G-API: Introduced a Text Detection sample

This sample models the Text Detection demo from OMZ:
https://github.com/openvinotoolkit/open_model_zoo/tree/2020.4/demos/text_detection_demo

Also: renamed cv::gapi::size() to cv::gapi::streaming::size()

modules/gapi/include/opencv2/gapi/core.hpp

@@ -508,19 +508,23 @@ namespace core {
             return in.withType(in.depth, in.chan).withSize(dsize);
         }
     };
+} // namespace core
 
-    G_TYPED_KERNEL(GSize, <GOpaque<Size>(GMat)>, "org.opencv.core.size") {
-        static GOpaqueDesc outMeta(const GMatDesc&) {
-            return empty_gopaque_desc();
-        }
-    };
+namespace streaming {
 
-    G_TYPED_KERNEL(GSizeR, <GOpaque<Size>(GOpaque<Rect>)>, "org.opencv.core.sizeR") {
-        static GOpaqueDesc outMeta(const GOpaqueDesc&) {
-            return empty_gopaque_desc();
-        }
-    };
-}
+// Operations for Streaming (declared in this header for convenience)
+G_TYPED_KERNEL(GSize, <GOpaque<Size>(GMat)>, "org.opencv.streaming.size") {
+    static GOpaqueDesc outMeta(const GMatDesc&) {
+        return empty_gopaque_desc();
+    }
+};
+
+G_TYPED_KERNEL(GSizeR, <GOpaque<Size>(GOpaque<Rect>)>, "org.opencv.streaming.sizeR") {
+    static GOpaqueDesc outMeta(const GOpaqueDesc&) {
+        return empty_gopaque_desc();
+    }
+};
+} // namespace streaming
 
 //! @addtogroup gapi_math
 //! @{
@@ -1753,9 +1757,10 @@ GAPI_EXPORTS GMat warpAffine(const GMat& src, const Mat& M, const Size& dsize, i
                              int borderMode = cv::BORDER_CONSTANT, const Scalar& borderValue = Scalar());
 //! @} gapi_transform
 
+namespace streaming {
 /** @brief Gets dimensions from Mat.
 
-@note Function textual ID is "org.opencv.core.size"
+@note Function textual ID is "org.opencv.streaming.size"
 
 @param src Input tensor
 @return Size (tensor dimensions).
@@ -1765,12 +1770,13 @@ GAPI_EXPORTS GOpaque<Size> size(const GMat& src);
 /** @overload
 Gets dimensions from rectangle.
 
-@note Function textual ID is "org.opencv.core.sizeR"
+@note Function textual ID is "org.opencv.streaming.sizeR"
 
 @param r Input rectangle.
 @return Size (rectangle dimensions).
 */
 GAPI_EXPORTS GOpaque<Size> size(const GOpaque<Rect>& r);
+} //namespace streaming
 } //namespace gapi
 } //namespace cv
 

modules/gapi/include/opencv2/gapi/gstreaming.hpp

@@ -111,6 +111,22 @@ public:
      */
     GAPI_WRAP void setSource(const gapi::wip::IStreamSource::Ptr& s);
 
+    /**
+     * @brief Constructs and specifies an input video stream for a
+     * single-input computation pipeline with the given parameters.
+     *
+     * Throws if pipeline is already running. Use stop() and then
+     * setSource() to run the graph on a new video stream.
+     *
+     * @overload
+     * @param args arguments used to contruct and initialize a stream
+     * source.
+     */
+    template<typename T, typename... Args>
+    void setSource(Args&&... args) {
+        setSource(cv::gapi::wip::make_src<T>(std::forward<Args>(args)...));
+    }
+
     /**
      * @brief Start the pipeline execution.
      *

modules/gapi/include/opencv2/gapi/infer/parsers.hpp

@@ -122,4 +122,16 @@ GAPI_EXPORTS std::tuple<GArray<Rect>, GArray<int>> parseYolo(const GMat& in,
 } // namespace gapi
 } // namespace cv
 
+// Reimport parseSSD & parseYolo under their initial namespace
+namespace cv {
+namespace gapi {
+namespace streaming {
+
+using cv::gapi::parseSSD;
+using cv::gapi::parseYolo;
+
+} // namespace streaming
+} // namespace gapi
+} // namespace cv
+
 #endif // OPENCV_GAPI_PARSERS_HPP