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Package mp4ff implements MP4 media file parsing and writing for AVC and HEVC video, AAC and AC-3 audio, and stpp and wvtt subtitles. It is focused on fragmented files as used for streaming in DASH, MSS and HLS fMP4, but can also decode and encode all boxes needed for progressive MP4 files. In particular, the tool mp4ff-crop can be used to crop a progressive file.

Command Line Tools

Some useful command line tools are available in cmd.

  1. mp4ff-info prints a tree of the box hierarchy of a mp4 file with information about the boxes. The level of detail can be increased with the option -l, like -l all:1 for all boxes or -l trun:1,stss:1 for specific boxes.
  2. mp4ff-pslister extracts and displays SPS and PPS for AVC or HEVC in a mp4 or a bytestream (Annex B) file. Partial information is printed for HEVC.
  3. mp4ff-nallister lists NALUs and picture types for video in progressive or fragmented file
  4. mp4ff-wvttlister lists details of wvtt (WebVTT in ISOBMFF) samples
  5. mp4ff-crop shortens a progressive mp4 file to a specified duration

You can install these tools by going to their respective directory and run go install . or directly from the repo with

go install github.com/edgeware/mp4ff/cmd/[email protected]

Example code

Example code is available in the examples directory. The examples and their functions are:

  1. initcreator creates typical init segments (ftyp + moov) for video and audio
  2. resegmenter reads a segmented file (CMAF track) and resegments it with other segment durations using fullSample
  3. segmenter takes a progressive mp4 file and creates init and media segments from it. This tool has been extended to support generation of segments with multiple tracks as well as reading and writing mdat in lazy mode
  4. multitrack parses a fragmented file with multiple tracks
  5. decrypt-cenc decrypts a segmented mp4 file encrypted in cenc mode


The library has functions for parsing (called Decode) and writing (Encode) in the package mp4ff/mp4. It also contains codec specific parsing of AVC/H.264 including complete parsing of SPS and PPS in the package mp4ff.avc. HEVC/H.265 parsing is less complete, and available as mp4ff.hevc.

Traditional multiplexed non-fragmented mp4 files can be parsed and decoded, but the focus is on fragmented mp4 files as used in DASH, HLS, and CMAF.

Beyond single-track fragmented files, support has been added to parse and generate multi-track fragmented files as can be seen in examples/segment and examples/multitrack.

The top level structure for both non-fragmented and fragmented mp4 files is mp4.File.

In a progressive (non-fragmented) mp4.File, the top level attributes Ftyp, Moov, and Mdat points to the corresponding boxes.

A fragmented mp4.File can be more or less complete, like a single init segment, one or more media segments, or a combination of both like a CMAF track which renders into a playable one-track asset. It can also have multiple tracks. For fragmented files, the following high-level attributes are used:

  • Init contains a ftyp and a moov box and provides the general metadata for a fragmented file. It corresponds to a CMAF header
  • Segments is a slice of MediaSegment which start with an optional styp box and contains one or more Fragments
  • Fragment is a mp4 fragment with exactly one moof box followed by a mdat box where the latter contains the media data. It can have one or more trun boxes containing the metadata for the samples.

All child boxes of container box such as MoovBox are listed in the Children attribute, but the most prominent child boxes have direct links with names which makes it possible to write a path such as


to access the (only) trun box in a fragment with only one traf box, or


to get the second trun of the second traf box (provided that they exist). Care must be taken to assert that none of the intermediate pointers are nil to avoid panic.

Creating new fragmented files

A typical use case is to a fragment consisting of an init segment followed by a series of media segments.

The first step is to create the init segment. This is done in three steps as can be seen in examples/initcreator:

init := mp4.CreateEmptyInit()
init.AddEmptyTrack(timescale, mediatype, language)
init.Moov.Trak.SetHEVCDescriptor("hvc1", vpsNALUs, spsNALUs, ppsNALUs)

Here the third step fills in codec-specific parameters into the sample descriptor of the single track. Multiple tracks are also available via the slice attribute Traks instead of Trak.

The second step is to start producing media segments. They should use the timescale that was set when creating the init segment. Generally, that timescale should be chosen so that the sample durations have exact values.

A media segment contains one or more fragments, where each fragment has a moof and a mdat box. If all samples are available before the segment is created, one can use a single fragment in each segment. Example code for this can be found in examples/segmenter.

A simple, but not optimal, way of creating a media segment is to first create a slice of FullSample with the data needed. The definition of mp4.FullSample is

	Sample: mp4.Sample{
		Flags uint32 // Flag sync sample etc
		Dur   uint32 // Sample duration in mdhd timescale
		Size  uint32 // Size of sample data
		Cto   int32  // Signed composition time offset
	DecodeTime uint64 // Absolute decode time (offset + accumulated sample Dur)
	Data       []byte // Sample data

The mp4.Sample part is what will be written into the trun box. DecodeTime is the media timeline accumulated time. The DecodeTime value of the first sample of a fragment, will be set as the BaseMediaDecodeTime in the tfdt box.

Once a number of such full samples are available, they can be added to a media segment like

seg := mp4.NewMediaSegment()
frag := mp4.CreateFragment(uint32(segNr), mp4.DefaultTrakID)
for _, sample := range samples {

This segment can finally be output to a w io.Writer as

err := seg.Encode(w)

For multi-track segments, the code is a bit more involved. Please have a look at examples/segmenter to see how it is done. A more optimal way of handling media sample is to handle them lazily, as explained next.

Lazy decoding and writing of mdat data

For video and audio, the dominating part of a mp4 file is the media data which is stored in one or more mdat boxes. In some cases, for example when segmenting large progressive files, it is much more memory efficient to just read the movie or fragment data from the moov or moof box and defer the reading of the media data from the mdat box to later.

For decoding, this is supported by running mp4.DecodeFile() in lazy mode as

parsedMp4, err = mp4.DecodeFile(ifd, mp4.WithDecodeMode(mp4.DecModeLazyMdat))

In this case, the media data of the mdat box will not be read, but only its size is being set. To read or copy the actual data corresponding to a sample, one must calculate the corresponding byte range and either call

func (m *MdatBox) ReadData(start, size int64, rs io.ReadSeeker) ([]byte, error)


func (m *MdatBox) CopyData(start, size int64, rs io.ReadSeeker, w io.Writer) (nrWritten int64, err error)

Example code for this, including lazy writing of mdat, can be found in examples/segmenter with the lazy mode set.

More efficient I/O using SliceReader and SliceWriter

The use of the interfaces io.Reader and io.Writer for reading and writing boxes gives a lot of flexibility, but is not optimal when it comes to memory allocation. In particular, the Read(p []byte) method needs a slice p of the proper size to read data, which leads to a lot of allocations and copying of data. In order to achieve better performance, it is advantageous to read the full top level boxes into one, or a few, slices and decode these.

To enable that mode, version 0.27 of the code introduces DecodeX(sr bits.SliceReader) methods to every box X where mp4ff.bits.SliceReader is an interface. For example, the TrunBox gets the method DecodeTrunSR(sr bits.SliceReader) in addition to its old DecodeTrun(r io.Reader) method. The bits.SliceReader interface provides methods to read all kinds of data structures from an underlying slice of bytes. It has an implementation bits.FixedSliceReader which uses a fixed-size slice as underlying slice, but one could consider implementing a growing version which would get its data from some external source.

The memory allocation and speed improvements achieved by this may vary, but should be substantial, especially compared to versions before 0.27 which used an extra io.LimitReader layer.


To investigate the efficiency of the new SliceReader and SliceWriter methods, benchmarks have been done. The benchmarks are defined in the file mp4/benchmarks_test.go and mp4/benchmarks_srw_test.go. For DecodeFile, one can see a big improvement by going from version 0.26 to version 0.27 which both use the io.Reader interface but another big increase by using the SliceReader source. The latter benchmarks are called BenchmarkDecodeFileSR but have here been given the same name, for easy comparison. Note that the allocations here refers to the heap allocations that are done inside the benchmark loop. Outside that loop, a slice is allocated to keep the input data.

For EncodeFile, one can see that v0.27 is actually worse than v0.26 when used with the io.Writer interface. That is because the code was restructured so that all writes go via the SliceWriter layer in order to reduce code duplication. However, if instead using the SliceWriter methods directly, there is a big relative gain in allocations as can be seen in the last column.

name \ time/op v0.26 v0.27 v0.27-srw
DecodeFile/1.m4s-16 21.9µs 6.7µs 2.6µs
DecodeFile/prog_8s.mp4-16 143µs 48µs 16µs
EncodeFile/1.m4s-16 1.70µs 2.14µs 1.50µs
EncodeFile/prog_8s.mp4-16 15.7µs 18.4µs 12.9µs
name \ alloc/op v0.26 v0.27 v0.27-srw
DecodeFile/1.m4s-16 120kB 28kB 2kB
DecodeFile/prog_8s.mp4-16 906kB 207kB 12kB
EncodeFile/1.m4s-16 1.16kB 1.39kB 0.08kB
EncodeFile/prog_8s.mp4-16 6.84kB 8.30kB 0.05kB
name \ allocs/op v0.26 v0.27 v0.27-srw
DecodeFile/1.m4s-16 98.0 42.0 34.0
DecodeFile/prog_8s.mp4-16 454 180 169
EncodeFile/1.m4s-16 15.0 15.0 3.0
EncodeFile/prog_8s.mp4-16 101 86 1

Box structure and interface

Most boxes have their own file named after the box, but in some cases, there may be multiple boxes that have the same content, and the code file then has a generic name like mp4/visualsampleentry.go.

The Box interface is specified in mp4/box.go. It does not contain decode (parsing) methods which have distinct names for each box type and are dispatched,

The mapping for decoding dispatch is given in the table mp4.decoders for the io.Reader methods and in mp4.decodersSR for the mp4ff.bits.SliceReader methods.

How to implement a new box

To implement a new box fooo, the following is needed.

Create a file fooo.go and create a struct type FoooBox.

FoooBox must implement the Box interface methods:

Encode(w io.Writer)
EncodeSW(sw bits.SliceWriter)  // new in v0.27.0

It also needs its own decode method DecodeFooo, which must be added in the decoders map in box.go, and the new in v0.27.0 DecodeFoooSR method in decodersSR. For a simple example, look at the PrftBox in prft.go.

A test file fooo_test.go should also have a test using the method boxDiffAfterEncodeAndDecode to check that the box information is equal after encoding and decoding.

Direct changes of attributes

Many attributes are public and can therefore be changed in freely. The advantage of this is that it is possible to write code that can manipulate boxes in many different ways, but one must be cautious to avoid breaking links to sub boxes or create inconsistent states in the boxes.

As an example, container boxes such as TrafBox have a method AddChild which adds a box to Children, its slice of children boxes, but also sets a specific member reference such as Tfdt to point to that box. If Children is manipulated directly, that link may not be valid.

Encoding modes and optimizations

For fragmented files, one can choose to either encode all boxes in a mp4.File, or only code the ones which are included in the init and media segments. The attribute that controls that is called FragEncMode. Another attribute EncOptimize controls possible optimizations of the file encoding process. Currently, there is only one possible optimization called OptimizeTrun. It can reduce the size of the TrunBox by finding and writing default values in the TfhdBox and omitting the corresponding values from the TrunBox. Note that this may change the size of all ancestor boxes of trun.

Sample Number Offset

Following the ISOBMFF standard, sample numbers and other numbers start at 1 (one-based). This applies to arguments of functions and methods. The actual storage in slices is zero-based, so sample nr 1 has index 0 in the corresponding slice.


The APIs should be fairly stable, but minor non-backwards-compatible changes may happen until version 1.


The main specification for the MP4 file format is the ISO Base Media File Format (ISOBMFF) standard ISO/IEC 14496-12 6th edition 2020. Some boxes are specified in other standards, as should be commented in the code.


MIT, see LICENSE.md.

Some code in pkg/mp4, comes from or is based on https://github.com/jfbus/mp4 which has Copyright (c) 2015 Jean-François Bustarret.

Some code in pkg/bits comes from or is based on https://github.com/tcnksm/go-casper/tree/master/internal/bits Copyright (c) 2017 Taichi Nakashima.


See Versions.md.


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