Merge pull request #16050 from dmatveev:dm/ocv42_gapi_doc_fixup

* G-API: Addressed various documentation issues

- Fixed various typos and missing references;
- Added brief documentaion on G_TYPED_KERNEL and G_COMPOUND_KERNEL macros;
- Briefly described GComputationT<>;
- Briefly described G-API data objects (in a group section).

* G-API: Some clean-ups in doxygen, also a chapter on Render API

* G-API: Expose more graph compilation arguments in the documentation

* G-API: Address documentation review comments

modules/gapi/samples/api_ref_snippets.cpp

@@ -9,6 +9,44 @@
 #include <opencv2/gapi/fluid/core.hpp>
 #include <opencv2/gapi/fluid/imgproc.hpp>
 
+static void typed_example()
+{
+    const cv::Size sz(32, 32);
+    cv::Mat
+        in_mat1        (sz, CV_8UC1),
+        in_mat2        (sz, CV_8UC1),
+        out_mat_untyped(sz, CV_8UC1),
+        out_mat_typed1 (sz, CV_8UC1),
+        out_mat_typed2 (sz, CV_8UC1);
+    cv::randu(in_mat1, cv::Scalar::all(0), cv::Scalar::all(255));
+    cv::randu(in_mat2, cv::Scalar::all(0), cv::Scalar::all(255));
+
+    //! [Untyped_Example]
+    // Untyped G-API ///////////////////////////////////////////////////////////
+    cv::GComputation cvtU([]()
+    {
+        cv::GMat in1, in2;
+        cv::GMat out = cv::gapi::add(in1, in2);
+        return cv::GComputation({in1, in2}, {out});
+    });
+    std::vector<cv::Mat> u_ins  = {in_mat1, in_mat2};
+    std::vector<cv::Mat> u_outs = {out_mat_untyped};
+    cvtU.apply(u_ins, u_outs);
+    //! [Untyped_Example]
+
+    //! [Typed_Example]
+    // Typed G-API /////////////////////////////////////////////////////////////
+    cv::GComputationT<cv::GMat (cv::GMat, cv::GMat)> cvtT([](cv::GMat m1, cv::GMat m2)
+    {
+        return m1+m2;
+    });
+    cvtT.apply(in_mat1, in_mat2, out_mat_typed1);
+
+    auto cvtTC =  cvtT.compile(cv::descr_of(in_mat1), cv::descr_of(in_mat2));
+    cvtTC(in_mat1, in_mat2, out_mat_typed2);
+    //! [Typed_Example]
+}
+
 G_TYPED_KERNEL(IAdd, <cv::GMat(cv::GMat)>, "test.custom.add") {
     static cv::GMatDesc outMeta(const cv::GMatDesc &in) { return in; }
 };
@@ -77,5 +115,8 @@ int main(int argc, char *argv[])
         , CustomRGB2YUV
         >();
     //! [kernels_snippet]
+
+    // Just call typed example with no input/output
+    typed_example();
     return 0;
 }

modules/gapi/samples/draw_example.cpp

@@ -0,0 +1,56 @@
+#include <opencv2/imgproc.hpp> // cv::FONT*, cv::LINE*, cv::FILLED
+#include <opencv2/highgui.hpp> // imwrite
+
+#include <opencv2/gapi.hpp>
+#include <opencv2/gapi/render.hpp>
+
+int main(int argc, char *argv[])
+{
+    if (argc < 2) {
+        std::cerr << "Filename requried" << std::endl;
+        return 1;
+    }
+
+    const auto font  = cv::FONT_HERSHEY_DUPLEX;
+    const auto blue  = cv::Scalar{ 255,   0,   0}; // B/G/R
+    const auto green = cv::Scalar{   0, 255,   0};
+    const auto coral = cv::Scalar{0x81,0x81,0xF1};
+    const auto white = cv::Scalar{ 255, 255, 255};
+    cv::Mat test(cv::Size(480, 160), CV_8UC3, white);
+
+    namespace draw = cv::gapi::wip::draw;
+    std::vector<draw::Prim> prims;
+    prims.emplace_back(draw::Circle{   // CIRCLE primitive
+            {400,72},                  // Position (a cv::Point)
+            32,                        // Radius
+            coral,                     // Color
+            cv::FILLED,                // Thickness/fill type
+            cv::LINE_8,                // Line type
+            0                          // Shift
+        });
+    prims.emplace_back(draw::Text{     // TEXT primitive
+            "Hello from G-API!",       // Text
+            {64,96},                   // Position (a cv::Point)
+            font,                      // Font
+            1.0,                       // Scale (size)
+            blue,                      // Color
+            2,                         // Thickness
+            cv::LINE_8,                // Line type
+            false                      // Bottom left origin flag
+        });
+    prims.emplace_back(draw::Rect{     // RECTANGLE primitive
+            {16,48,400,72},            // Geometry (a cv::Rect)
+            green,                     // Color
+            2,                         // Thickness
+            cv::LINE_8,                // Line type
+            0                          // Shift
+        });
+    prims.emplace_back(draw::Mosaic{   // MOSAIC primitive
+            {320,96,128,32},           // Geometry (a cv::Rect)
+            16,                        // Cell size
+            0                          // Decimation
+        });
+    draw::render(test, prims);
+    cv::imwrite(argv[1], test);
+    return 0;
+}