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# Webp JavaScript decoder
```
__ __ ____ ____ ____ __ ____
/ \\/ \ _ \ _ \ _ \ (__)/ __\
\ / __/ _ \ __/ _) \_ \
\__\__/_____/____/_/ /____/____/
```
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://emscripten.org/docs/getting_started/downloads.html#installation-instructions-using-the-emsdk-recommended
After installation, you should have some global variable positioned to the
location of the SDK. In particular, $EMSDK should point to the top-level
directory containing Emscripten tools.
- configure the project 'WEBP_JS' with CMake using:
```shell
cd webp_js && \
emcmake cmake -DWEBP_BUILD_WEBP_JS=ON \
../
```
- compile webp.js using 'emmake 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.
```shell
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 2.0.18,
latest-upstream is recommended) and of your WASM-enabled browser to run this
version.
## Caveats
- 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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