OpenCV.js does not work from a web worker if WASM threads are enabled

,
1

OpenCV.js does not work for me when built with WASM threads enabled (--threads option active) and used in a web worker.

The problem occurs when using OpenCV.js compiled with emscripten (3.1.57, or latest)

$ git clone https://github.com/opencv/opencv.git
$ git checkout 4.10.0

Now build opencv.js with, eg. emscripten 3.1.57 (or latest emscripten version ; I tried a few versions, all produce the same result):

$ emcmake python3 ./opencv/platforms/js/build_js.py build_wasmthreads --build_wasm --threads

opencv.js has been generated in the build_wasmthreads/bin folder.

Next, make sure SharedArrayBuffer is enabled regardless of cross-origin header status, to be on the safe side ; for this, I use --enable-features=SharedArrayBuffer on Chrome.

Now try the following:

Case 1)

  • load opencv.js in a web page using a <script> tag
  • await cv; This line of code will succeed.

Case 2)

  • load opencv.js in a web worker using importScripts().
  • await cv; This line of code will fail with the following error:

Refused to execute script from ‘http://localhost/undefined’ because its MIME type (‘text/html’) is not executable, and strict MIME type checking is enabled

Note that OpenCV.js will always work fine when WASM threads are disabled (–threads option not used).

Shall a specific emscripten version be used for WASM threads to work in web workers?

2

Looks like you’ve got the problem pinned down.

I’d recommend filing an issue on OpenCV’s github, or adding your findings if such a one already exists.