Summary

This PR brings cv::gapi::streaming::sync_policy enum which allows to control input streaming sources synchronization when passed as compile arg. Currently possible values are dont_sync and drop. If dont_sync or nothing is passed, framework will behave as previously. If drop is passed, framework will run the whole graph only on inputs which timestamps are equal, dropping all inappropriate frames. Need to note that const (non-video) sources are not taken into account when comparing timestamps since their output is always the same.

Changes overview

For each input our streaming executor creates an Emitter which is responsible for pulling data from the source. There is 1:1 correspondence between emitters and GComputation inputs. For const sources ConstEmitter-s are created, for video sources - VideoEmitter-s. For each emitter framework creates a thread which simply pulls data from the according source and pushes this data further to all its readers (islands). There are queues placed between emitters and their readers to maintain pipelining, so emitter simply pushes to according queues while islands are popping from these queues.

To add input timestamp synchronization to this picture a new Synchronizer entity was introduced. If synchronization is required, it adds additional layer of queues between emitters which need to be synchronized and their readers. A synchronization thread is created to pull data from emitter queues and compare their timestamps, dropping some data till it gets all input data from all emitters having the same timestamp value. After that this synchronized data is pushed further to the islands and the actual calculations are launched.

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

Build configuration

Xforce_builders_only=linux,docs
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-2021.1.0:20.04
build_image:Custom Win=openvino-2021.1.0
build_image:Custom Mac=openvino-2021.2.0

test_modules:Custom=gapi,python2,python3,java
test_modules:Custom Win=gapi,python2,python3,java
test_modules:Custom Mac=gapi,python2,python3,java

buildworker:Custom=linux-1
# disabled due high memory usage: test_opencl:Custom=ON
test_opencl:Custom=OFF
test_bigdata:Custom=1
test_filter:Custom=*