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README.webp_js

README.webp_js 2.5 KB
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  5. 

  6. Description:
    7. 

  7. ============
    8. 

  8. 

  9. This file describes the compilation of libwebp into a JavaScript decoder
    10. 

  10. using Emscripten and CMake.
    11. 

  11. 

  12.  - install the Emscripten SDK following the procedure described at:
    13. 

  13.    https://emscripten.org/docs/getting_started/downloads.html#installation-instructions-using-the-emsdk-recommended
    14. 

  14.    After installation, you should have some global variable positioned to the
    15. 

  15.    location of the SDK. In particular, $EMSDK should point to the
    16. 

  16.    top-level directory containing Emscripten tools.
    17. 

  17. 

  18.  - configure the project 'WEBP_JS' with CMake using:
    19. 

  19. 

  20.  cd webp_js && \
    21. 

  21.  emcmake cmake -DWEBP_BUILD_WEBP_JS=ON \
    22. 

  22.        ../
    23. 

  23. 

  24.  - compile webp.js using 'emmake make'.
    25. 

  25. 

  26.  - that's it! Upon completion, you should have the webp.js and
    27. 

  27.    webp.wasm files generated.
    28. 

  28. 

  29. The callable JavaScript function is WebPToSDL(), which decodes a raw WebP
    30. 

  30. bitstream into a canvas. See webp_js/index.html for a simple usage sample
    31. 

  31. (see below for instructions).
    32. 

  32. 

  33. Demo HTML page:
    34. 

  34. ===============
    35. 

  35. 

  36.    The HTML page webp_js/index.html requires an HTTP server to serve the WebP
    37. 

  37.    image example. It's easy to just use Python for that.
    38. 

  38. 

  39. cd webp_js && python -m SimpleHTTPServer 8080
    40. 

  40. 

  41. and then navigate to http://localhost:8080 in your favorite browser.
    42. 

  42. 

  43. 

  44. Web-Assembly (WASM) version:
    45. 

  45. ============================
    46. 

  46. 

  47.   CMakeLists.txt is configured to build the WASM version when using
    48. 

  48.   the option WEBP_BUILD_WEBP_JS=ON. The compilation step will assemble
    49. 

  49.   the files 'webp_wasm.js', 'webp_wasm.wasm' in the webp_js/ directory.
    50. 

  50.   See webp_js/index_wasm.html for a simple demo page using the WASM version
    51. 

  51.   of the library.
    52. 

  52. 

  53.   You will need a fairly recent version of Emscripten (at least 2.0.18,
    54. 

  54.   latest-upstream is recommended) and of your WASM-enabled browser to run this
    55. 

  55.   version.
    56. 

  56. 

  57. Caveat:
    58. 

  58. =======
    59. 

  59. 

  60.   - First decoding using the library is usually slower, due to just-in-time
    61. 

  61.     compilation.
    62. 

  62. 

  63.   - Some versions of llvm produce the following compile error when SSE2 is
    64. 

  64.     enabled.
    65. 

  65. 

  66. "Unsupported:   %516 = bitcast <8 x i16> %481 to i128
    67. 

  67.  LLVM ERROR: BitCast Instruction not yet supported for integer types larger than 64 bits"
    68. 

  68. 

  69.     The corresponding Emscripten bug is at:
    70. 

  70.     https://github.com/kripken/emscripten/issues/3788
    71. 

  71. 

  72.     Therefore, SSE2 optimization is currently disabled in CMakeLists.txt.
    73. 

  73. 

  74.   - If WEBP_ENABLE_SIMD is set to 1 the JavaScript version (webp.js) will be
    75. 

  75.     disabled as wasm2js does not support SIMD.
    76.