Mxnet doesn't install for R if OpenCV is build on linux

[shawnbrar]

I am an avid R user and wanted to build Mxnet for R on Ubuntu 18.04.
I have also built (not installed through apt-get) OpenCV with its contributed modules.
For Mxnet, I had installed Intel's MKL library through their repository and had built Intel MKL-DNN library.
I wanted to build Mxnet for CPU. Below is the cmake command with it's flags that I use to make the cmake files in the build directory.

cmake -GNinja -DUSE_CUDA=OFF -DUSE_MKL_IF_AVAILABLE=ON -DCMAKE_CUDA_COMPILER_LAUNCHER=ccache -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DCMAKE_BUILD_TYPE=Release -DUSE_MKLDNN=1 -DUSE_OPENCV=ON ..

Using this, get the following output:-

---

-- The C compiler identification is GNU 7.4.0
-- The CXX compiler identification is GNU 7.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- CMAKE_CROSSCOMPILING FALSE
-- CMAKE_HOST_SYSTEM_PROCESSOR x86_64
-- CMAKE_SYSTEM_PROCESSOR x86_64
-- CMAKE_SYSTEM_NAME Linux
-- CMake version '3.16.2' using generator 'Ninja'
-- Performing Test SUPPORT_CXX11
-- Performing Test SUPPORT_CXX11 - Success
-- Performing Test SUPPORT_CXX0X
-- Performing Test SUPPORT_CXX0X - Success
-- Performing Test SUPPORT_MSSE3
-- Performing Test SUPPORT_MSSE3 - Success
-- Performing Test SUPPORT_MSSE2
-- Performing Test SUPPORT_MSSE2 - Success
-- Determining F16C support
-- Performing Test COMPILER_SUPPORT_MF16C
-- Performing Test COMPILER_SUPPORT_MF16C - Success
-- F16C enabled
-- MKL-DNN compat: set DNNL_BUILD_EXAMPLES to MKLDNN_BUILD_EXAMPLES with value OFF
-- MKL-DNN compat: set DNNL_BUILD_TESTS to MKLDNN_BUILD_TESTS with value OFF
-- MKL-DNN compat: set DNNL_ENABLE_JIT_PROFILING to MKLDNN_ENABLE_JIT_PROFILING with value OFF
-- MKL-DNN compat: set DNNL_LIBRARY_TYPE to MKLDNN_LIBRARY_TYPE with value `STATIC-- MKL-DNN compat: set DNNL_ARCH_OPT_FLAGS to MKLDNN_ARCH_OPT_FLAGS with value
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE
-- Found OpenMP_C: -fopenmp (found version "4.5")
-- Found OpenMP_CXX: -fopenmp (found version "4.5")
-- Found OpenMP: TRUE (found version "4.5")
-- GPU support is disabled
-- Found Doxygen: /usr/bin/doxygen (found version "1.8.13") found components: doxygen dot
-- Found Git: /usr/bin/git (found version "2.17.1")
-- Intel(R) VTune(TM) Amplifier JIT profiling disabled
-- Found MKL: /opt/intel/mkl/include
-- Found MKL (include: /opt/intel/mkl/include, lib: /opt/intel/mkl/lib/intel64/libmkl_rt.so
-- Found PkgConfig: /usr/bin/pkg-config (found version "0.29.1")
-- Found Jemalloc: /usr/lib/x86_64-linux-gnu/libjemalloc.so
-- Using JEMalloc malloc
-- Found OpenCV: /usr/local (found version "4.2.0") found components: core highgui imgproc imgcodecs
-- OpenCV 4.2.0 found (/usr/local/lib/cmake/opencv4)
-- OpenCV_LIBS=opencv_core;opencv_highgui;opencv_imgproc;opencv_imgcodecs
-- Found OpenMP_C: -fopenmp
-- Found OpenMP_CXX: -fopenmp
-- Found OpenMP: TRUE
-- Performing Test OPENMP_HAVE_WERROR_FLAG
-- Performing Test OPENMP_HAVE_WERROR_FLAG - Success
-- Performing Test OPENMP_HAVE_STD_GNUPP11_FLAG
-- Performing Test OPENMP_HAVE_STD_GNUPP11_FLAG - Success
-- Performing Test OPENMP_HAVE_STD_CPP11_FLAG
-- Performing Test OPENMP_HAVE_STD_CPP11_FLAG - Success
-- Found PythonInterp: /usr/bin/python (found version "2.7.17")
-- Cannot find llvm-lit.
-- Please put llvm-lit in your PATH, set OPENMP_LLVM_LIT_EXECUTABLE to its full path, or point OPENMP_LLVM_TOOLS_DIR to its directory.
CMake Warning at 3rdparty/openmp/cmake/OpenMPTesting.cmake:22 (message):
The check targets will not be available!
Call Stack (most recent call first):
3rdparty/openmp/cmake/OpenMPTesting.cmake:40 (find_standalone_test_dependencies)
3rdparty/openmp/CMakeLists.txt:49 (include)

-- Performing Test LIBOMP_HAVE_FNO_EXCEPTIONS_FLAG
-- Performing Test LIBOMP_HAVE_FNO_EXCEPTIONS_FLAG - Success
-- Performing Test LIBOMP_HAVE_FNO_RTTI_FLAG
-- Performing Test LIBOMP_HAVE_FNO_RTTI_FLAG - Success
-- Performing Test LIBOMP_HAVE_X_CPP_FLAG
-- Performing Test LIBOMP_HAVE_X_CPP_FLAG - Success
-- Performing Test LIBOMP_HAVE_WCAST_QUAL_FLAG
-- Performing Test LIBOMP_HAVE_WCAST_QUAL_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_UNUSED_FUNCTION_FLAG
-- Performing Test LIBOMP_HAVE_WNO_UNUSED_FUNCTION_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_UNUSED_LOCAL_TYPEDEF_FLAG
-- Performing Test LIBOMP_HAVE_WNO_UNUSED_LOCAL_TYPEDEF_FLAG - Failed
-- Performing Test LIBOMP_HAVE_WNO_UNUSED_VALUE_FLAG
-- Performing Test LIBOMP_HAVE_WNO_UNUSED_VALUE_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_UNUSED_VARIABLE_FLAG
-- Performing Test LIBOMP_HAVE_WNO_UNUSED_VARIABLE_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_SWITCH_FLAG
-- Performing Test LIBOMP_HAVE_WNO_SWITCH_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_COVERED_SWITCH_DEFAULT_FLAG
-- Performing Test LIBOMP_HAVE_WNO_COVERED_SWITCH_DEFAULT_FLAG - Failed
-- Performing Test LIBOMP_HAVE_WNO_DEPRECATED_REGISTER_FLAG
-- Performing Test LIBOMP_HAVE_WNO_DEPRECATED_REGISTER_FLAG - Failed
-- Performing Test LIBOMP_HAVE_WNO_SIGN_COMPARE_FLAG
-- Performing Test LIBOMP_HAVE_WNO_SIGN_COMPARE_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_GNU_ANONYMOUS_STRUCT_FLAG
-- Performing Test LIBOMP_HAVE_WNO_GNU_ANONYMOUS_STRUCT_FLAG - Failed
-- Performing Test LIBOMP_HAVE_WNO_UNKNOWN_PRAGMAS_FLAG
-- Performing Test LIBOMP_HAVE_WNO_UNKNOWN_PRAGMAS_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_MISSING_FIELD_INITIALIZERS_FLAG
-- Performing Test LIBOMP_HAVE_WNO_MISSING_FIELD_INITIALIZERS_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_MISSING_BRACES_FLAG
-- Performing Test LIBOMP_HAVE_WNO_MISSING_BRACES_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_COMMENT_FLAG
-- Performing Test LIBOMP_HAVE_WNO_COMMENT_FLAG - Success
-- Performing Test LIBOMP_HAVE_WNO_SELF_ASSIGN_FLAG
-- Performing Test LIBOMP_HAVE_WNO_SELF_ASSIGN_FLAG - Failed
-- Performing Test LIBOMP_HAVE_WNO_VLA_EXTENSION_FLAG
-- Performing Test LIBOMP_HAVE_WNO_VLA_EXTENSION_FLAG - Failed
-- Performing Test LIBOMP_HAVE_WNO_FORMAT_PEDANTIC_FLAG
-- Performing Test LIBOMP_HAVE_WNO_FORMAT_PEDANTIC_FLAG - Failed
-- Performing Test LIBOMP_HAVE_WSTRINGOP_OVERFLOW_FLAG
-- Performing Test LIBOMP_HAVE_WSTRINGOP_OVERFLOW_FLAG - Success
-- Performing Test LIBOMP_HAVE_MSSE2_FLAG
-- Performing Test LIBOMP_HAVE_MSSE2_FLAG - Success
-- Performing Test LIBOMP_HAVE_FTLS_MODEL_FLAG
-- Performing Test LIBOMP_HAVE_FTLS_MODEL_FLAG - Success
-- Performing Test LIBOMP_HAVE_MMIC_FLAG
-- Performing Test LIBOMP_HAVE_MMIC_FLAG - Failed
-- Performing Test LIBOMP_HAVE_M32_FLAG
-- Performing Test LIBOMP_HAVE_M32_FLAG - Failed
-- Performing Test LIBOMP_HAVE_X_FLAG
-- Performing Test LIBOMP_HAVE_X_FLAG - Success
-- Performing Test LIBOMP_HAVE_WARN_SHARED_TEXTREL_FLAG
-- Performing Test LIBOMP_HAVE_WARN_SHARED_TEXTREL_FLAG - Success
-- Performing Test LIBOMP_HAVE_AS_NEEDED_FLAG
-- Performing Test LIBOMP_HAVE_AS_NEEDED_FLAG - Success
-- Performing Test LIBOMP_HAVE_VERSION_SCRIPT_FLAG
-- Performing Test LIBOMP_HAVE_VERSION_SCRIPT_FLAG - Success
-- Performing Test LIBOMP_HAVE_STATIC_LIBGCC_FLAG
-- Performing Test LIBOMP_HAVE_STATIC_LIBGCC_FLAG - Success
-- Performing Test LIBOMP_HAVE_Z_NOEXECSTACK_FLAG
-- Performing Test LIBOMP_HAVE_Z_NOEXECSTACK_FLAG - Success
-- Performing Test LIBOMP_HAVE_FINI_FLAG
-- Performing Test LIBOMP_HAVE_FINI_FLAG - Success
-- Found Perl: /usr/bin/perl (found version "5.26.1")
-- Performing Test LIBOMP_HAVE_VERSION_SYMBOLS
-- Performing Test LIBOMP_HAVE_VERSION_SYMBOLS - Success
-- Performing Test LIBOMP_HAVE___BUILTIN_FRAME_ADDRESS
-- Performing Test LIBOMP_HAVE___BUILTIN_FRAME_ADDRESS - Success
-- Performing Test LIBOMP_HAVE_WEAK_ATTRIBUTE
-- Performing Test LIBOMP_HAVE_WEAK_ATTRIBUTE - Success
-- Looking for include files windows.h, psapi.h
-- Looking for include files windows.h, psapi.h - not found
-- Looking for EnumProcessModules in psapi
-- Looking for EnumProcessModules in psapi - not found
-- LIBOMP: Operating System -- Linux
-- LIBOMP: Target Architecture -- x86_64
-- LIBOMP: Build Type -- Release
-- LIBOMP: Library Kind -- SHARED
-- LIBOMP: Library Type -- normal
-- LIBOMP: Fortran Modules -- FALSE
-- LIBOMP: Build -- 20140926
-- LIBOMP: Use Stats-gathering -- FALSE
-- LIBOMP: Use Debugger-support -- FALSE
-- LIBOMP: Use ITT notify -- TRUE
-- LIBOMP: Use OMPT-support -- TRUE
-- LIBOMP: Use OMPT-optional -- TRUE
-- LIBOMP: Use Adaptive locks -- TRUE
-- LIBOMP: Use quad precision -- TRUE
-- LIBOMP: Use TSAN-support -- FALSE
-- LIBOMP: Use Hwloc library -- FALSE
-- Looking for sqrt in m
-- Looking for sqrt in m - found
-- Looking for __atomic_load_1
-- Looking for __atomic_load_1 - not found
-- Looking for __atomic_load_1 in atomic
-- Looking for __atomic_load_1 in atomic - found
-- check-libomp does nothing.
-- check-ompt does nothing.
-- check-openmp does nothing.
USE_LAPACK is ON
CMake Warning at 3rdparty/googletest/googletest/CMakeLists.txt:47 (project):
VERSION keyword not followed by a value or was followed by a value that
expanded to nothing.

-- Found GTest: gtest
-- Looking for clock_gettime in rt
-- Looking for clock_gettime in rt - found
-- Looking for fopen64
-- Looking for fopen64 - not found
-- Looking for C++ include cxxabi.h
-- Looking for C++ include cxxabi.h - found
-- Looking for nanosleep
-- Looking for nanosleep - found
-- Looking for backtrace
-- Looking for backtrace - found
-- backtrace facility detected in default set of libraries
-- Found Backtrace: /usr/include
-- Check if the system is big endian
-- Searching 16 bit integer
-- Looking for sys/types.h
-- Looking for sys/types.h - found
-- Looking for stdint.h
-- Looking for stdint.h - found
-- Looking for stddef.h
-- Looking for stddef.h - found
-- Check size of unsigned short
-- Check size of unsigned short - done
-- Using unsigned short
-- Check if the system is big endian - little endian
-- /home/shawnbrar/Documents/Books/mxnet/3rdparty/dmlc-core/cmake/build_config.h.in -> include/dmlc/build_config.h
-- Found OpenMP_C: -fopenmp
-- Found OpenMP_CXX: -fopenmp
-- Configuring done
-- Generating done
-- Build files have been written to: /home/shawnbrar/Documents/Books/mxnet/build

---

So it finds OpenCV and gives the following output:
-- Found OpenCV: /usr/local (found version "4.2.0") found components: core highgui imgproc imgcodecs -- OpenCV 4.2.0 found (/usr/local/lib/cmake/opencv4) -- OpenCV_LIBS=opencv_core;opencv_highgui;opencv_imgproc;opencv_imgcodecs

After this I build the Mxnet library using the following command:
ninja -v
This properly compiles my Mxnet library and also creates a libmxnet.so in the build directory.

After this I make the R-package with the following command
make -f R-package/Makefile rpkg

This gives me the following error :
im2rec.cc:39:10: fatal error: opencv2/opencv.hpp: No such file or directory #include <opencv2/opencv.hpp>

However, after reading this, I installed libopencv-dev using the following command
sudo apt-get install libopencv-dev
This installed some binaries. After this is installed, I again make the R-package with the same command that I used earlier. But this makes the R package properly.

Why does this happen? Why isn't mxnet using the OpenCV which I had earlier built earlier?

Thanks

[leezu]

You likely didn't set up your CPATH, LIBRARY_PATH and LD_LIBRARY_PATH correctly. Could you post the value of each?

Ie run echo CPATH etc

[shawnbrar]

I actually found the problem. When I type in make -f R-package/Makefile rpkg, the following output appears :
make -f R-package/Makefile rpkg

mkdir -p R-package/inst/libs
cp src/io/image_recordio.h R-package/src
if [ -d "lib" ]; then \
	cp -rf lib/libmxnet.so R-package/inst/libs; \
	if [ -e "lib/libtvm_runtime.so" ]; then \
		cp -rf lib/libtvm_runtime.so R-package/inst/libs; \
	fi; \
else \
	cp -rf build/libmxnet.so R-package/inst/libs; \
	if [ -e "build/libtvm_runtime.so" ]; then \
		cp -rf build/libtvm_runtime.so R-package/inst/libs; \
	fi; \
fi
mkdir -p R-package/inst/include
cp -rl include/* R-package/inst/include
Rscript -e "if(!require(devtools)){install.packages('devtools', repo = 'https://cloud.r-project.org/')}"
Loading required package: devtools
Loading required package: usethis
Rscript -e "if(!require(roxygen2)||packageVersion('roxygen2') < '6.1.1'){install.packages('roxygen2', repo = 'https://cloud.r-project.org/')}"
Loading required package: roxygen2
Rscript -e "library(devtools); library(methods); options(repos=c(CRAN='https://cloud.r-project.org/')); install_deps(pkg='R-package', dependencies = TRUE)"
Loading required package: usethis

cp R-package/dummy.NAMESPACE R-package/NAMESPACE  # NAMESPACE will be replaced by devtools::document later
echo "import(Rcpp)" >> R-package/NAMESPACE
R CMD INSTALL R-package
* installing to library ‘/home/shawnbrar/R/x86_64-pc-linux-gnu-library/3.6’
* installing *source* package ‘mxnet’ ...
** using staged installation
** libs
make[1]: Entering directory '/home/shawnbrar/Documents/Libs/mxnet/R-package/src'
g++ -std=gnu++11 -I"/usr/share/R/include" -DNDEBUG -I../inst/include `pkg-config --cflags opencv` `Rscript -e 'Rcpp:::CxxFlags()'` -I"/home/shawnbrar/R/x86_64-pc-linux-gnu-library/3.6/Rcpp/include"   -fpic  -g -O2 -fdebug-prefix-map=/build/r-base-t3diwe/r-base-3.6.2=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g  -c im2rec.cc -o im2rec.o
Package opencv was not found in the pkg-config search path.
Perhaps you should add the directory containing `opencv.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opencv' found
im2rec.cc:39:10: fatal error: opencv2/opencv.hpp: No such file or directory
 #include <opencv2/opencv.hpp>
          ^~~~~~~~~~~~~~~~~~~~
compilation terminated.
/usr/lib/R/etc/Makeconf:175: recipe for target 'im2rec.o' failed
make[1]: *** [im2rec.o] Error 1
make[1]: Leaving directory '/home/shawnbrar/Documents/Libs/mxnet/R-package/src'
ERROR: compilation failed for package ‘mxnet’
* removing ‘/home/shawnbrar/R/x86_64-pc-linux-gnu-library/3.6/mxnet’
* restoring previous ‘/home/shawnbrar/R/x86_64-pc-linux-gnu-library/3.6/mxnet’
R-package/Makefile:5: recipe for target 'rpkg' failed
make: *** [rpkg] Error 1

In this the following is important to see :
g++ -std=gnu++11 -I"/usr/share/R/include" -DNDEBUG -I../inst/include 'pkg-config --cflags opencv'

It says pkg-config --cflags opencv
Whereas it should be pkg-config --cflags opencv4.

What can I do to make this change? Should I rename my opencv4.pc file to opencv.pc?

[shawnbrar]

I renamed all of my opencv4.pc files to opencv.pc. This solution worked for me.

Providing with a screenshot

[leezu]

It says pkg-config --cflags opencv
Whereas it should be pkg-config --cflags opencv4

@shawnbrar Do you have any reference for this statement? If so, we need to update

https://github.com/apache/incubator-mxnet/blob/b295d13491895a5ac9bd4ca186e9260da10774ef/R-package/src/Makevars#L2-L4

accordingly. However, I tested the changes with pkg-config --cflags opencv and it works fine on my Ubuntu 18.04.. There is likely something wrong in the way you installed OpenCV.

Let's reopen this issue until your reference above is clarified.

[leezu]

@shawnbrar how did you generate opencv4.pc? I tried compiling OpenCV 4.2 from source but didn't get a opencv4.pc. How did you compile OpenCV to obtain it?

In any case, this issue can be fixed by the following patch:

modified   R-package/src/Makevars
@@ -1,4 +1,4 @@
 CXX_STD = CXX11
-PKG_LIBS = $(LAPACK_LIBS) $(BLAS_LIBS) `pkg-config --libs opencv`
-PKG_CFLAGS = `pkg-config --cflags opencv`
-PKG_CPPFLAGS = -I../inst/include `pkg-config --cflags opencv` `Rscript -e 'Rcpp:::CxxFlags()'`
+PKG_LIBS = $(LAPACK_LIBS) $(BLAS_LIBS) `(pkg-config --cflags opencv || pkg-config --cflags opencv4)`
+PKG_CFLAGS = `(pkg-config --cflags opencv || pkg-config --cflags opencv4)`
+PKG_CPPFLAGS = -I../inst/include `(pkg-config --cflags opencv || pkg-config --cflags opencv4)` `Rscript -e 'Rcpp:::CxxFlags()'`

[shawnbrar]

It says pkg-config --cflags opencv
Whereas it should be pkg-config --cflags opencv4

@shawnbrar Do you have any reference for this statement? If so, we need to update

https://github.com/apache/incubator-mxnet/blob/b295d13491895a5ac9bd4ca186e9260da10774ef/R-package/src/Makevars#L2-L4

accordingly. However, I tested the changes with pkg-config --cflags opencv and it works fine on my Ubuntu 18.04.. There is likely something wrong in the way you installed OpenCV.

Let's reopen this issue until your reference above is clarified.

So earlier when you had pointed out that I should run some echo commands in my terminal, I did not get any output in my terminal. So I went ahead and searched for opencv in my system. I found out that I had three different versions on opencv on my computer. Two of which were OpenCV 4.2.0-dev (the one which I had compiled) and OpenCV 3.2 (which I had installed from apt-get). The third version was OpenCV 4.1.1 which I think I might have compiled earlier. Since I am not so great at properly uninstalling softwares in Ubuntu, I formatted my hard-drive and did a fresh install of Ubuntu. Then, I built OpenCV 4.2.0-dev using the following commands :

---

sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install libavcodec-dev libavformat-dev libswscale-dev libgstreamer-plugins-base1.0-dev libgstreamer1.0-dev
sudo apt-get install libgtk-3-dev
sudo apt purge libx264-dev x264
sudo apt-get install build-essential cmake pkg-config libjpeg8-dev libtiff5-dev libpng12-0
sudo apt install v4l-utils libv4l-dev libgphoto2-dev libgstreamer1.0-0 libgstreamer1.0-dev x264
cd /usr/include/linux/
sudo ln -s -f ../libv4l1-videodev.h videodev.h
cd ~/opencv
cd ./opencv && mkdir build && cd build
sudo apt install libavcodec-dev libavformat-dev libswscale-dev libxvidcore-dev
sudo apt install libgtk-3-dev libatlas-base-dev gfortran python3.6-dev python2.7-dev libtiff-dev

cmake -D CMAKE_BUILD_TYPE=RELEASE -D WITH_V4L=ON -D CMAKE_INSTALL_PREFIX=/usr/local -D INSTALL_PYTHON_EXAMPLES=ON -D INSTALL_C_EXAMPLES=OFF -D PYTHON_EXECUTABLE=/usr/bin/python3.6 -D BUILD_EXAMPLES=ON -D OPENCV_EXTRA_MODULES_PATH=../../opencv_contrib/modules -D OPENCV_GENERATE_PKGCONFIG=ON ..
make -j4
make
sudo make install
sudo ldconfig

---

This time I had added an extra flag in the cmake command : -D OPENCV_GENERATE_PKGCONFIG=ON. The official documentation of OpenCV does say that if one wants to generate pkg-config info, one should add this flag when running CMake. This is quoted from the step number 6 in their official installation instructions at the following link :
OpenCV-4.2.0-dev_Linux_Installation

The official documentation also says "Installed as opencv4, usage: pkg-config --cflags --libs opencv4"

After compilation I ran the command pkg-config --cflags --libs opencv4, the OpenCV 4 flags and libraries were returning their proper output.

Next I installed R and Rstudio.
Next I installed Intel's MKL and build MKL-DNN.
After that I build MXNet with the following cmake options:
cmake -GNinja -DUSE_CUDA=OFF -DUSE_MKL_IF_AVAILABLE=ON -DCMAKE_CUDA_COMPILER_LAUNCHER=ccache -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DCMAKE_BUILD_TYPE=Release -DUSE_MKLDNN=1 -DUSE_OPENCV=ON ..
ninja -v

After this just to check if the library was built properly, I installed it for Python-3. In python3, I ran import mxnet, and it was working. All the modules and functions were appearing.

So then I ran the following command to make the R-Package :
make -f R-package/Makefile rpkg

It ran and the problem was still there. It still gave the error :
Package opencv was not found in the pkg-config search path. Perhaps you should add the directory containing 'opencv.pc' to the PKG_CONFIG_PATH environment variable No package 'opencv' found

So I ran the following command to see my PKG_CONFIG_PATH:
pkg-config --variable pc_path pkg-config
The output was as following:
/usr/local/lib/x86_64-linux-gnu/pkgconfig:/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig:/usr/lib/x86_64-linux-gnu/pkgconfig:/usr/lib/pkgconfig:/usr/share/pkgconfig

Since there were six paths and I could only find one opencv4.pc file, I copied it to all the other paths. Then, I ran the command to make the R-Package, but the error still persisted. But then I also looked at the error this time and saw that while building the R-Package, it said pkg-config --cflags opencv. But according to the OpenCV's documentation and my own computer's configuration it should've been pkg-config --cflags opencv4.

This is when, I asked my second question if I should rename my opencv4.pc file to opencv.pc
#17359 (comment)

After a few minutes, I decided to renamed all the opencv4.pc files to opencv.pc but I did not change any of the file's contents. And I was able to build my R-Package successfully.
Then I posted that screenshot showing the functions in the mxnet library.(#17359 (comment))

I don't know why your opencv's build makes the file opencv.pc whereas mine builds opencv4.pc. And I even checked the doumentation for OpenCV 4.2.0. Even this one quotes "Installed as opencv4, usage: pkg-config --cflags --libs opencv4". OpenCV-4.2.0_Linux_Installation

Maybe you also have a version of opencv which you might have installed through apt-get. Please do check this.

[leezu]

Thanks @shawnbrar for the elaboration. The issue will be fixed by #17404 shortly.

[shawnbrar]

You're welcome. I've been dying to try this library in R since mid 2019. But wasn't able to build it properly earlier.

[leezu]

@shawnbrar I fixed some issues with the R build recently, so at least the build should work well now. Please report any further issues you fine here on github

[shawnbrar]

@shawnbrar I fixed some issues with the R build recently, so at least the build should work well now. Please report any further issues you fine here on github

@leezu, were you able to find out whether the build on your Ubuntu was using OpenCV 4 or OpenCV 3?

[leezu]

It was with OpenCV 3. The fix in #17404 will apply to Ubuntu 20.04 and current Debian Testing, as well as when manually compiling OpenCV 4 from source and installing it correctly.