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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. |
