Check key-frame alignment with MP4Box

Dear adaptive streaming followers,

Many of you reported issues when packaging your encoded content to respectively HLS or MPEG-DASH using GPAC (resp. the MP42TS and MP4Box tools). Most of the time, this is due to misalignment of key-frames across the different encoded qualities. In this article, we are going to show how to check key-frame alignment.

A packager like MP4Box doesn’t re-encode your content

MP4Box does two things for you:

  1. Import: It understands your media to import it to the MP4 container. For example: ‘MP4Box -add video.h264 -add audio.aac av.mp4’
    When MP4Box doesn’t understand the format, you may want to specify manually this step with NHML.
  2. Manipulates: MP4Box manipulates the MP4 container (e.g. edit, fragment, cut, dash, encrypt, etc.). One key feature of the MP4 container is the ability to manipulate content without any knowledge about the content format. Theoretically it means that MP4Box can package some MPEG-DASH content even for a codec it wouldn’t know.

But in any case MP4Box does not re-encode the content. For that, please use an encoder (such as FFmpeg – see references at the end of the article). It is your responsibility, as the content editor, to feed MP4Box with some appropriate content at the encoder level.

If your content is not prepared correctly, MP4Box works on a best-effort basis and may (or may not) do its job. MP4Box may or may not print warnings. But some players (like dash.js for MPEG-DASH) may silently fail with the packaged content.

Command-line to get quick summary of key-frames intervals

You will get the average key-frame interval computed for the track with ID TRACK_ID:

Having different numbers for the average GOP length on different files mean that your GOP size differ and key-frames won’t be aligned across various qualities: you will have to re-encode. However there may be cases where the average GOP length is the same, but slight variations may occur resulting in misalignment when DASHing.

Command-line to get complete key-frames list and indexes to check alignment

You’ll get:

Then do it with another source file, and compare:

Additional resources for encoding properly

Some tutorials are available at:


Open positions in our lab

The GPAC team at Telecom ParisTech has two new open positions:

We also have an open PhD position on peer-to-peer streaming in web browsers, description available here.

If you would like to join us in our offices in Paris and participate in our open-source and R&D activities, don’t hesitate to contact us!


GPAC: build MP4Box only (all platforms)

Dear readers,

As many of you know, GPAC is a full framework providing authoring tools, packagers, streamers, a player and now some js stuff. This can make GPAC cumbersome to build.

However many of you are mainly interested in MP4Box which is easy to build. Whether you are on Linux, Windows or MacOS, here is the definite guide to build MP4Box easily :)

Edit 2015/11/03: added MSVC MP4Box minimal build.

Build MP4Box with Visual Studio

We published a minimal solution to build MP4Box.exe only, with zero dependency. No DLL to distribute.

You need Visual Studio 2010 or more recent (a pop-up will ask you to upgrade your projects, just accept). Open build/msvc10/gpac_mp4box_mini.sln and build (Debug or Release).

That’s all!

Build MP4Box with Make

On your system, you need the following tools installed on your system:

  • git
  • gcc
  • make
  • libpthread-dev (should be available on any development system)

In your favorite terminal, type:

To execute MP4Box, assuming you are still in the same directory (i.e. the root of the GPAC repository), you will find the MP4Box executable at:

To install MP4Box on your system:

Check installation

now when you type

you should see

which is the default install folder for the version we have just built. You can tweak this with the ‘-prefix=’ configure option.

If you see /usr/bin/MP4Box (no “local/” in here), uninstall GPAC from your local packager. On MacOS, go to Finder and unmount the app on the left panel, or uninstall it from your package manager (‘port’ or ‘homebrew’ or ‘fink’).

Update to a newer revision

Of course you want to keep updated with the latest build without having to download the full repository again or re-execute the configuration when not necessary (it may be necessary sometimes, see below “Clean your build” if things go unexpectedly):

That’s all!

Clean your build

If things go wrong and you suspect there is an issue in your configuration, the fastest way to restart a clean build is:


Cross-compiling GPAC is quite standard, and requires only a modification at the ‘configure’ step. Use ‘–extra-cflags=’ and ‘–extra-ldflags=’ to add your environmment flags:

For example, to use the ‘x86_64-w64-mingw32’ toolchain to cross-build from Linux to Windows:

You can find more examples in our TravisCI script.

Next Step

We have started a multimedia component-level build system called Zenbuild. Zenbuild builds FFmpeg/libav, VLC or GPAC with most of their features enabled (librtmp, jack, openHEVC, etc.). You can start using Zenbuild, it is fully operational!

Using GDB in Visual Studio

Dear followers,

One of the target platform for GPAC is Windows. Developing for Windows in a comfortable manner requires Visual Studio, in particular for debugging. Microsoft recently announced great changes that will make Visual Studio users (including the GPAC developers) happy:

Edit: update from Microsoft on 2015/11/18.

Setup for local debugging

Install software:

  • Install Visual Studio 2015. Install the “Tools for Visual C++ Mobile Development” in a sub-menu (disabled by default).
  • Install gdb/gdbserver. Use MinGW-w64 binaries and put them in your PATH (you can rely on the excellent MSYS2 for a full development environment).
    /!\ Legacy MinGW and Cygwin gdb binaries are known to fail /!\


  • Use your usual build system and gcc or clang compiler. Don’t forget to enable debugging!


  • In your command-line, launch:
  • In Visual Studio:
    • Open the “command window” (either press CTRL+ALT+A or click: View menu -> Other Windows -> Command Window)
    • Launch:

That’s all! Once Debug.GDBLaunch is attached your program will run. Breakpoints are effective. And you can enjoy the Visual Studio debugging experience:


Setup for remote debugging

This is an extension of the gdbserver we used above.  It allows to debug mobile (android/iOS) or any platform (linux, …) executables from a remote

For specific remote parameters, see:

GPAC scalable and hybrid demo, including French Tennis Open 2015

Dear readers,

For the last few years, the GPAC tools backed demos at the French Tennis Open (Roland Garros) in its demo corner called RG Labs. This year, through the H2B2VS project, we participated in a demonstration showing the use of Hybrid Broadcast Broadband delivery for Ultra HD video content.

The video was encoded using the SHVC format with two layers. The encoder was MPEG reference software. The base HEVC layer, corresponding to HD content, was delivered using MPEG-2 TS, produced by our MP42TS tool. The enhancement layer, enhancing the spatial resolution to UHD, was delivered using MPEG-DASH generated by MP4Box. Both streams were synchronized based on the recent MPEG-2 TS TEMI extensions (inserted by MP42TS in the mux) and played by MP4Client, rendered on this nice curved 4K TV. You can find the press release by France TV here.


The GPAC player is very powerful. It features a graphical statistics module able to display and control the player. Look at the screenshot below. It can also be used to choose your HLS or MPEG-DASH quality or limit the download bandwidth to simulate a saturated server environment.


During the first week of June we were also present at the MediaSync 2015 workshop for talk and demos, as well as invited in the demo session of TVX 2015. We presented some demonstrations developed during the H2B2VS project:

  • HD/UHD hybrid delivery, same demo as the one in RG labs, bug using Tears of Steel (left on the picture).
  • Sign language picture-in-picture service enhancement, with main content on broadcast and sign language video on broadband using MPEG-DASH (right on the picture).
  • Dynamic switch between broadcast and broadband versions of the same content, to allow trick modes in a broadcast (time shifting, rewind, fast forward).


The demonstrations were all using MP42TS for broadcast génération, MP4Box for live or onDemand DASH packaging and MP4Client for the playback. Complete instructions for reproducing the test bed are available here.

Use MP4Box to dump and analyze broadcast TS streams

Today I want to share with you a few tips regarding MP4Box. MP4Box can manipulate MP4 files, but it has also capabilities regarding broadcast streams.

Dump a broadcast TS in a file

Command syntax:

For example:

Dump from a specific network interface

MP4Box (and also other GPAC tools such as the MP4Client player) provides a ‘-ifce’ option:

Use ‘ifconfig’ (Unix) or ‘ipconfig’ (Windows) to retrieve your IP address on the interface of your choice. Example:

Troubleshoot Windows: MP4Box (or any tool) cannot dump my multicast

1) Clicking the Start button and then click on Control Panel. In the search box, type firewall, and then click Windows Firewall.

2) On the left panel, click on “Allow a program or feature through the Windows Firewall”.

3) Make sure all columns are checked for your MP4Box programs.

Enjoy :)

Leaving SVN and SourceForge, using Git and GitHub

Dear followers,

In a previous post we mentionned that we had created an official account on GitHub but at that time the migration was not complete. We still used internally the SVN repository hosted on SourceForge and in particular for our BuildBot to generate the Nightly Builds. The git repository was synced with the SVN repository.

Since last week, we have migrated all our tools to use the git repository hosted on GitHub as the official source code of GPAC. The SVN repository at SourceForge will not be maintained anymore. In fact, recent commits have already been made to GitHub which are not present on the SVN.

As part of that migration, we have changed the numbering of GPAC’s binaries. New builds now use a numbering in the form of 0.5.2-DEV-<number-of-commits-since-last-release>-<latest-git-commit-hash>-<git-branch>.

Note also that GPAC is now also automatically built on TravisCI when a commit is pushed to the repository or when a pull request is made. We hope this move to git and GitHub will help improving the code. So don’t hesitate to fork, fix and make pull requests.

The GPAC team.

2015: meet us at FOMS and FOSDEM

Dear followers,

We wish you a happy new year from the whole GPAC team!

GPAC people are attending FOSDEM this year. FOSDEM is a conference which allows “open source communities a place to meet, share ideas and collaborate”. It takes place every year in Brussels, Belgium. It happens that FOMS also occurs at the same time and city. So both organizing teams agreed to make a joint track called “Open Media”.

Cyril Concolato and Romain Bouqueau will give a talk there about the latest multimedia technologies and GPAC. More information at:

If you’re interested and have questions, meet us or ask questions on our forums.

GPAC is now on GitHub

Dear readers,

GPAC is now officially available on GitHub, where you can find:

Please contribute on GitHub as soon as… now!

Of course this is only the beginning. We’d be happy to have your opinion on these changes. The more GitHub requests, feedback and pull-requests we have, the more likely we’ll migrate completely to GitHub :)