Build OpenCV projects using Bazel on Linux

OpenCV Packages

Many Linux distributions have OpenCV packages. Debian-based ones:

OS	OpenCV
Debian 9 (stretch)	2.4
Debian 10 (buster)	3.2
Debian 11 (bullseye)	4.5
Ubuntu 16.04 LTS	2.4
Ubuntu 18.04 LTS	3.2
Ubuntu 20.04 LTS	4.2
Ubuntu 21.04	4.5

Install OpenCV:

sudo apt-get install -y \
  libopencv-core-dev \
  libopencv-highgui-dev \
  libopencv-calib3d-dev \
  libopencv-features2d-dev \
  libopencv-imgproc-dev \
  libopencv-video-dev

Check where OpenCV header files are located:

dpkg -L libopencv-core-dev | grep 'include/'

Checek where OpenCV shared libraries are located:

dpkg -L libopencv-core-dev | grep 'lib/'

Get the version of currently installed OpenCV:

apt-cache show libopencv-core-dev

Inspect a list of default GCC include directories:

echo | gcc -xc++ -E -v -

Bazel uses the same list of default include directories:

cat $(bazel info output_base)/external/local_config_cc/builtin_include_directory_paths

Inspect a list of default GCC library directories:

gcc -print-search-dirs | grep 'libraries:'

OpenCV 2/3 Packages

Header files are installed to /usr/include/opencv2/. /usr/include is a default include directory (-I flag is not needed).

Library files are installed to /usr/lib/aarch64-linux-gnu/. This is a default library directory (-L flag is not needed). Link specific OpenCV libraries by adding -l flags:

-lopencv_core
-lopencv_calib3d
-lopencv_features2d
-lopencv_highgui
-lopencv_imgcodecs
-lopencv_imgproc
-lopencv_video
-lopencv_videoio

There are both static .a and shared .so versions of each library. Shared library is chosen by default. Usage of only shared libraries can be more explicit:

-l:libopencv_core.so
-l:libopencv_calib3d.so
-l:libopencv_features2d.so
-l:libopencv_highgui.so
-l:libopencv_imgcodecs.so
-l:libopencv_imgproc.so
-l:libopencv_video.so
-l:libopencv_videoio.so

Pay attention that sometimes (e.g. manual compilation) OpenCV libraries can go to /usr/local/lib. This is not a default library directory for the compiler! You have to pass -L/usr/local/lib explicitly in this case.

OpneCV 4 Packages

Header files are installed to /usr/include/opencv4/opencv2 and /usr/include/<multiarch>/opencv4/opencv2. Pass additional include directories to the compiler:

-I/usr/include/opencv4/
-I/usr/include/$(shell gcc -print-multiarch)/opencv4/

Library files are installed at the same location as for OpenCV 2/3.

Bazel Compilation

Pass any compiler flag to Bazel using --copt=<option> and linker flag by using--linkopt=<option>.

To compile OpenCV 2/3 project:

bazel build \
  --linkopt=-l:libopencv_core.so \
  --linkopt=-l:libopencv_calib3d.so \
  --linkopt=-l:libopencv_features2d.so \
  --linkopt=-l:libopencv_highgui.so \
  --linkopt=-l:libopencv_imgcodecs.so \
  --linkopt=-l:libopencv_imgproc.so \
  --linkopt=-l:libopencv_video.so \
  --linkopt=-l:libopencv_videoio.so \
  <bazel-target>

To compile OpenCV 4 project:

bazel build \
  --copt=-I/usr/include/opencv4/ \
  --copt=-I/usr/include/$(shell gcc -print-multiarch)/opencv4/ \
  --linkopt=-l:libopencv_core.so \
  --linkopt=-l:libopencv_calib3d.so \
  --linkopt=-l:libopencv_features2d.so \
  --linkopt=-l:libopencv_highgui.so \
  --linkopt=-l:libopencv_imgcodecs.so \
  --linkopt=-l:libopencv_imgproc.so \
  --linkopt=-l:libopencv_video.so \
  --linkopt=-l:libopencv_videoio.so \
  <bazel-target>

Sometimes it is preferable to avoid passing special command flags to Bazel and use dependency mechanism instead. Passing linker flags is easy:

cc_library(
  name = "opencv",
  linkopts = [
    "-l:libopencv_core.so",
    "-l:libopencv_calib3d.so",
    "-l:libopencv_features2d.so",
    "-l:libopencv_highgui.so",
    "-l:libopencv_imgcodecs.so",
    "-l:libopencv_imgproc.so",
    "-l:libopencv_video.so",
    "-l:libopencv_videoio.so",
  ],
)

Include directories are more complicated for OpenCV 4 (OpenCV 2/3 needs nothing). You have to define new_local_repository in the WORKSPACE file and point it to the /usr/include:

new_local_repository(
    name = "opencv_linux",
    path = "/usr/include",
    build_file_content = """
cc_library(
  name = "opencv",
  hdrs = glob([
      "opencv4/opencv2/**/*.h*",
      "x86_64-linux-gnu/opencv4/opencv2/cvconfig.h",
  ])
  includes = [
      "opencv4",
      "x86_64-linux-gnu/opencv4"
  ],
  linkopts = [
    "-l:libopencv_core.so",
    "-l:libopencv_calib3d.so",
    "-l:libopencv_features2d.so",
    "-l:libopencv_highgui.so",
    "-l:libopencv_imgcodecs.so",
    "-l:libopencv_imgproc.so",
    "-l:libopencv_video.so",
    "-l:libopencv_videoio.so",
  ],
)
"""
)

The tricky part here is a multiarch x86_64-linux-gnu value which can be different for cross-compilation.

Bazel Cross-Compilation

Cross-compilation is done using crosstool project. Corresponding dependency should be added to the WORKSPACE file.

Pass --crosstool_top, --compiler, and --cpu to cross-compile:

# Cross-compile to arm64
bazel build --crosstool_top=@crosstool//:toolchains \
            --compiler=gcc \
            --cpu=aarch64 \
            <bazel-target>

# Cross-compile to armhf
bazel build --crosstool_top=@crosstool//:toolchains \
            --compiler=gcc \
            --cpu=armv7a \
            <bazel-target>

Mapping between arch/multiarch and Bazel:

arch	multiarch	Bazel --cpu	Bazel --cpu + crosstool
amd64	x86_64-linux-gnu	k8	k8
arm64	aarch64-linux-gnu	aarch64	aarch64
armhf	arm-linux-gnueabihf	N/A	armv7a

Get arch value:

dpkg --print-architecture

Get multiarch value:

dpkg-architecture -qDEB_HOST_MULTIARCH
gcc -print-multiarch

Docker Environment

Get docker container shell with OpenCV 3 installed:

DOCKER_IMAGE=debian:buster ./docker.sh

Get docker container shell with OpenCV 4 installed:

DOCKER_IMAGE=debian:bullseye ./docker.sh

Get docker container shell for cross-compiling OpenCV 4 to armhf (e.g. Raspberry Pi):

DOCKER_IMAGE=debian:bullseye DOCKER_FILE=docker/Dockerfile.armhf ./docker.sh
container$ cd opencv_cross
container$ make build-armhf

Get docker container shell for cross-compiling OpenCV 4 to arm64 (e.g. Coral Dev Board):

DOCKER_IMAGE=debian:bullseye DOCKER_FILE=docker/Dockerfile.arm64 ./docker.sh
container$ cd opencv_cross
container$ make build-arm64

Run commands directly in the container by passing arguments to docker.sh:

DOCKER_IMAGE=debian:bullseye ./docker.sh dpkg -L libopencv-core-dev