VSCode is unable to compile OpenCV simple example even after adding the path to configuration

[wbadry]

Bug type: Language Service

Describe the bug

• OS and Version: 20.04.2 LTS
• VS Code Version: 1.56.2
• C/C++ Extension Version: 1..4.0
• Other extensions you installed (and if the issue persists after disabling them): ROS 0.6.8

I tried to use VSCode to compile a simple check for OpenCV. I already have the latest version of OpenCV

$ apt show libopencv-dev
Package: libopencv-dev
Version: 4.2.0+dfsg-5
Priority: optional
Section: universe/libdevel
Source: opencv
Origin: Ubuntu 

The library is installed as expected in /usr/include/opencv4

I made a simple C++ program

#include <opencv2/core.hpp>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <iostream>

int main()
{
    cv::Mat img = cv::imread("../images/starry_night.jpg", cv::IMREAD_COLOR);

    if(img.empty())
    {
        std::cout << "Could not read the image ! " << std::endl;
        return 1;
    }

    imshow("Display window", img);
    int k = cv::waitKey(0); // Wait for a keystroke in the window
    
    if(k == 's')
    {
        cv::imwrite("starry_night.png", img);
    }
    //! [imsave]


    return 0;
}

The extension was able to detect the library location, and it was added to c_cpp_properties.json :

{
    "configurations": [
        {
            "name": "Linux",
            "includePath": [
                "${workspaceFolder}/**",
                "/usr/include/opencv4"
            ],
            "defines": [],
            "compilerPath": "/usr/bin/gcc",
            "cStandard": "gnu17",
            "cppStandard": "gnu++14",
            "intelliSenseMode": "linux-gcc-x64"
        }
    ],
    "version": 4

I then configured a building task.json (tried cpp, g++, same result)

{
	"version": "2.0.0",
	"tasks": [
		{
			"type": "cppbuild",
			"label": "C/C++: g++ build active file",
			"command": "/usr/bin/g++",
			"args": [
				"-g",
				"${file}",
				"-o",
				"${fileDirname}/${fileBasenameNoExtension}"
			],
			"options": {
				"cwd": "${fileDirname}"
			},
			"problemMatcher": [
				"$gcc"
			],
			"group": {
				"kind": "build",
				"isDefault": true
			},
			"detail": "compiler: /usr/bin/g++"
		}
	]
}

The problem

The compiler is unable to detect the OpenCV library and reported it is not found.

> Executing task: C/C++: g++ build active file <

Starting build...
/usr/bin/g++ -g /home/wbadry/Desktop/simplecv/testing.cpp -o /home/wbadry/Desktop/simplecv/testing
/home/wbadry/Desktop/simplecv/testing.cpp:1:10: fatal error: opencv2/core.hpp: No such file or directory
    1 | #include <opencv2/core.hpp>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.

Build finished with error(s).
The terminal process failed to launch (exit code: -1).

I would appreciate it if there is an extra step to be done as I expected the VSCode extension to compile it with no problem. I believe I miss some flags.

Your help is appreciated. Thanks

[wbadry]

After googling for a day,

{
	"version": "2.0.0",
	"tasks": [
		{
			"type": "cppbuild",
			"label": "C/C++: g++ build active file",
			"command": "/usr/bin/g++",
			"args": [
				"-g",
				"${file}",
				"-o",
				"${fileDirname}/${fileBasenameNoExtension}",
				"`pkg-config", "--cflags", "--libs" ,"opencv4`"
			],
			"options": {
				"cwd": "${fileDirname}"
			},
			"problemMatcher": [
				"$gcc"
			],
			"group": {
				"kind": "build",
				"isDefault": true
			},
			"detail": "compiler: /usr/bin/g++"
		}
	]
}

adding "pkg-config", "--cflags", "--libs" ,"opencv4" was the key to allow linker to find the library.