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2    /  \\/  \  _ \  _ \  _ \   (__)/  __\
3    \       /  __/ _  \  __/   _)  \_   \
4     \__\__/_____/____/_/     /____/____/
5
6Description:
7============
8
9This file describes the compilation of libwebp into a JavaScript decoder
10using Emscripten and CMake.
11
12 - install the Emscripten SDK following the procedure described at:
13   https://kripken.github.io/emscripten-site/docs/getting_started/downloads.html
14   After installation, you should have some global variable positioned to the
15   location of the SDK. In particular, $EMSCRIPTEN should point to the
16   top-level directory containing Emscripten tools.
17
18 - make sure the file $EMSCRIPTEN/cmake/Modules/Platform/Emscripten.cmake is
19   accessible. This is the toolchain file used by CMake to invoke Emscripten.
20
21 - configure the project 'WEBP_JS' with CMake using:
22
23 cd webp_js && \
24 cmake -DWEBP_BUILD_WEBP_JS=ON \
25       -DEMSCRIPTEN_GENERATE_BITCODE_STATIC_LIBRARIES=1 \
26       -DCMAKE_TOOLCHAIN_FILE=$EMSCRIPTEN/cmake/Modules/Platform/Emscripten.cmake \
27       ../
28
29 - compile webp.js using 'make'.
30
31 - that's it! Upon completion, you should have the webp.js and
32   webp.js.mem files generated.
33
34The callable JavaScript function is WebPToSDL(), which decodes a raw WebP
35bitstream into a canvas. See webp_js/index.html for a simple usage sample
36(see below for instructions).
37
38Demo HTML page:
39===============
40
41   The HTML page webp_js/index.html requires an HTTP server to serve the WebP
42   image example. It's easy to just use Python for that.
43
44cd webp_js && python -m SimpleHTTPServer 8080
45
46and then navigate to http://localhost:8080 in your favorite browser.
47
48
49Web-Assembly (WASM) version:
50============================
51
52  CMakeLists.txt is configured to build the WASM version when using
53  the option WEBP_BUILD_WEBP_JS=ON. The compilation step will assemble
54  the files 'webp_wasm.js', 'webp_wasm.wasm' in the webp_js/ directory.
55  See webp_js/index_wasm.html for a simple demo page using the WASM version
56  of the library.
57
58  You will need a fairly recent version of Emscripten (at least 1.37.8) and of
59  your WASM-enabled browser to run this version. Consider it very experimental!
60
61Caveat:
62=======
63
64  - First decoding using the library is usually slower, due to just-in-time
65    compilation.
66
67  - Some versions of llvm produce the following compile error when SSE2 is
68    enabled.
69
70"Unsupported:   %516 = bitcast <8 x i16> %481 to i128
71 LLVM ERROR: BitCast Instruction not yet supported for integer types larger than 64 bits"
72
73    The corresponding Emscripten bug is at:
74    https://github.com/kripken/emscripten/issues/3788
75
76    Therefore, SSE2 optimization is currently disabled in CMakeLists.txt.
77