dany/libwebp
1
0
Fork 0
mirror of https://github.com/webmproject/libwebp.git synced 2024-11-20 04:18:26 +01:00
Code Issues Actions Packages Projects Releases Wiki Activity
e3ba2b1f5b
libwebp/webp_js
History
James Zern 19b1a71c48 webp_js/README.md,cosmetics: reflow some lines 
after:
cb90f76b Update wasm instructions.

Change-Id: I8d937d08dba98c23c4a298ba1165e4b1f553fea9
2022-10-06 14:10:03 -07:00
..
index_wasm.html fix memory leak in SDL_Init() 2017-07-31 12:47:02 -07:00
index.html fix memory leak in SDL_Init() 2017-07-31 12:47:02 -07:00
README.md webp_js/README.md,cosmetics: reflow some lines 2022-10-06 14:10:03 -07:00
test_webp_js.webp WASM support 2017-03-21 14:07:22 +01:00
test_webp_wasm.webp WASM support 2017-03-21 14:07:22 +01:00

README.md

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:

    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:

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.