Crate webm_iterable

This crate was built to ease parsing files encoded in a Matroska container, such as WebMs or MKVs.

The main content provided by this crate is the MatroskaSpec enum. Otherwise, this crate simply provides type aliases in the form of WebmIterator and WebmWriter.

Example Usage

The following examples show how to read, modify, and write webm files using this library.

Example 1

The following example reads a media file into memory and decodes it.

use std::fs::File;
use webm_iterable::WebmIterator;
 
fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut src = File::open("media/test.webm").unwrap();
    let tag_iterator = WebmIterator::new(&mut src, &[]);
 
    for tag in tag_iterator {
        println!("[{:?}]", tag?);
    }
 
    Ok(())
}

Example 2

This next example does the same thing, but keeps track of the number of times each tag appears in the file.

use std::fs::File;
use std::collections::HashMap;
use webm_iterable::WebmIterator;
use webm_iterable::matroska_spec::EbmlTag;
 
fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut src = File::open("media/test.webm").unwrap();
    let tag_iterator = WebmIterator::new(&mut src, &[]);
    let mut tag_counts = HashMap::new();
 
    for tag in tag_iterator {
        let count = tag_counts.entry(tag?.get_id()).or_insert(0);
        *count += 1;
    }
     
    println!("{:?}", tag_counts);
    Ok(())
}

Example 3

This example grabs the audio from a webm and stores the result in a new file. The logic in this example is rather advanced - an explanation follows the code.

use std::fs::File;
use std::convert::TryInto;
 
use webm_iterable::{
    WebmIterator, 
    WebmWriter,
    matroska_spec::{MatroskaSpec, Master, Block, EbmlSpecification, EbmlTag},
};
 
fn main() -> Result<(), Box<dyn std::error::Error>> {
    // 1
    let mut src = File::open("media/audiosample.webm").unwrap();
    let tag_iterator = WebmIterator::new(&mut src, &[MatroskaSpec::TrackEntry(Master::Start)]);
    let mut dest = File::create("media/audioout.webm").unwrap();
    let mut tag_writer = WebmWriter::new(&mut dest);
    let mut stripped_tracks = Vec::new();
 
    // 2
    for tag in tag_iterator {
        let tag = tag?;
        match tag {
            // 3
            MatroskaSpec::TrackEntry(master) => {
                let children = master.get_children();
                let is_audio_track = |tag: &MatroskaSpec| {
                    if let MatroskaSpec::TrackType(val) = tag {
                        return *val != 2;
                    } else {
                        false
                    }
                };
 
                if children.iter().any(is_audio_track) {
                    let track_number_variant = children.iter().find(|c| matches!(c, MatroskaSpec::TrackNumber(_))).expect("should have a track number child");
                    let track_number = track_number_variant.as_unsigned_int().expect("TrackNumber is an unsigned int variant");
                    stripped_tracks.push(*track_number);
                } else {
                    tag_writer.write(&MatroskaSpec::TrackEntry(Master::Full(children)))?;
                }
            },
            // 4
            MatroskaSpec::Block(ref data) => {
                let data: &[u8] = &data;
                let block: Block = data.try_into()?;
                if !stripped_tracks.iter().any(|t| *t == block.track) {
                    tag_writer.write(&tag)?;
                }
            },
            MatroskaSpec::SimpleBlock(ref data) => {
                let data: &[u8] = &data;
                let block: Block = data.try_into()?;
                if !stripped_tracks.iter().any(|t| *t == block.track) {
                    tag_writer.write(&tag)?;
                }
            },
            // 5
            _ => {
                tag_writer.write(&tag)?;
            }
        }
    }
     
    Ok(())
}

In the above example, we (1) build our iterator and writer based on local file paths and declare useful local variables, (2) iterate over the tags in the webm file, (3) identify any tracks that are not audio and store their numbers in the stripped_tracks variable; if they are audio, we write the “TrackEntry” out, (4) only write block data for tracks that are audio, and (5) write all other tags to the output destination.

Notes

• Notice the second parameter passed into the WebmIterator::new() function. This parameter tells the decoder which “master” tags should be read as Master::Full variants rather than the standard Master::Start and Master::End variants. This greatly simplifies our iteration loop logic as we don’t have to maintain an internal buffer for the “TrackEntry” tags that we are interested in processing.

Modules

errors

Potential errors that can occur when reading or writing WebM data.

matroska_spec

Provides the MatroskaSpec enum, which implements EbmlSpecification and EbmlTag.

Type Definitions

WebmIterator

Alias for ebml_iterable::TagIterator using MatroskaSpec as the generic type.

WebmWriter

Alias for ebml_iterable::TagWriter.