Undefined symbol using emcmake (emscripten and opencv) - opencv

Question

I'm trying to build a minimalist project using OpenCV in the browser.
I've compiled OpenCV on my computer.
Here is the C++ code (josef.cpp):
#include <iostrem>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/opencv.hpp>
int main(int argc, char **argv)
{
printf("Hello here\n");
cv::Mat m;
std::cout << m;
}
Here is the CMakeLists.txt :
cmake_minimum_required(VERSION 2.8)
project( josef )
# Give here the directory in which we have
# the file OpenCVConfig.cmake
set(OpenCV_DIR "/usr/local/lib/cmake/opencv4")
find_package( OpenCV REQUIRED )
add_executable( josef josef.cpp )
target_link_libraries( josef ${OpenCV_LIBS} )
Compiling with
cmake .
make
works perfectly well and provides an executable which produces the expected result:
Hello here
[]
My problem arises when trying with emscripten.
emcmake cmake .
produces lot of warnings like that one:
CMake Warning (dev) at /usr/local/lib/cmake/opencv4/OpenCVModules.cmake:393 (add_library):
ADD_LIBRARY called with SHARED option but the target platform does not
support dynamic linking. Building a STATIC library instead. This may lead
to problems.
Call Stack (most recent call first):
/usr/local/lib/cmake/opencv4/OpenCVConfig.cmake:126 (include)
CMakeLists.txt:7 (find_package)
This warning is for project developers. Use -Wno-dev to suppress it.
Then
emmake make
fails on undefined symbols:
error: undefined symbol: _ZN2cv3Mat10deallocateEv
warning: Link with `-s LLD_REPORT_UNDEFINED` to get more information on undefined symbols
warning: To disable errors for undefined symbols use `-s ERROR_ON_UNDEFINED_SYMBOLS=0`
My question is : what do I wrong ? How to tell emscripten where are the famous missing symbols (these are the file .so if I understood correctly) ?

Answer 1

This is partially a hack solution.
In the file /usr/local/lib/cmake/opencv4/OpenCVModules.cmake, change every instances of "SHARED" by "STATIC".
In your cmake file, add the line
file( GLOB opencv_js "/path/to/opencvjs/opencv/build_js/lib/*.a")
This part supposes that you compiled opencv by hand.
then refer to my answer to my next question. (in short: use a canvas, do not use cv::imdecode.)