Merge pull request #18391 from dmatveev:dm/gframe_00_new_type

* G-API: Make GFrame a new (distinct) G-type, not an alias to GMat

- The underlying host type is still cv::Mat, a new cv::MediaFrame
  type is to be added as a separate PR

* Fix warnings and review comments

- Somewhow there was a switch() without a default: clause in Fluid

modules/gapi/include/opencv2/gapi/gcommon.hpp

@@ -84,6 +84,7 @@ enum class GShape: int
     GSCALAR,
     GARRAY,
     GOPAQUE,
+    GFRAME,
 };
 
 struct GCompileArg;

modules/gapi/include/opencv2/gapi/gframe.hpp

@@ -0,0 +1,59 @@
+// 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.
+//
+// Copyright (C) 2020 Intel Corporation
+
+
+#ifndef OPENCV_GAPI_GFRAME_HPP
+#define OPENCV_GAPI_GFRAME_HPP
+
+#include <ostream>
+#include <memory>                 // std::shared_ptr
+
+#include <opencv2/gapi/opencv_includes.hpp>
+#include <opencv2/gapi/gcommon.hpp> // GShape
+
+#include <opencv2/gapi/gmat.hpp>
+#include <opencv2/gapi/own/assert.hpp>
+
+// TODO GAPI_EXPORTS or so
+namespace cv
+{
+// Forward declaration; GNode and GOrigin are an internal
+// (user-inaccessible) classes.
+class GNode;
+struct GOrigin;
+
+/** \addtogroup gapi_data_objects
+ * @{
+ */
+class GAPI_EXPORTS_W_SIMPLE GFrame
+{
+public:
+    GAPI_WRAP GFrame();                       // Empty constructor
+    GFrame(const GNode &n, std::size_t out);  // Operation result constructor
+
+    GOrigin& priv();                        // Internal use only
+    const GOrigin& priv()  const;           // Internal use only
+
+private:
+    std::shared_ptr<GOrigin> m_priv;
+};
+/** @} */
+
+/**
+ * \addtogroup gapi_meta_args
+ * @{
+ */
+struct GAPI_EXPORTS GFrameDesc
+{
+};
+static inline GFrameDesc empty_gframe_desc() { return GFrameDesc{}; }
+/** @} */
+
+GAPI_EXPORTS std::ostream& operator<<(std::ostream& os, const cv::GFrameDesc &desc);
+
+} // namespace cv
+
+#endif // OPENCV_GAPI_GFRAME_HPP

modules/gapi/include/opencv2/gapi/gmat.hpp

@@ -2,7 +2,7 @@
 // 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.
 //
-// Copyright (C) 2018 Intel Corporation
+// Copyright (C) 2018-2020 Intel Corporation
 
 
 #ifndef OPENCV_GAPI_GMAT_HPP
@@ -65,12 +65,6 @@ public:
     using GMat::GMat;
 };
 
-class GAPI_EXPORTS GFrame : public GMat
-{
-public:
-    using GMat::GMat;
-};
-
 /** @} */
 
 /**

modules/gapi/include/opencv2/gapi/gtype_traits.hpp

@@ -15,6 +15,7 @@
 #include <opencv2/gapi/gscalar.hpp>
 #include <opencv2/gapi/garray.hpp>
 #include <opencv2/gapi/gopaque.hpp>
+#include <opencv2/gapi/gframe.hpp>
 #include <opencv2/gapi/streaming/source.hpp>
 #include <opencv2/gapi/gcommon.hpp>