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Copyright (C) 2022, Axis Communications AB, Lund, Sweden. All Rights Reserved.

A guide to building and running OpenCV in an ACAP application

This guide explains how to build OpenCV from source and bundle it for use in an ACAP application. The example application runs a background subtraction operation on video from the camera for a very simple motion detection application to demonstrate how to integrate the broad functionality of OpenCV.

File structure

building-opencv
├── app
│   ├── example.cpp - The application running OpenCV code
│   ├── imgprovider.cpp - Convenience functions for VDO
│   ├── imgprovider.h - imgprovider headers
│   ├── LICENSE
│   ├── Makefile - The Makefile specifying how the ACAP should be built
│   └── manifest.json - A file specifying execution-related options for the ACAP
├── Dockerfile - Specification of the container used to build the ACAP
├── README.md
└── sources.list - Text file specifying repositories for armhf packages

Instructions

Quick start

Note

Depending on the network you are connected to, you may need to add proxy settings. The file that needs these settings is: ~/.docker/config.json. For reference please see https://docs.docker.com/network/proxy and a script for Axis devices in the ACAP documentation.

  1. Standing in your working directory run the following commands:

    On armv7hf architecture

    docker build --tag <APP_IMAGE> .

    On aarch64 architecture

    docker build --tag <APP_IMAGE> --build-arg ARCH=aarch64 .

    <APP_IMAGE> is the name to tag the image with, e.g., opencv-app:1.0

    Copy the result from the container image to a local directory build:

    docker cp $(docker create <APP_IMAGE>):/opt/app ./build

  2. You should now have a build directory. In it is the .eap file that is your application. Upload the application to your camera.

  3. Start the application. In the App log, a printout from the application should be seen. The same log with continuous scroll can be seen by SSHing to the camera and running journalctl -f. The printout shows whether the application has detected movement in the image or not:

    opencv_app[0]: starting opencv_app
opencv_app[2211]: Running OpenCV example with VDO as video source
opencv_app[2211]: Creating VDO image provider and creating stream 1024 x 576
opencv_app[2211]: Dump of vdo stream settings map =====
opencv_app[2211]: chooseStreamResolution: We select stream w/h=1024 x 576 based on VDO channel info.
opencv_app[2211]: Start fetching video frames from VDO
opencv_app[2211]: Motion detected: YES
opencv_app[2211]: Motion detected: YES

Walk-through of application

The Dockerfile suits as a good overview of the build process;

  1. First the OpenCV libraries are built with the help of CMake and the ACAP SDK libraries, especially libc and libstdc++.
  2. The OpenCV libraries are then copied to the application directory under lib.
  3. Finally the ACAP application is built with the build instructions in the Makefile where it links to and bundles the OpenCV libraries to the application.

Building OpenCV libraries

OpenCV libraries are built with CMake and some special steps are made to get a correct build:

  • The ACAP SDK is sourced to get cross compilation variables like CC and CXX which contains the path to the SDK libraries.
  • To get correct cross compilation settings, CMAKE_TOOLCHAIN_FILE is set to the architecture specific file provided by OpenCV.
  • CMake picks up environment variables like CC and CXX but seems to not work in combination with CMAKE_TOOLCHAIN_FILE. To get CMake to pick up the cross compiler and SDK library path, these variables are split and set explicitly in CMAKE_{C,CXX}_COMPILER and CMAKE_{C,CXX}_FLAGS respectively.

Other noteworthy options are the ones related to NEON and VFPV3, which are optimizations available for the platform that can greatly speed up CPU operations. This is only necessary for armv7hf, in aarch64 these options are implicitly present.

The other configuration options are there to make the OpenCV installation quite stripped of functionality not needed for the example application. However, you will likely have to change these options to accommodate your custom application.

Building ACAP application

The build instructions of the ACAP application are found in the Makefile. Note the use of the option -Wl,--no-as-needed,-rpath,'$$ORIGIN/lib' which will set the runtime share library search path and is where the bundled libraries will be found when the application is installed on a device.

ACAP application using OpenCV

The example application source code using OpenCV can be seen in example.cpp. It is a C++ application which uses the OpenCV MOG2 background subtraction and noise filtering to detect changes in the image in order to perform motion detection.

The code is documented to give a clear understanding of what steps are needed to grab frames from the camera and perform operations on them.

The output of the application can be seen through the App log or by running journalctl -f while connected through SSH to the device.

License

Apache License 2.0

References