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82 lines
2.6 KiB
Markdown
82 lines
2.6 KiB
Markdown
# WebP JavaScript decoder
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```
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__ __ ____ ____ ____ __ ____
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/ \\/ \ _ \ _ \ _ \ (__)/ __\
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\ / __/ _ \ __/ _) \_ \
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\__\__/_____/____/_/ /____/____/
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```
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This file describes the compilation of libwebp into a JavaScript decoder using
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Emscripten and CMake.
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- install the Emscripten SDK following the procedure described at:
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https://emscripten.org/docs/getting_started/downloads.html#installation-instructions-using-the-emsdk-recommended
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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, $EMSDK should point to the top-level
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directory containing Emscripten tools.
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- configure the project 'WEBP_JS' with CMake using:
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```shell
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cd webp_js && \
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emcmake cmake -DWEBP_BUILD_WEBP_JS=ON \
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../
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```
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- compile webp.js using 'emmake make'.
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- that's it! Upon completion, you should have the 'webp.js', 'webp.js.mem',
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'webp_wasm.js' and 'webp_wasm.wasm' 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 (see
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below for instructions).
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## Demo HTML page
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The HTML page webp_js/index.html requires the built files 'webp.js' and
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'webp.js.mem' to be copied to webp_js/. An HTTP server to serve the WebP image
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example is also needed. With Python, just run:
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```shell
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cd webp_js && python3 -m http.server 8080
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```
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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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CMakeLists.txt is configured to build the WASM version when using the option
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WEBP_BUILD_WEBP_JS=ON. The compilation step will assemble the files
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'webp_wasm.js' and 'webp_wasm.wasm' that you then need to copy to the webp_js/
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directory.
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See webp_js/index_wasm.html for a simple demo page using the WASM version of the
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library.
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You will need a fairly recent version of Emscripten (at least 2.0.18,
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latest-upstream is recommended) and of your WASM-enabled browser to run this
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version.
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## Caveats
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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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```
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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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```
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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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- If WEBP_ENABLE_SIMD is set to 1 the JavaScript version (webp.js) will be
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disabled as wasm2js does not support SIMD.
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