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    \       /  __/ _  \  __/   _)  \_   \

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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://kripken.github.io/emscripten-site/docs/getting_started/downloads.html

   After installation, you should have some global variable positioned to the

   location of the SDK. In particular, $EMSCRIPTEN should point to the

   top-level directory containing Emscripten tools.

 - make sure the file $EMSCRIPTEN/cmake/Modules/Platform/Emscripten.cmake is

   accessible. This is the toolchain file used by CMake to invoke Emscripten.

 - configure the project 'WEBP_JS' with CMake using:

 cd webp_js && \

 cmake -DWEBP_BUILD_WEBP_JS=ON \

       -DEMSCRIPTEN_GENERATE_BITCODE_STATIC_LIBRARIES=1 \

       -DCMAKE_TOOLCHAIN_FILE=$EMSCRIPTEN/cmake/Modules/Platform/Emscripten.cmake \

       ../

 - compile webp.js using 'make'.

 - that's it! Upon completion, you should have the webp.js and

   webp.js.mem 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 1.37.8) and of

  your WASM-enabled browser to run this version. Consider it very experimental!

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.