Sounds interesting but it's a little unclear what's running at what layer - it sounds like the JS code from OpenCV.js is now running in webassembly? And how much of OpenCV is still running in native code, e.g. in prebuilt OpenCV libraries?
From what I can tell, the OpenCV C code is directly compiled to wasm bytecode; the only JS part is some helper code to let you easily call wasm functions from your own code.
