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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. |