2019-08-20 19:24:22 +00:00
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/ \\/ \ _ \ _ \ _ \ (__)/ __\
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\ / __/ _ \ __/ _) \_ \
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\__\__/_____/____/_/ /____/____/
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Description:
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============
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This file describes the compilation of libwebp into a JavaScript decoder
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using Emscripten and CMake.
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- install the Emscripten SDK following the procedure described at:
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https://kripken.github.io/emscripten-site/docs/getting_started/downloads.html
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After installation, you should have some global variable positioned to the
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location of the SDK. In particular, $EMSCRIPTEN should point to the
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top-level directory containing Emscripten tools.
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- make sure the file $EMSCRIPTEN/cmake/Modules/Platform/Emscripten.cmake is
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accessible. This is the toolchain file used by CMake to invoke Emscripten.
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2019-10-07 20:56:30 +00:00
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If $EMSCRIPTEN is unset search for Emscripten.cmake under $EMSDK and set
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$EMSCRIPTEN accordingly, for example:
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unix-like environments: export EMSCRIPTEN=$EMSDK/fastcomp/emscripten
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windows: set EMSCRIPTEN=%EMSDK%\fastcomp\emscripten
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2019-08-20 19:24:22 +00:00
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- configure the project 'WEBP_JS' with CMake using:
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cd webp_js && \
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cmake -DWEBP_BUILD_WEBP_JS=ON \
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-DEMSCRIPTEN_GENERATE_BITCODE_STATIC_LIBRARIES=1 \
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-DCMAKE_TOOLCHAIN_FILE=$EMSCRIPTEN/cmake/Modules/Platform/Emscripten.cmake \
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../
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- compile webp.js using 'make'.
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- that's it! Upon completion, you should have the webp.js and
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webp.js.mem files generated.
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The callable JavaScript function is WebPToSDL(), which decodes a raw WebP
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bitstream into a canvas. See webp_js/index.html for a simple usage sample
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(see below for instructions).
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Demo HTML page:
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===============
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The HTML page webp_js/index.html requires an HTTP server to serve the WebP
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image example. It's easy to just use Python for that.
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cd webp_js && python -m SimpleHTTPServer 8080
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and then navigate to http://localhost:8080 in your favorite browser.
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Web-Assembly (WASM) version:
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============================
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CMakeLists.txt is configured to build the WASM version when using
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the option WEBP_BUILD_WEBP_JS=ON. The compilation step will assemble
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the files 'webp_wasm.js', 'webp_wasm.wasm' in the webp_js/ directory.
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See webp_js/index_wasm.html for a simple demo page using the WASM version
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of the library.
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You will need a fairly recent version of Emscripten (at least 1.37.8) and of
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your WASM-enabled browser to run this version. Consider it very experimental!
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Caveat:
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=======
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- First decoding using the library is usually slower, due to just-in-time
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compilation.
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- Some versions of llvm produce the following compile error when SSE2 is
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enabled.
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"Unsupported: %516 = bitcast <8 x i16> %481 to i128
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LLVM ERROR: BitCast Instruction not yet supported for integer types larger than 64 bits"
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The corresponding Emscripten bug is at:
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https://github.com/kripken/emscripten/issues/3788
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Therefore, SSE2 optimization is currently disabled in CMakeLists.txt.
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