cv::imread and other does not working with emscripten? - c++

Question

I have following C++ code with opencv. It is basic code for display image.
I'm trying to compile it using emscripten but it seems that emscripten can't load some function of opencv. For example the cv::Mat is ok.
Test for build_js went all right.
Do you know what I'm doing wrong?
Code:
#include <stdio.h>
#include <opencv2/opencv.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <emscripten/emscripten.h>
int main(int argc, char** argv )
{
if ( argc != 2 )
{
printf("usage: DisplayImage.out <Image_Path>\n");
return -1;
}
cv::Mat image;
image = cv::imread( argv[1], 1 );
if ( !image.data )
{
printf("No image data \n");
return -1;
}
cv::namedWindow("Display Image");
cv::imshow("Display Image", image);
cv::waitKey(0);
return 0;
}
Error message:
/home/vasek/project_test/opencv/opencvgcccmake_emscript/DisplayImage.cpp:15:13: error: use of undeclared identifier 'imread'
image = cv::imread( argv[1], 1 );
^
/home/vasek/project_test/opencv/opencvgcccmake_emscript/DisplayImage.cpp:21:9: error: no member named 'namedWindow' in namespace 'cv'
cv::namedWindow("Display Image");
~~~~^
/home/vasek/project_test/opencv/opencvgcccmake_emscript/DisplayImage.cpp:22:9: error: no member named 'imshow' in namespace 'cv'
cv::imshow("Display Image", image);
~~~~^
/home/vasek/project_test/opencv/opencvgcccmake_emscript/DisplayImage.cpp:23:9: error: no member named 'waitKey' in namespace 'cv'
cv::waitKey(0);
~~~~^
4 errors generated.
cmake command:
emcmake cmake -DOpenCV_DIR=/home/vasek/tools/opencv/build_js ..
Edit:
None of the function work exept cv::Mat.

Answer 1

This is not a problem with includes, since opencv2/opencv.hpp automagically pulls in all the module top-level headers if the modules are available. The obvious conclusion is that those modules are not available in your build of OpenCV, and now the question is why is that the case?
As it turns out, some of the OpenCV functionality is not suitable for Web, and opencv.js is built without it.
The functions cv::namedWindow, cv::imshow and cv::waitKey belong to the highgui module. The implementation of this module is platform-dependent (at build time you have to select the backend that will be used, be it WinAPI, WinRT, Gtk, Qt, cocoa, etc.). Furthermore, since (AFAIK) the JavaScript code in the browser runs in a sandbox, it would be impossible to access to APIs anyway.
The function cv::imread belongs to the imgcodecs module, yet another one that is not built in this case. I don't know the exact reason for this (perhaps due to some of it directly accessing filesystem), and the issue https://github.com/opencv/opencv/issues/17535 indicates that the third party codecs haven't been integrated into opencv.js and some mechanisms are missing from the binding generator.