What is the best way to build C++ code that uses the OpenCV library using Bazel? I.e., what would the BUILD rules look like?
Bazel.io has docs for external dependencies but it's not very clear.
There are a couple of options. The easiest way is probably to install locally in the way the OpenCV site recommends:
git clone https://github.com/Itseez/opencv.git
cd opencv/
mkdir build install
cd build
cmake -D CMAKE_BUILD_TYPE=Release -D CMAKE_INSTALL_PREFIX=/path/to/opencv/install ..
make install
Then add the following to your WORKSPACE file:
new_local_repository(
name = "opencv",
path = "/path/to/opencv/install",
build_file = "opencv.BUILD",
)
Create opencv.BUILD
in the same directory as WORKSPACE with the following:
cc_library(
name = "opencv",
srcs = glob(["lib/*.so*"]),
hdrs = glob(["include/**/*.hpp"]),
includes = ["include"],
visibility = ["//visibility:public"],
linkstatic = 1,
)
Then your code can depend on @opencv//:opencv
to link in the .so's under lib/ and reference the headers under include/.
However, this isn't very portable. If you want a portable solution (and you're feeling ambitious), you could add the OpenCV git repo to your workspace and download & build it. Something like:
# WORKSPACE
new_git_repository(
name = "opencv",
remote = "https://github.com/Itseez/opencv.git",
build_file = "opencv.BUILD",
tag = "3.1.0",
)
And make opencv.BUILD something like:
cc_library(
name = "core",
visibility = ["//visibility:public"],
srcs = glob(["modules/core/src/**/*.cpp"]),
hdrs = glob([
"modules/core/src/**/*.hpp",
"modules/core/include/**/*.hpp"]
) + [":module-includes"],
)
genrule(
name = "module-includes",
cmd = "echo '#define HAVE_OPENCV_CORE' > $@",
outs = ["opencv2/opencv_modules.hpp"],
)
...
Then your code could depend on more specific targets, e.g., @opencv//:core
.
As a third option, you declare both cmake and OpenCV in your WORKSPACE file and use a genrule to run cmake on OpenCV from within Bazel.