WebP encoder module of the JPG to WebP tool
===========================================

This folder holds a build of libwebp's encoder, without any change to its
source code. libwebp is distributed under a BSD license (3-clause), with
an additional patent grant from Google:

- libwebp 1.6.0
  Copyright (c) 2010, Google Inc. All rights reserved.
  https://chromium.googlesource.com/webm/libwebp

The build also contains the Emscripten 6.0.12 runtime (MIT or
University of Illinois/NCSA license) and musl, its C library (Copyright
(c) 2005-2020 Rich Felker, et al., MIT license), including math code
under Sun Microsystems' fdlibm notice.


When it is used
---------------

Chrome, Edge and Firefox write WebP images themselves and never load
this module. Safari cannot, on Mac, iPhone or iPad, and every browser on
iPhone and iPad uses Safari's engine. There, the page loads this module
in its conversion worker and writes each image with libwebp's simple
API, with libwebp's default settings, as Chrome does: WebPEncodeRGBA at
the quality chosen below 100, and WebPEncodeLosslessRGBA at 100. The
image never leaves the device.


Files
-----

libwebp.js, libwebp.wasm
    The module: libwebp's encoder (src/enc, the src/dsp and src/utils
    files it uses, and sharpyuv) compiled with Emscripten, plain C with
    no SIMD and no threads. SHA-256 in SHA256SUMS.

source/libwebp-1.6.0.tar.gz
    The exact source code of this build: the release file published by
    the WebM Project at
    https://storage.googleapis.com/downloads.webmproject.org/releases/webp/
    (SHA-256 e4ab7009bf0629fd11982d4c2aa83964cf244cffba7347ecd39019a9e38c4564),
    signed with the "WebP release signing key", fingerprint
    6B0E 6B70 976D E303 EDF2  F601 F9C3 D6BD B823 2B5D.

source/build.sh
    The build script, with every option and flag used.

COPYING-libwebp.txt, PATENTS-libwebp.txt, AUTHORS-libwebp.txt
    libwebp's license, Google's patent grant and libwebp's authors.

LICENSE-emscripten.txt, LICENSE-musl.txt, NOTICE-fdlibm.txt
    The licenses and notices of the Emscripten runtime, musl and the
    fdlibm math code.


Rebuilding
----------

1. Install and activate emsdk 6.0.12 (https://emscripten.org) and curl.
2. Copy source/libwebp-1.6.0.tar.gz into <work-folder>: build.sh checks
   its SHA-256 and does not download it again.
3. Run: source/build.sh <work-folder> <output-folder>

Built this way, in separate folders of the same machine, the output was
byte-identical to libwebp.js and libwebp.wasm here (same SHA-256). The
system libraries that come prebuilt with emsdk embed their own build
paths, so another emsdk setup may give small differences.


Replacing the library
---------------------

The tool loads libwebp.js as an ES module inside its Web Worker, fetches
libwebp.wasm, and calls the module's default export with { wasmBinary }.
It then uses only these exports: _WebPEncodeRGBA, _WebPEncodeLosslessRGBA,
_WebPFree, _WebPGetEncoderVersion, _malloc, _free and the HEAPU8 and
HEAPU32 views of the module's memory. Any build of libwebp with those
exports can take the place of these two files.