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libwebp/webp_js/README.md
Maryla 54e61a3864 Markdownify libwebp docs and reorganize them. 
Break the main README into into multiple pages in the doc/ directory,
except for the tests, swig and webp_js docs which are in the corresponding
directories.
The webp mux doc is merged into the API doc and the tools doc.

Change-Id: Ia407617dd88094f4662841d37947cfef80799914
2022-02-15 15:31:56 +00:00

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Webp JavaScript decoder

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This file describes the compilation of libwebp into a JavaScript decoder using Emscripten and CMake.

  • install the Emscripten SDK following the procedure described at: https://emscripten.org/docs/getting_started/downloads.html#installation-instructions-using-the-emsdk-recommended After installation, you should have some global variable positioned to the location of the SDK. In particular, $EMSDK should point to the top-level directory containing Emscripten tools.

  • configure the project 'WEBP_JS' with CMake using:

    cd webp_js && \
emcmake cmake -DWEBP_BUILD_WEBP_JS=ON \
      ../

  • compile webp.js using 'emmake make'.

  • that's it! Upon completion, you should have the webp.js and webp.wasm files generated.

The callable JavaScript function is WebPToSDL(), which decodes a raw WebP bitstream into a canvas. See webp_js/index.html for a simple usage sample (see below for instructions).

Demo HTML page

The HTML page webp_js/index.html requires an HTTP server to serve the WebP image example. It's easy to just use Python for that.

cd webp_js && python -m SimpleHTTPServer 8080

and then navigate to http://localhost:8080 in your favorite browser.

Web-Assembly (WASM) version:

CMakeLists.txt is configured to build the WASM version when using the option WEBP_BUILD_WEBP_JS=ON. The compilation step will assemble the files 'webp_wasm.js', 'webp_wasm.wasm' in the webp_js/ directory. See webp_js/index_wasm.html for a simple demo page using the WASM version of the library.

You will need a fairly recent version of Emscripten (at least 2.0.18, latest-upstream is recommended) and of your WASM-enabled browser to run this version.

Caveats

  • First decoding using the library is usually slower, due to just-in-time compilation.

  • Some versions of llvm produce the following compile error when SSE2 is enabled.

    "Unsupported:   %516 = bitcast <8 x i16> %481 to i128
LLVM ERROR: BitCast Instruction not yet supported for integer types larger than 64 bits"

    The corresponding Emscripten bug is at: https://github.com/kripken/emscripten/issues/3788

    Therefore, SSE2 optimization is currently disabled in CMakeLists.txt.

  • If WEBP_ENABLE_SIMD is set to 1 the JavaScript version (webp.js) will be disabled as wasm2js does not support SIMD.