libass-webos-legacy

A lightweight ASS subtitle rendering pipeline for legacy LG webOS browsers, built around libass with event-driven caching and performance-focused scheduling.

Build

The project ships two targets because webOS spans two very different Chromium versions:

| Target | Chromium | webOS | Emscripten SDK | | ------ | -------- | ------ | ----------------- | | modern | 68 | 4 / 5+ | emsdk:4.0.13 | | legacy | 38 | 3 | emsdk:3.1.73 |

Each target needs a different Emscripten toolchain, so a separate Docker image is built for each. The Dockerfile accepts an EMSDK_VERSION build-arg to select the SDK without editing the file between builds.

build.sh

Everything (Docker image → WASM compile → webpack bundle) is driven by a single script:

./build.sh [modern|legacy|all] [full|quick]

Defaults: all full.

| Target | Effect | | -------- | -------------------------------------------------------------------------- | | modern | Modern target only. | | legacy | Legacy target only. | | all | Both, sequentially. |

| Mode | Effect | | ------- | --------------------------------------------------------------------------------------------- | | full | Rebuild the Docker image with --no-cache, then make clean-libs && make <target> + webpack.| | quick | Reuse the existing image (build it if missing), skip clean-libs, then make <target> + webpack. |

Examples:

./build.sh                    # both targets, from scratch
./build.sh all quick          # both targets, reuse existing images and C libs
./build.sh modern             # modern only, from scratch
./build.sh legacy quick       # legacy only, incremental

Artifacts:

• build/js/{modern,legacy}/worker.{min,debug}.js — raw emcc output.
• dist/{modern,legacy}/ — webpack bundles ready to publish.

Manual invocation

If you need to bypass the script, the underlying steps are:

# Modern (Chrome 68)
docker buildx build -t libass-webos-legacy \
    --build-arg EMSDK_VERSION=4.0.13 \
    --no-cache --network=host .
docker run --rm -v "$PWD:/work" libass-webos-legacy \
    bash -lc "make clean-libs && make modern"

# Legacy (Chrome 38)
docker buildx build -t libass-webos-legacy-old \
    --build-arg EMSDK_VERSION=3.1.73 \
    --no-cache --network=host .
docker run --rm -v "$PWD:/work" libass-webos-legacy-old \
    bash -lc "make clean-libs && make legacy"

# JS bundle
npm install
npm run webpack                # or webpack-modern / webpack-legacy

make clean-libs is needed before switching between targets because the two toolchains produce incompatible object files. Use make clean for a full reset (deletes both build/ and the emitted JS).

Layout

• src/ — C++ bindings and JS runtime (libass.cpp, libass.js, worker.js, LRUCache.js).
• lib/ — vendored C dependencies (libass, freetype, harfbuzz, fribidi, brotli).
• webpack/{common,modern,legacy}/ — bundler configs, one config pair per target.
• build/ — CMake output (owned by the Docker container).
• dist/{modern,legacy}/ — final JS bundles produced by webpack.
• docs/ — design documents.