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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.
- If $EMSCRIPTEN is unset search for Emscripten.cmake under $EMSDK and set
- $EMSCRIPTEN accordingly, for example:
- unix-like environments: export EMSCRIPTEN=$EMSDK/upstream/emscripten
- windows: set EMSCRIPTEN=%EMSDK%\upstream\emscripten
- - configure the project 'WEBP_JS' with CMake using:
- cd webp_js && \
- cmake -DWEBP_BUILD_WEBP_JS=ON \
- -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.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 1.37.8,
- 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.
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