alass is a command line tool to synchronize subtitles to movies.
It can automatically correct
- constant offsets
- splits due to advertisement breaks, directors cut, ...
- different framerates
The alignment process is not only fast and accurate, but also language-agnostic. This means you can align subtitles to movies in different languages.
alass stands for "Automatic Language-Agnostic Subtitle Synchronization". The theory and algorithms
are documented in my bachelor's thesis
and summarized in my bachelor's presentation.
Executable for Windows (64-bit)
Get the latest executable from here! Just download and extract the archive. The file
alass.bat is the command line tool.
Executable for Linux (64-bit)
Get the latest executable from here! To run the executable,
ffprobe have to be installed.
You can change their paths with the environment variables
The most basic command is:
$ alass movie.mp4 incorrect_subtitle.srt output.srt
You can also use
alass to align the incorrect subtitle to a different subtitle:
$ alass reference_subtitle.ssa incorrect_subtitle.srt output.srt
You can additionally adjust how much the algorithm tries to avoid introducing or removing a break:
# split-penalty is a value between 0 and 1000 (default 7) $ alass reference_subtitle.ssa incorrect_subtitle.srt output.srt --split-penalty 10
Values between 5 and 20 are the most useful. Anything above 20 misses some important splits and anything below 5 introduces many unnecessary splits.
If you only want to shift the subtitle, without introducing splits, you can use
# synchronizing the subtitles in this mode is very fast $ alass movie.mp4 incorrect_subtitle.srt output.srt --no-splits
Currently supported are
.idx files. Every common video format is supported for the reference file.
Performance and Results
The extraction of the audio from a video takes about 10 to 20 seconds. Computing the alignment usually takes between 5 and 10 seconds.
The alignment is usually perfect -
the percentage of "good subtitles" is about 88% to 98%, depending on how strict you classify a "good subtitle".
Downloading random subtitles
OpenSubtitles.org had an error rate of about 50%
(sample size N=118).
Of all subtitle lines (not subtitle files) in the tested database,
- 50% were within 50ms of target position
- 80% were within 100ms of target position
- 90% were within 400ms of target position
- 95% were within 800ms of target position
compared to a (possibly not perfect) reference subtitle.
How to compile the binary
Install Rust and Cargo then run:
# this will create the lastest release in ~/.cargo/bin/alass-cli $ cargo install alass-cli
The voice-activity module this project uses is written in C. Therefore a C compiler (
clang) is needed to compile this project.
alass-cli with video files,
ffprobe have to be installed. It is used to extract the raw audio data. You can set the paths used by
alass using the environment variables
Building from Source
If you want to build and run the project from source code:
$ git clone https://github.com/kaegi/alass $ cd alass $ cargo build $ cargo run -- movie.mp4 input.srt output.srt
All parameters are shown for
cargo build can also be used for
cargo install and
FFmpeg as a library
You can also link
ffmpeg as a dynamic library during compile time. The library implementation can extract the audio about 2 to 3 seconds faster. Unfortunately it is harder to compile, the error handling is only very basic and might still have bugs.
You have to remove "
# FFMPEG-LIB" from every line that starts with it in
alass-cli/Cargo.toml. Then use:
# Important: you have to be inside `alass-cli`! Otherwise the parameters get ignored. $ cargo build --no-default-features --features ffmpeg-library
For Linux users: It is recommended to add the folder path to your system path as well as setup an alias for
alass-cli. Add this to your
~/.bashrc (or the setup file of your favorite shell):
export PATH="$PATH:$HOME/.cargo/bin" alias alass="alass-cli"
cargo workspace contains two projects:
alass-corewhich provides the algorithm
It is targeted at developers who want to use the same algorithm in their project.
alass-cliwhich is the official command line tool
It is target at end users who want to correct their subtitles.
Open README from
This program was called
aligner in the past. This made it nearly impossible to find on a search engine, so
alass was chosen instead.