OpenCV: Build OpenCV.js

| OpenCV 3.4.2 Open Source Computer Vision

Build OpenCV.js

## Installing Emscripten

Emscripten is an LLVM-to-JavaScript compiler. We will use Emscripten to build OpenCV.js.

To Install Emscripten, follow instructions of Emscripten SDK.

For example:

./emsdk update
./emsdk install latest
./emsdk activate latest

Note
  To compile to WebAssembly, you need to install and activate Binaryen with the `emsdk` command. Please refer to Developer's Guide for more details.

After install, ensure the `EMSCRIPTEN` environment is setup correctly.

For example:

source ./emsdk_env.sh
echo ${EMSCRIPTEN}

## Obtaining OpenCV Source Code

You can use the latest stable OpenCV version or you can grab the latest snapshot from our Git repository.

### Obtaining the Latest Stable OpenCV Version

- Go to our releases page.
- Download the source archive and unpack it.

### Obtaining the Cutting-edge OpenCV from the Git Repository

Launch Git client and clone OpenCV repository.

For example:

git clone https://github.com/opencv/opencv.git

Note
  It requires `git` installed in your development environment.

## Building OpenCV.js from Source

1. To build `opencv.js`, execute python script `<opencv_src_dir>/platforms/js/build_js.py <build_dir>`.

   For example, to build in `build_js` directory:

   cd opencv
   python ./platforms/js/build_js.py build_js
   Note
     It requires `python` and `cmake` installed in your development environment.

2. The build script builds asm.js version by default. To build WebAssembly version, append `--build_wasm` switch.

   For example, to build wasm version in `build_wasm` directory:

   python ./platforms/js/build_js.py build_wasm --build_wasm

3. [optional] To build documents, append `--build_doc` option.

   For example:

   python ./platforms/js/build_js.py build_js --build_doc
   Note
     It requires `doxygen` installed in your development environment.

4. [optional] To build tests, append `--build_test` option.

   For example:

   python ./platforms/js/build_js.py build_js --build_test

   To run tests, launch a local web server in <build_dir>/bin folder. For example, node http-server which serves on `localhost:8080`.

   Navigate the web browser to `http://localhost:8080/tests.html`, which runs the unit tests automatically.

   You can also run tests using Node.js.

   For example:

   cd bin
   npm install
   node tests.js
   Note
     It requires `node` installed in your development environment.