forked from Green-Sky/tomato
Green Sky
67653bbe50
git-subtree-dir: external/libwebp/libwebp git-subtree-split: dd7364c3cefe0f5c0b3c18c3b1887d353f90fc1f
82 lines
2.6 KiB
Markdown
82 lines
2.6 KiB
Markdown
# 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', 'webp.js.mem',
|
|
'webp_wasm.js' and 'webp_wasm.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 the built files 'webp.js' and
|
|
'webp.js.mem' to be copied to webp_js/. An HTTP server to serve the WebP image
|
|
example is also needed. With Python, just run:
|
|
|
|
```shell
|
|
cd webp_js && python3 -m http.server 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' and 'webp_wasm.wasm' that you then need to copy to 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.
|