README.webp_js

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Description:
============

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.

Caveat:
=======

  - 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.