Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

Overview

Changes

• Added struct GTypeInfo which stores GShape & GKind.
• Implemented function collectInfo which collect GTypeInfo from input/output node handles in the ade::Graph, such kind of information are needed to:
  • Properly allocate outputs ( Python GComputation.apply() doesn't take output args, so need to allocate their inside)
  • Properly extract inputs ( Need to know how to represent values coming from python. Example: If we have tuple contains 4 values and can be represented as cv::Scalar or cv::Rect)
• Added CV_POINT2F to GKind to handle GArray<cv::Point2f>
• Wrapped GGoodFeatures operation
• Wrapped GArray<cv::Point2f> as GArrayP2f into python (GArray is supported only for output)

Implementation details

GArray is a template class, so it can't be wrapped to python as is. The solution is wrap all instances corresponding in GKind. Now only instance for GArray<cv::Point2f> (cv::GShape::CV_POINT2F) is wrapped. For this, need to create using on GArraycv::Point2f`:

using GArrayP2f = cv::GArray<cv::Point2f>

and put this:

class GAPI_EXPORTS_W_SIMPLE GArrayP2f { };

to shadow_gapi.hpp to force python generate code for this instance.
Such principle will be used for wrapping other GArray & GOpaque types.

Kind cv::GKind::CV_UNKNOWN isn't planned to be supported, yet

Build configuration

force_builders=Custom,Custom Win,Custom Mac
build_gapi_standalone:Linux x64=ade-0.1.1f
build_gapi_standalone:Win64=ade-0.1.1f
build_gapi_standalone:Mac=ade-0.1.1f
build_gapi_standalone:Linux x64 Debug=ade-0.1.1f
Xbuild_image:Custom=centos:7
Xbuildworker:Custom=linux-1
build_gapi_standalone:Custom=ade-0.1.1f
build_image:Custom=ubuntu-openvino-2020.3.0:16.04
build_image:Custom Win=openvino-2020.3.0
build_image:Custom Mac=openvino-2020.3.0
test_modules:Custom=gapi
test_modules:Custom Win=gapi
test_modules:Custom Mac=gapi
buildworker:Custom=linux-1
# disabled due high memory usage: test_opencl:Custom=ON
test_opencl:Custom=OFF
test_bigdata:Custom=1
test_filter:Custom=*