptureFile_)
+            return fn_createCaptureFile_(filename);
+        return Ptr<IVideoCapture>();
+    }
+    Ptr<IVideoWriter> createWriter(const std::string &filename, int fourcc, double fps, const cv::Size &sz, bool isColor) const CV_OVERRIDE
+    {
+        if (fn_createWriter_)
+            return fn_createWriter_(filename, fourcc, fps, sz, isColor);
+        return Ptr<IVideoWriter>();
+    }
+}; // StaticBackend
+
+class StaticBackendFactory : public IBackendFactory
+{
+protected:
+    Ptr<StaticBackend> backend;
+
+public:
+    StaticBackendFactory(FN_createCaptureFile createCaptureFile, FN_createCaptureCamera createCaptureCamera, FN_createWriter createWriter)
+        : backend(makePtr<StaticBackend>(createCaptureFile, createCaptureCamera, createWriter))
+    {
+        // nothing
+    }
+
+    ~StaticBackendFactory() CV_OVERRIDE {}
+
+    Ptr<IBackend> getBackend() const CV_OVERRIDE
+    {
+        return backend.staticCast<IBackend>();
+    }
+};
+
+Ptr<IBackendFactory> createBackendFactory(FN_createCaptureFile createCaptureFile,
+                                          FN_createCaptureCamera createCaptureCamera,
+                                          FN_createWriter createWriter)
+{
+    return makePtr<StaticBackendFactory>(createCaptureFile, createCaptureCamera, createWriter).staticCast<IBackendFactory>();
+}
+
+} // namespace

modules/videoio/src/cap_ffmpeg.cpp

@@ -351,23 +351,25 @@ cv::Ptr<cv::IVideoWriter> cvCreateVideoWriter_FFMPEG_proxy(const std::string& fi
 
 #include "plugin_api.hpp"
 
-CV_EXTERN_C int cv_domain()
-{
-    return cv::CAP_FFMPEG;
-}
+namespace cv {
 
-CV_EXTERN_C bool cv_open_capture(const char * filename, int, void * &handle)
+static
+CvResult CV_API_CALL cv_capture_open(const char* filename, int camera_index, CV_OUT CvPluginCapture* handle)
 {
+    if (!handle)
+        return CV_ERROR_FAIL;
+    *handle = NULL;
     if (!filename)
-        return false;
-    cv::CvCapture_FFMPEG_proxy *cap = 0;
+        return CV_ERROR_FAIL;
+    CV_UNUSED(camera_index);
+    CvCapture_FFMPEG_proxy *cap = 0;
     try
     {
-        cap = new cv::CvCapture_FFMPEG_proxy(filename);
+        cap = new CvCapture_FFMPEG_proxy(filename);
         if (cap->isOpened())
         {
-            handle = cap;
-            return true;
+            *handle = (CvPluginCapture)cap;
+            return CV_ERROR_OK;
         }
     }
     catch (...)
@@ -375,94 +377,104 @@ CV_EXTERN_C bool cv_open_capture(const char * filename, int, void * &handle)
     }
     if (cap)
         delete cap;
-    return false;
+    return CV_ERROR_FAIL;
 }
 
-CV_EXTERN_C bool cv_get_cap_prop(void * handle, int prop, double & val)
+static
+CvResult CV_API_CALL cv_capture_release(CvPluginCapture handle)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
+    CvCapture_FFMPEG_proxy* instance = (CvCapture_FFMPEG_proxy*)handle;
+    delete instance;
+    return CV_ERROR_OK;
+}
+
+
+static
+CvResult CV_API_CALL cv_capture_get_prop(CvPluginCapture handle, int prop, CV_OUT double* val)
+{
+    if (!handle)
+        return CV_ERROR_FAIL;
+    if (!val)
+        return CV_ERROR_FAIL;
     try
     {
-        cv::CvCapture_FFMPEG_proxy *instance = static_cast<cv::CvCapture_FFMPEG_proxy*>(handle);
-        val = instance->getProperty(prop);
-        return true;
+        CvCapture_FFMPEG_proxy* instance = (CvCapture_FFMPEG_proxy*)handle;
+        *val = instance->getProperty(prop);
+        return CV_ERROR_OK;
     }
     catch (...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_set_cap_prop(void * handle, int prop, double val)
+static
+CvResult CV_API_CALL cv_capture_set_prop(CvPluginCapture handle, int prop, double val)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
     try
     {
-        cv::CvCapture_FFMPEG_proxy *instance = static_cast<cv::CvCapture_FFMPEG_proxy*>(handle);
-        return instance->setProperty(prop, val);
+        CvCapture_FFMPEG_proxy* instance = (CvCapture_FFMPEG_proxy*)handle;
+        return instance->setProperty(prop, val) ? CV_ERROR_OK : CV_ERROR_FAIL;
     }
     catch(...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_grab(void * handle)
+static
+CvResult CV_API_CALL cv_capture_grab(CvPluginCapture handle)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
     try
     {
-        cv::CvCapture_FFMPEG_proxy *instance = static_cast<cv::CvCapture_FFMPEG_proxy*>(handle);
-        return instance->grabFrame();
+        CvCapture_FFMPEG_proxy* instance = (CvCapture_FFMPEG_proxy*)handle;
+        return instance->grabFrame() ? CV_ERROR_OK : CV_ERROR_FAIL;
     }
     catch(...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_retrieve(void * handle, int idx, cv_retrieve_cb_t * callback, void * userdata)
+static
+CvResult CV_API_CALL cv_capture_retrieve(CvPluginCapture handle, int stream_idx, cv_videoio_retrieve_cb_t callback, void* userdata)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
     try
     {
-        cv::CvCapture_FFMPEG_proxy *instance = static_cast<cv::CvCapture_FFMPEG_proxy*>(handle);
-        cv::Mat img;
+        CvCapture_FFMPEG_proxy* instance = (CvCapture_FFMPEG_proxy*)handle;
+        Mat img;
         // TODO: avoid unnecessary copying
-        if (instance->retrieveFrame(idx, img))
-            return callback(img.data, img.step, img.cols, img.rows, img.channels(), userdata);
-        return false;
+        if (instance->retrieveFrame(stream_idx, img))
+            return callback(stream_idx, img.data, img.step, img.cols, img.rows, img.channels(), userdata);
+        return CV_ERROR_FAIL;
     }
     catch(...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_release_capture(void * handle)
-{
-    if (!handle)
-        return false;
-    cv::CvCapture_FFMPEG_proxy *instance = static_cast<cv::CvCapture_FFMPEG_proxy*>(handle);
-    delete instance;
-    return true;
-}
-
-CV_EXTERN_C bool cv_open_writer(const char * filename, int fourcc, double fps, int width, int height, int isColor, void * &handle)
+static
+CvResult CV_API_CALL cv_writer_open(const char* filename, int fourcc, double fps, int width, int height, int isColor,
+                                    CV_OUT CvPluginWriter* handle)
 {
-    cv::Size sz(width, height);
-    cv::CvVideoWriter_FFMPEG_proxy* wrt = 0;
+    Size sz(width, height);
+    CvVideoWriter_FFMPEG_proxy* wrt = 0;
     try
     {
-        wrt = new cv::CvVideoWriter_FFMPEG_proxy(filename, fourcc, fps, sz, isColor != 0);
+        wrt = new CvVideoWriter_FFMPEG_proxy(filename, fourcc, fps, sz, isColor != 0);
         if(wrt && wrt->isOpened())
         {
-            handle = wrt;
-            return true;
+            *handle = (CvPluginWriter)wrt;
+            return CV_ERROR_OK;
         }
     }
     catch(...)
@@ -470,43 +482,79 @@ CV_EXTERN_C bool cv_open_writer(const char * filename, int fourcc, double fps, i
     }
     if (wrt)
         delete wrt;
-    return false;
+    return CV_ERROR_FAIL;
+}
+
+static
+CvResult CV_API_CALL cv_writer_release(CvPluginWriter handle)
+{
+    if (!handle)
+        return CV_ERROR_FAIL;
+    CvVideoWriter_FFMPEG_proxy* instance = (CvVideoWriter_FFMPEG_proxy*)handle;
+    delete instance;
+    return CV_ERROR_OK;
 }
 
-CV_EXTERN_C bool cv_get_wri_prop(void*, int, double&)
+static
+CvResult CV_API_CALL cv_writer_get_prop(CvPluginWriter /*handle*/, int /*prop*/, CV_OUT double* /*val*/)
 {
-    return false;
+    return CV_ERROR_FAIL;
 }
 
-CV_EXTERN_C bool cv_set_wri_prop(void*, int, double)
+static
+CvResult CV_API_CALL cv_writer_set_prop(CvPluginWriter /*handle*/, int /*prop*/, double /*val*/)
 {
-    return false;
+    return CV_ERROR_FAIL;
 }
 
-CV_EXTERN_C bool cv_write(void * handle, const unsigned char * data, int step, int width, int height, int cn)
+static
+CvResult CV_API_CALL cv_writer_write(CvPluginWriter handle, const unsigned char *data, int step, int width, int height, int cn)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
     try
     {
-        cv::CvVideoWriter_FFMPEG_proxy * instance = static_cast<cv::CvVideoWriter_FFMPEG_proxy*>(handle);
-        cv::Mat img(cv::Size(width, height), CV_MAKETYPE(CV_8U, cn), const_cast<uchar*>(data), step);
+        CvVideoWriter_FFMPEG_proxy* instance = (CvVideoWriter_FFMPEG_proxy*)handle;
+        Mat img(Size(width, height), CV_MAKETYPE(CV_8U, cn), const_cast<uchar*>(data), step);
         instance->write(img);
-        return true;
+        return CV_ERROR_OK;
     }
     catch(...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_release_writer(void * handle)
+static const OpenCV_VideoIO_Plugin_API_preview plugin_api_v0 =
 {
-    if (!handle)
-        return false;
-    cv::CvVideoWriter_FFMPEG_proxy * instance = static_cast<cv::CvVideoWriter_FFMPEG_proxy*>(handle);
-    delete instance;
-    return true;
+    {
+        sizeof(OpenCV_VideoIO_Plugin_API_preview), ABI_VERSION, API_VERSION,
+        CV_VERSION_MAJOR, CV_VERSION_MINOR, CV_VERSION_REVISION, CV_VERSION_STATUS,
+        "FFmpeg OpenCV Video I/O plugin"
+    },
+    /*  1*/CAP_FFMPEG,
+    /*  2*/cv_capture_open,
+    /*  3*/cv_capture_release,
+    /*  4*/cv_capture_get_prop,
+    /*  5*/cv_capture_set_prop,
+    /*  6*/cv_capture_grab,
+    /*  7*/cv_capture_retrieve,
+    /*  8*/cv_writer_open,
+    /*  9*/cv_writer_release,
+    /* 10*/cv_writer_get_prop,
+    /* 11*/cv_writer_set_prop,
+    /* 12*/cv_writer_write
+};
+
+} // namespace
+
+const OpenCV_VideoIO_Plugin_API_preview* opencv_videoio_plugin_init_v0(int requested_abi_version, int requested_api_version, void* /*reserved=NULL*/) CV_NOEXCEPT
+{
+    if (requested_abi_version != 0)
+        return NULL;
+    if (requested_api_version != 0)
+        return NULL;
+    return &cv::plugin_api_v0;
 }
 
 #endif // BUILD_PLUGIN

modules/videoio/src/cap_ffmpeg_api.hpp

@@ -6,13 +6,17 @@ extern "C"
 {
 #endif
 
-#if defined _WIN32
+#ifndef OPENCV_FFMPEG_API
+#if defined(__OPENCV_BUILD) || defined(BUILD_PLUGIN)
+#   define OPENCV_FFMPEG_API
+#elif defined _WIN32
 #   define OPENCV_FFMPEG_API __declspec(dllexport)
 #elif defined __GNUC__ && __GNUC__ >= 4
 #   define OPENCV_FFMPEG_API __attribute__ ((visibility ("default")))
 #else
 #   define OPENCV_FFMPEG_API
 #endif
+#endif
 
 enum
 {

modules/videoio/src/cap_gstreamer.cpp

@@ -1629,13 +1629,16 @@ void handleMessage(GstElement * pipeline)
 
 #include "plugin_api.hpp"
 
-CV_EXTERN_C int cv_domain()
-{
-    return cv::CAP_GSTREAMER;
-}
+namespace cv {
 
-CV_EXTERN_C bool cv_open_capture(const char * filename, int camera_index, void * &handle)
+static
+CvResult CV_API_CALL cv_capture_open(const char* filename, int camera_index, CV_OUT CvPluginCapture* handle)
 {
+    if (!handle)
+        return CV_ERROR_FAIL;
+    *handle = NULL;
+    if (!filename)
+        return CV_ERROR_FAIL;
     GStreamerCapture *cap = 0;
     try
     {
@@ -1647,8 +1650,8 @@ CV_EXTERN_C bool cv_open_capture(const char * filename, int camera_index, void *
             res = cap->open(camera_index);
         if (res)
         {
-            handle = cap;
-            return true;
+            *handle = (CvPluginCapture)cap;
+            return CV_ERROR_OK;
         }
     }
     catch (...)
@@ -1656,84 +1659,94 @@ CV_EXTERN_C bool cv_open_capture(const char * filename, int camera_index, void *
     }
     if (cap)
         delete cap;
-    return false;
+    return CV_ERROR_FAIL;
 }
 
-CV_EXTERN_C bool cv_get_cap_prop(void * handle, int prop, double & val)
+static
+CvResult CV_API_CALL cv_capture_release(CvPluginCapture handle)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
+    GStreamerCapture* instance = (GStreamerCapture*)handle;
+    delete instance;
+    return CV_ERROR_OK;
+}
+
+
+static
+CvResult CV_API_CALL cv_capture_get_prop(CvPluginCapture handle, int prop, CV_OUT double* val)
+{
+    if (!handle)
+        return CV_ERROR_FAIL;
+    if (!val)
+        return CV_ERROR_FAIL;
     try
     {
-        GStreamerCapture * instance = static_cast<GStreamerCapture*>(handle);
-        val = instance->getProperty(prop);
-        return true;
+        GStreamerCapture* instance = (GStreamerCapture*)handle;
+        *val = instance->getProperty(prop);
+        return CV_ERROR_OK;
     }
-    catch(...)
+    catch (...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_set_cap_prop(void * handle, int prop, double val)
+static
+CvResult CV_API_CALL cv_capture_set_prop(CvPluginCapture handle, int prop, double val)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
     try
     {
-        GStreamerCapture * instance = static_cast<GStreamerCapture*>(handle);
-        return instance->setProperty(prop, val);
+        GStreamerCapture* instance = (GStreamerCapture*)handle;
+        return instance->setProperty(prop, val) ? CV_ERROR_OK : CV_ERROR_FAIL;
     }
     catch(...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_grab(void * handle)
+static
+CvResult CV_API_CALL cv_capture_grab(CvPluginCapture handle)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
     try
     {
-        GStreamerCapture * instance = static_cast<GStreamerCapture*>(handle);
-        return instance->grabFrame();
+        GStreamerCapture* instance = (GStreamerCapture*)handle;
+        return instance->grabFrame() ? CV_ERROR_OK : CV_ERROR_FAIL;
     }
     catch(...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_retrieve(void * handle, int idx, cv_retrieve_cb_t * callback, void * userdata)
+static
+CvResult CV_API_CALL cv_capture_retrieve(CvPluginCapture handle, int stream_idx, cv_videoio_retrieve_cb_t callback, void* userdata)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
     try
     {
-        GStreamerCapture * instance = static_cast<GStreamerCapture*>(handle);
+        GStreamerCapture* instance = (GStreamerCapture*)handle;
         Mat img;
         // TODO: avoid unnecessary copying - implement lower level GStreamerCapture::retrieve
-        if (instance->retrieveFrame(idx, img))
-            return callback(img.data, img.step, img.cols, img.rows, img.channels(), userdata);
-        return false;
+        if (instance->retrieveFrame(stream_idx, img))
+            return callback(stream_idx, img.data, img.step, img.cols, img.rows, img.channels(), userdata);
+        return CV_ERROR_FAIL;
     }
     catch(...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_release_capture(void * handle)
-{
-    if (!handle)
-        return false;
-    GStreamerCapture * instance = static_cast<GStreamerCapture*>(handle);
-    delete instance;
-    return true;
-}
-
-CV_EXTERN_C bool cv_open_writer(const char * filename, int fourcc, double fps, int width, int height, int isColor, void * &handle)
+static
+CvResult CV_API_CALL cv_writer_open(const char* filename, int fourcc, double fps, int width, int height, int isColor,
+                                    CV_OUT CvPluginWriter* handle)
 {
     CvVideoWriter_GStreamer* wrt = 0;
     try
@@ -1742,8 +1755,8 @@ CV_EXTERN_C bool cv_open_writer(const char * filename, int fourcc, double fps, i
         CvSize sz = { width, height };
         if(wrt && wrt->open(filename, fourcc, fps, sz, isColor))
         {
-            handle = wrt;
-            return true;
+            *handle = (CvPluginWriter)wrt;
+            return CV_ERROR_OK;
         }
     }
     catch(...)
@@ -1751,45 +1764,81 @@ CV_EXTERN_C bool cv_open_writer(const char * filename, int fourcc, double fps, i
     }
     if (wrt)
         delete wrt;
-    return false;
+    return CV_ERROR_FAIL;
 }
 
-CV_EXTERN_C bool cv_get_wri_prop(void*, int, double&)
+static
+CvResult CV_API_CALL cv_writer_release(CvPluginWriter handle)
 {
-    return false;
+    if (!handle)
+        return CV_ERROR_FAIL;
+    CvVideoWriter_GStreamer* instance = (CvVideoWriter_GStreamer*)handle;
+    delete instance;
+    return CV_ERROR_OK;
 }
 
-CV_EXTERN_C bool cv_set_wri_prop(void*, int, double)
+static
+CvResult CV_API_CALL cv_writer_get_prop(CvPluginWriter /*handle*/, int /*prop*/, CV_OUT double* /*val*/)
 {
-    return false;
+    return CV_ERROR_FAIL;
+}
+
+static
+CvResult CV_API_CALL cv_writer_set_prop(CvPluginWriter /*handle*/, int /*prop*/, double /*val*/)
+{
+    return CV_ERROR_FAIL;
 }
 
-CV_EXTERN_C bool cv_write(void * handle, const unsigned char * data, int step, int width, int height, int cn)
+static
+CvResult CV_API_CALL cv_writer_write(CvPluginWriter handle, const unsigned char *data, int step, int width, int height, int cn)
 {
     if (!handle)
-        return false;
+        return CV_ERROR_FAIL;
     try
     {
-        CvVideoWriter_GStreamer * instance = static_cast<CvVideoWriter_GStreamer*>(handle);
+        CvVideoWriter_GStreamer* instance = (CvVideoWriter_GStreamer*)handle;
         CvSize sz = { width, height };
         IplImage img;
         cvInitImageHeader(&img, sz, IPL_DEPTH_8U, cn);
         cvSetData(&img, const_cast<unsigned char*>(data), step);
-        return instance->writeFrame(&img);
+        return instance->writeFrame(&img) ? CV_ERROR_OK : CV_ERROR_FAIL;
     }
     catch(...)
     {
-        return false;
+        return CV_ERROR_FAIL;
     }
 }
 
-CV_EXTERN_C bool cv_release_writer(void * handle)
+static const OpenCV_VideoIO_Plugin_API_preview plugin_api_v0 =
 {
-    if (!handle)
-        return false;
-    CvVideoWriter_GStreamer * instance = static_cast<CvVideoWriter_GStreamer*>(handle);
-    delete instance;
-    return true;
+    {
+        sizeof(OpenCV_VideoIO_Plugin_API_preview), ABI_VERSION, API_VERSION,
+        CV_VERSION_MAJOR, CV_VERSION_MINOR, CV_VERSION_REVISION, CV_VERSION_STATUS,
+        "GStreamer OpenCV Video I/O plugin"
+    },
+    /*  1*/CAP_GSTREAMER,
+    /*  2*/cv_capture_open,
+    /*  3*/cv_capture_release,
+    /*  4*/cv_capture_get_prop,
+    /*  5*/cv_capture_set_prop,
+    /*  6*/cv_capture_grab,
+    /*  7*/cv_capture_retrieve,
+    /*  8*/cv_writer_open,
+    /*  9*/cv_writer_release,
+    /* 10*/cv_writer_get_prop,
+    /* 11*/cv_writer_set_prop,
+    /* 12*/cv_writer_write
+};
+
+} // namespace
+
+const OpenCV_VideoIO_Plugin_API_preview* opencv_videoio_plugin_init_v0(int requested_abi_version, int requested_api_version, void* /*reserved=NULL*/) CV_NOEXCEPT
+{
+    if (requested_abi_version != 0)
+        return NULL;
+    if (requested_api_version != 0)
+        return NULL;
+    return &cv::plugin_api_v0;
 }
 
 #endif // BUILD_PLUGIN

modules/videoio/src/plugin_api.cpp

-// This file is part of OpenCV project.
-// It is subject to the license terms in the LICENSE file found in the top-level directory
-// of this distribution and at http://opencv.org/license.html.
-
-#ifdef BUILD_PLUGIN
-
-#include "plugin_api.hpp"
-#include "opencv2/core/version.hpp"
-
-void cv_get_version(int & major, int & minor, int & patch, int & api, int & abi)
-{
-    major = CV_VERSION_MAJOR;
-    minor = CV_VERSION_MINOR;
-    patch = CV_VERSION_REVISION;
-    api = API_VERSION;
-    abi = ABI_VERSION;
-}
-
-#endif // BUILD_PLUGIN

modules/videoio/src/plugin_api.hpp

@@ -5,63 +5,164 @@
 #ifndef PLUGIN_API_HPP
 #define PLUGIN_API_HPP
 
+#include <opencv2/core/cvdef.h>
+#include <opencv2/core/llapi/llapi.h>
+
 // increase for backward-compatible changes, e.g. add new function
 // Main API <= Plugin API -> plugin is compatible
-#define API_VERSION 1
+#define API_VERSION 0 // preview
 // increase for incompatible changes, e.g. remove function argument
 // Main ABI == Plugin ABI -> plugin is compatible
-#define ABI_VERSION 1
+#define ABI_VERSION 0 // preview
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-// common
-typedef void cv_get_version_t(int & major, int & minor, int & patch, int & api, int & abi);
-typedef int cv_domain_t();
-
-// capture
-typedef bool cv_open_capture_t(const char * filename, int camera_index, void * &handle);
-typedef bool cv_get_cap_prop_t(void * handle, int prop, double & val);
-typedef bool cv_set_cap_prop_t(void * handle, int prop, double val);
-typedef bool cv_grab_t(void * handle);
-// callback function type
-typedef bool cv_retrieve_cb_t(unsigned char * data, int step, int width, int height, int cn, void * userdata);
-typedef bool cv_retrieve_t(void * handle, int idx, cv_retrieve_cb_t * cb, void * userdata);
-typedef bool cv_release_capture_t(void * handle);
-
-// writer
-typedef bool cv_open_writer_t(const char * filename, int fourcc, double fps, int width, int height, int isColor, void * &handle);
-typedef bool cv_get_wri_prop_t(void * handle, int prop, double & val);
-typedef bool cv_set_wri_prop_t(void * handle, int prop, double val);
-typedef bool cv_write_t(void * handle, const unsigned char * data, int step, int width, int height, int cn);
-typedef bool cv_release_writer_t(void * handle);
+typedef CvResult (CV_API_CALL *cv_videoio_retrieve_cb_t)(int stream_idx, unsigned const char* data, int step, int width, int height, int cn, void* userdata);
+
+typedef struct CvPluginCapture_t* CvPluginCapture;
+typedef struct CvPluginWriter_t* CvPluginWriter;
+
+typedef struct OpenCV_VideoIO_Plugin_API_preview
+{
+    OpenCV_API_Header api_header;
+
+    /** OpenCV capture ID (VideoCaptureAPIs)
+    @note API-ENTRY 1, API-Version == 0
+     */
+    int captureAPI;
+
+    /** @brief Open video capture
+
+    @param filename File name or NULL to use camera_index instead
+    @param camera_index Camera index (used if filename == NULL)
+    @param[out] handle pointer on Capture handle
+
+    @note API-CALL 2, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Capture_open)(const char* filename, int camera_index, CV_OUT CvPluginCapture* handle);
+
+    /** @brief Release Capture handle
+
+    @param handle Capture handle
+
+    @note API-CALL 3, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Capture_release)(CvPluginCapture handle);
+
+    /** @brief Get property value
+
+    @param handle Capture handle
+    @param prop Property index
+    @param[out] val property value
+
+    @note API-CALL 4, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Capture_getProperty)(CvPluginCapture handle, int prop, CV_OUT double* val);
+
+    /** @brief Set property value
+
+    @param handle Capture handle
+    @param prop Property index
+    @param val property value
+
+    @note API-CALL 5, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Capture_setProperty)(CvPluginCapture handle, int prop, double val);
+
+    /** @brief Grab frame
+
+    @param handle Capture handle
+
+    @note API-CALL 6, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Capture_grab)(CvPluginCapture handle);
+
+    /** @brief Retrieve frame
+
+    @param handle Capture handle
+    @param stream_idx stream index to retrieve (BGR/IR/depth data)
+    @param callback retrieve callback (synchronous)
+    @param userdata callback context data
+
+    @note API-CALL 7, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Capture_retreive)(CvPluginCapture handle, int stream_idx, cv_videoio_retrieve_cb_t callback, void* userdata);
+
+
+    /** @brief Try to open video writer
+
+    @param filename File name or NULL to use camera_index instead
+    @param camera_index Camera index (used if filename == NULL)
+    @param[out] handle pointer on Writer handle
+
+    @note API-CALL 8, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Writer_open)(const char* filename, int fourcc, double fps, int width, int height, int isColor,
+                                         CV_OUT CvPluginWriter* handle);
+
+    /** @brief Release Writer handle
+
+    @param handle Writer handle
+
+    @note API-CALL 9, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Writer_release)(CvPluginWriter handle);
+
+    /** @brief Get property value
+
+    @param handle Capture handle
+    @param prop Property index
+    @param[out] val property value
+
+    @note API-CALL 10, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Writer_getProperty)(CvPluginWriter handle, int prop, CV_OUT double* val);
+
+    /** @brief Set property value
+
+    @param handle Capture handle
+    @param prop Property index
+    @param val property value
+
+    @note API-CALL 11, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Writer_setProperty)(CvPluginWriter handle, int prop, double val);
+
+    /** @brief Write frame
+
+    @param handle Capture handle
+    @param data Capture handle
+    @param step step in bytes
+    @param width frame width in pixels
+    @param height frame height
+    @param cn number of channels per pixel
+
+    @note API-CALL 12, API-Version == 0
+     */
+    CvResult (CV_API_CALL *Writer_write)(CvPluginWriter handle, const unsigned char *data, int step, int width, int height, int cn);
+
+} OpenCV_VideoIO_Plugin_API_preview;
 
 #ifdef BUILD_PLUGIN
 
+#ifndef CV_PLUGIN_EXPORTS
 #if (defined _WIN32 || defined WINCE || defined __CYGWIN__)
 #  define CV_PLUGIN_EXPORTS __declspec(dllexport)
 #elif defined __GNUC__ && __GNUC__ >= 4
 #  define CV_PLUGIN_EXPORTS __attribute__ ((visibility ("default")))
 #endif
+#endif
 
-CV_PLUGIN_EXPORTS cv_get_version_t cv_get_version;
-CV_PLUGIN_EXPORTS cv_domain_t cv_domain;
-
-CV_PLUGIN_EXPORTS cv_open_capture_t cv_open_capture;
-CV_PLUGIN_EXPORTS cv_get_cap_prop_t cv_get_cap_prop;
-CV_PLUGIN_EXPORTS cv_set_cap_prop_t cv_set_cap_prop;
-CV_PLUGIN_EXPORTS cv_grab_t cv_grab;
-CV_PLUGIN_EXPORTS cv_retrieve_t cv_retrieve;
-CV_PLUGIN_EXPORTS cv_release_capture_t cv_release_capture;
-
-CV_PLUGIN_EXPORTS cv_open_writer_t cv_open_writer;
-CV_PLUGIN_EXPORTS cv_get_wri_prop_t cv_get_wri_prop;
-CV_PLUGIN_EXPORTS cv_set_wri_prop_t cv_set_wri_prop;
-CV_PLUGIN_EXPORTS cv_write_t cv_write;
-CV_PLUGIN_EXPORTS cv_release_writer_t cv_release_writer;
+CV_PLUGIN_EXPORTS
+const OpenCV_VideoIO_Plugin_API_preview* CV_API_CALL opencv_videoio_plugin_init_v0
+        (int requested_abi_version, int requested_api_version, void* reserved /*NULL*/) CV_NOEXCEPT;
 
-#endif
+#else  // BUILD_PLUGIN
+typedef const OpenCV_VideoIO_Plugin_API_preview* (CV_API_CALL *FN_opencv_videoio_plugin_init_t)
+        (int requested_abi_version, int requested_api_version, void* reserved /*NULL*/);
+#endif  // BUILD_PLUGIN
 
 
 #ifdef __cplusplus