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https://github.com/kaegi/alass

"Automatic Language-Agnostic Subtitle Synchronization"
https://github.com/kaegi/alass

algorithm alignment api incorrect-subtitles subtitle tool

Last synced: about 2 months ago
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"Automatic Language-Agnostic Subtitle Synchronization"

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README 

        
# Introduction



`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](documentation/thesis.pdf)

and summarized in my [bachelor's presentation](documentation/slides.pdf).



## Executable for Windows (64-bit)



Get the latest executable from [here](https://github.com/kaegi/alass/releases)! 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](https://github.com/kaegi/alass/releases)! To run the executable, `ffmpeg` and

`ffprobe` have to be installed.

You can change their paths with the environment variables

`ALASS_FFMPEG_PATH` (default `ffmpeg`) and `ALASS_FFPROBE_PATH` (default `ffprobe`). 



## Usage



The most basic command is:



```bash

$ alass movie.mp4 incorrect_subtitle.srt output.srt

```



You can also use `alass` to align the incorrect subtitle to a different subtitle:



```bash

$ 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:



```bash

# 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 `--no-splits`:



```bash

# synchronizing the subtitles in this mode is very fast

$ alass movie.mp4 incorrect_subtitle.srt output.srt --no-splits

```



Currently supported are `.srt`, `.ssa`/`.ass` and `.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

from `OpenSubtitles.org` had an error rate of about 50%

(sample size N=118).

Of all subtitle _lines_ (not subtitle files) in the tested database,

after synchronization



 - 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](https://www.rust-lang.org/en-US/install.html) then run:



```bash

# 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 (`gcc` or `clang`) is needed to compile this project.



To use `alass-cli` with video files, `ffmpeg` and `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 `ALASS_FFMPEG_PATH` (default `ffmpeg`) and `ALASS_FFPROBE_PATH` (default `ffprobe`). 



### Building from Source 



If you want to build and run the project from source code:



```bash

$ git clone https://github.com/kaegi/alass

$ cd alass

$ cargo build

$ cargo run -- movie.mp4 input.srt output.srt

```



### Configuration



All parameters are shown for `cargo build` can also be used for `cargo install` and `cargo run`.



#### 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:



```bash

# Important: you have to be inside `alass-cli`! Otherwise the parameters get ignored.

$ cargo build --no-default-features --features ffmpeg-library

```



### Alias Setup



*For Linux users:* It is recommended to add the folder path to your system path as well as setup an alias for `alass` to `alass-cli`. Add this to your `~/.bashrc` (or the setup file of your favorite shell):



```bash

export PATH="$PATH:$HOME/.cargo/bin"

alias alass="alass-cli"

```



## Folder structure



This `cargo` workspace contains two projects:



  - `alass-core` which provides the algorithm

  

    It is targeted at *developers* who want to use the same algorithm in their project.



  - `alass-cli` which is the official command line tool



    It is target at *end users* who want to correct their subtitles.



## Library Documentation



[Open README](./alass-core/README.md) from `alass-core`.



## Notes



This program was called `aligner` in the past. This made it nearly impossible to find on a search engine, so `alass` was chosen instead.