Track Strategies

Track strategies return options for each track (audio or video) for the engine to understand how and if this track should be transcoded, and whether the whole process should be aborted.

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Transcoder.into(filePath)
    .setVideoTrackStrategy(videoStrategy)
    .setAudioTrackStrategy(audioStrategy)
    // ...

The point of TrackStrategy is to inspect the input android.media.MediaFormat and return the output android.media.MediaFormat, filled with required options.

Basic Strategies

This library offers track specific strategies that help with audio and video options (see Audio Strategies and Video Strategies). In addition, we have a few built-in strategies that can work for both audio and video:

PassThroughTrackStrategy

A TrackStrategy that asks the encoder to keep this track as is, by returning the same input format. Note that this is risky, as the input track format might not be supported my the MP4 container.

This will set the TrackStatus to TrackStatus.PASS_THROUGH.

RemoveTrackStrategy

A TrackStrategy that asks the encoder to remove this track from the output container, by returning null. For instance, this can be used as an audio strategy to remove audio from video/audio streams.

This will set the TrackStatus to TrackStatus.REMOVING.

Audio Strategies

The default internal strategy for audio is a DefaultAudioStrategy, which converts the audio stream to AAC format with the specified number of channels and sample rate.

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val strategy: DefaultAudioStrategy = DefaultAudioStrategy.builder()
    .channels(DefaultAudioStrategy.CHANNELS_AS_INPUT)
    .channels(1)
    .channels(2)
    .sampleRate(DefaultAudioStrategy.SAMPLE_RATE_AS_INPUT)
    .sampleRate(44100)
    .sampleRate(30000)
    .bitRate(DefaultAudioStrategy.BITRATE_UNKNOWN)
    .bitRate(bitRate)
    .build()

Transcoder.into(filePath)
    .setAudioTrackStrategy(strategy)
    // ...

Take a look at the source code to understand how to manage the android.media.MediaFormat object.

Video Strategies

The default internal strategy for video is a DefaultVideoStrategy, which converts the video stream to AVC format and is very configurable. The class helps in defining an output size. If the output size does not match the aspect ratio of the input stream size, Transcoder will crop part of the input so it matches the final ratio.

Video Size

We provide helpers for common tasks:

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// Sets an exact size. If aspect ratio does not match, cropping will take place.
val strategy = DefaultVideoStrategy.exact(1080, 720).build()

// Keeps the aspect ratio, but scales down the input size with the given fraction.
val strategy = DefaultVideoStrategy.fraction(0.5F).build()

// Ensures that each video size is at most the given value - scales down otherwise.
val strategy = DefaultVideoStrategy.atMost(1000).build()

// Ensures that minor and major dimension are at most the given values - scales down otherwise.
val strategy = DefaultVideoStrategy.atMost(500, 1000).build()

In fact, all of these will simply call DefaultVideoStrategy.Builder(resizer) with a special resizer. We offer handy resizers:

You can also group resizers through MultiResizer, which applies resizers in chain:

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// First scales down, then ensures size is at most 1000. Order matters!
val resizer: Resizer = MultiResizer()
resizer.addResizer(FractionResizer(0.5F))
resizer.addResizer(AtMostResizer(1000))

// First makes it 16:9, then ensures size is at most 1000. Order matters!
val resizer: Resizer = MultiResizer()
resizer.addResizer(AspectRatioResizer(16F / 9F))
resizer.addResizer(AtMostResizer(1000))

This option is already available through the DefaultVideoStrategy builder, so you can do:

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val strategy: DefaultVideoStrategy = DefaultVideoStrategy.Builder()
    .addResizer(AspectRatioResizer(16F / 9F))
    .addResizer(FractionResizer(0.5F))
    .addResizer(AtMostResizer(1000))
    .build()

Other options

You can configure the DefaultVideoStrategy with other options unrelated to the video size:

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val strategy: DefaultVideoStrategy = DefaultVideoStrategy.Builder()
    .bitRate(bitRate)
    .bitRate(DefaultVideoStrategy.BITRATE_UNKNOWN) // tries to estimate
    .frameRate(frameRate) // will be capped to the input frameRate
    .keyFrameInterval(interval) // interval between key-frames in seconds
    .build()

Compatibility

As stated pretty much everywhere, not all codecs/devices/manufacturers support all sizes/options. This is a complex issue which is especially important for video strategies, as a wrong size can lead to a transcoding error or corrupted file.

Android platform specifies requirements for manufacturers through the Compatibility test suite (CTS). Only a few codecs and sizes are strictly required to work.

We collect common presets in the DefaultVideoStrategies class:

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Transcoder.into(filePath)
    .setVideoTrackStrategy(DefaultVideoStrategies.for720x1280()) // 16:9
    .setVideoTrackStrategy(DefaultVideoStrategies.for360x480()) // 4:3
    // ...