Difference between pages "Install/Partitioning" and "FLOP:FFmpeg"

< Install(Difference between pages)
(Old-School (BIOS/MBR) Method)
 
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<noinclude>
+
{{FLOP
{{InstallPart|the process of partitioning and filesystem creation}}
+
|Created on=2015/01/31
</noinclude>
+
|Summary=Funtoo Linux prefers FFmpeg. Some enlightenment about our choice and why we prefer this  or could switch to alternative in future.
=== Prepare Hard Disk ===
+
|Author=Oleg, Mgorny
 +
|Maintainer=Oleg, Mgorny
 +
|Reference Bug=FL-844
 +
}}
 +
== Introduction ==
 +
FFmpeg and Libav are library sets for multimedia decoding (and more). Both libraries expose similar API and features.
  
==== Introduction ====
+
Both project have common origins and diverged only recently. The developers share the same bad coding practices causing permanent lack of API and ABI stability, therefore requiring frequent rebuilds of reverse dependencies.
  
In earlier times, there was only one way to boot a PC-compatible computer. All of our desktops and servers had a standard BIOS, all our hard drives used Master Boot Records, and were partitioned using the MBR partition scheme. And we liked it that way!
+
Worse than that, after the split projects use colliding SONAMEs for libraries with potentially different ABI. This means that after switching from one implementation to another, the reverse dependencies may become broken instantly (preserved-libs doesn't help) and need to be rebuilt ASAP.
  
Then, along came EFI and UEFI, which are new-style firmware designed to boot systems, along with GPT partition tables to support disks larger than 2.2TB. All of the sudden, we had a variety of options to boot Linux systems, turning what once was a one-method-fits-all approach into something a lot more complex.
+
Many packages for video decoding, are done via FFmpeg or Libav. Differences between FFmpeg and Libav can have a major impact on its behavior: the number of files it can decode, whether it decodes correctly, what video and audio filters are provided, network behavior, and more.
  
Let's take a moment to review the boot options available to you. This Install Guide uses, and recommends, the old-school method of BIOS booting and using an MBR. It works. There's nothing wrong with it. If your system disk is 2TB or smaller in size, it won't prevent you from using all of your disk's capacity, either.
+
== Current status ==
 +
=== Gentoo ===
 +
Gentoo supports both ffmpeg and libav, with a weak preference towards libav. The preference is caused by package order in virtual/ffmpeg — when no other circumstance affects the package choice, Portage will prefer libav. However, if ffmpeg is already installed or a package incompatible with libav is requested, Portage will use ffmpeg instead.
  
But, there are some situations where the old-school method isn't optimal. If you have a system disk >2TB in size, then MBR partitions won't allow you to access all your storage. So that's one reason. Another reason is that there are some so-called "PC" systems out there that don't support BIOS booting anymore, and force you to use UEFI to boot. So, out of compassion for people who fall into this predicament, this Install Guide documents UEFI booting too.
+
There are two major technical issues with this design:
 +
# there is no technically correct way of forcing rebuilds on ABI changes — subslot dependencies do not work with virtuals or || () deps,
 +
# there is no way of forcing rebuilds when switching from libav to ffmpeg, and the other way around.
  
Our recommendation is still to go old-school unless you have reason not to. We call this method the '''BIOS + GRUB (MBR)''' method. It's the traditional method of setting up a PC-compatible system to boot Linux.
+
=== Funtoo ===
 +
Funtoo supports only FFmpeg. It is forced by Funtoo version of virtual/ffmpeg. While this provides the ability to avoid the Gentoo issues, virtual still breaks ABI rebuilds.
  
If you need to use UEFI to boot, we recommend not using the MBR at all for booting, as some systems support this, but others don't. Instead, we recommend using UEFI to boot GRUB, which in turn will load Linux. We refer to this method as the '''UEFI + GRUB (GPT)''' method.
+
Decision made by Oleg, forced by #funtoo community and bugtracker reports.
  
And yes, there are even more methods, some of which are documented on the [[Boot Methods]] page. We used to recommend a '''BIOS + GRUB (GPT)''' method but it is not consistently supported across a wide variety of hardware.
+
== Future status ==
 +
=== Gentoo ===
 +
There is a planned Gentoo change which will eventually replace virtual/ffmpeg and explicit || () deps with 'libav' USE flag. The flag will be added to all packages that support both FFmpeg and libav. When the flag is enabled, the package will use libav; otherwise it will use FFmpeg. The choice of flag name is forced by the fact that USE=ffmpeg is already used as generic ffmpeg-or-libav flag.
  
'''The big question is -- which boot method should you use?''' Here's how to tell.
+
This change fixes both Gentoo issues:
 +
# USE-conditional dependencies allow subslot dependencies to force rebuilds on ABI changes,
 +
# provider change will force rebuild because of USE flag change.
  
;Principle 1 - Old School: If you can reliably boot System Rescue CD and it shows you an initial light blue menu, you are booting the CD using the BIOS, and it's likely that you can thus boot Funtoo Linux using the BIOS. So, go old-school and use BIOS booting, ''unless'' you have some reason to use UEFI, such as having a >2.2TB system disk. In that case, see Principle 2, as your system may also support UEFI booting.
+
The change may also eventually make it possible to install FFmpeg and libav side-by-side. Until then, the flag state would involve 'strong' preference of one implementation over the other, and user will have to change USE=libav as a global flag. '''Installing a package that supports only one of the two implementations will result in blocker that needs to be handled manually'''.
  
;Principle 2 - New School: If you can reliably boot System Rescue CD and it shows you an initial black and white menu -- congratulations, your system is configured to support UEFI booting. This means that you are ready to install Funtoo Linux to boot via UEFI. Your system may still support BIOS booting, but just be trying UEFI first. You can poke around in your BIOS boot configuration and play with this.
+
=== Funtoo ===
 
+
If Funtoo decides to keep supporting FFmpeg only, it only needs to mask libav in the profiles. Then dependencies on updated packages will unconditionally use FFmpeg. Eventually Funtoo will want to remove virtual/ffmpeg and depend on media-video/ffmpeg:0= directly in forked packages.
;What's the Big Difference between Old School and New School?: Here's the deal. If you go with old-school MBR partitions, your <code>/boot</code> partition will be an ext2 filesystem, and you'll use <code>fdisk</code> to create your MBR partitions. If you go with new-school GPT partitions and UEFI booting, your <code>/boot</code> partition will be a vfat filesystem, because this is what UEFI is able to read, and you will use <code>gdisk</code> to create your GPT partitions. And you'll install GRUB a bit differently. That's about all it comes down to, in case you were curious.
+
 
+
{{Note|'''Some motherboards may appear to support UEFI, but don't.''' Do your research. For example, the Award BIOS in my Gigabyte GA-990FXA-UD7 rev 1.1 has an option to enable UEFI boot for CD/DVD. '''This is not sufficient for enabling UEFI boot for hard drives and installing Funtoo Linux.''' UEFI must be supported for both removable media (so you can boot System Rescue CD using UEFI) as well as fixed media (so you can boot your new Funtoo Linux installation.) It turns out that later revisions of this board (rev 3.0) have a new BIOS that fully supports UEFI boot.  This may point to a third principle -- know thy hardware.}}
+
 
+
==== Old-School (BIOS/MBR) Method ====
+
 
+
{{Note|Use this method if you are booting using your BIOS, and if your System Rescue CD initial boot menu was light blue. If you're going to use the new-school method, [[#New-School (UEFI/GPT) Method|click here to jump down to UEFI/GPT.]]}}
+
 
+
===== Preparation =====
+
 
+
First, it's a good idea to make sure that you've found the correct hard disk to partition. Try this command and verify that <code>/dev/sda</code> is the disk that you want to partition:
+
 
+
<console>
+
# ##i##fdisk -l /dev/sda
+
 
+
Disk /dev/sda: 640.1 GB, 640135028736 bytes, 1250263728 sectors
+
Units = sectors of 1 * 512 = 512 bytes
+
Sector size (logical/physical): 512 bytes / 512 bytes
+
I/O size (minimum/optimal): 512 bytes / 512 bytes
+
Disk label type: gpt
+
 
+
 
+
#        Start          End    Size  Type            Name
+
1        2048  1250263694  596.2G  Linux filesyste Linux filesystem
+
</console>
+
 
+
Now, it's recommended that you erase any existing MBR or GPT partition tables on the disk, which could confuse the system's BIOS at boot time. We do this using <code>sgdisk</code>:
+
{{fancywarning|This will make any existing partitions inaccessible! You are '''strongly''' cautioned and advised to backup any critical data before proceeding.}}
+
 
+
<console>
+
# ##i##sgdisk --zap-all /dev/sda
+
 
+
Creating new GPT entries.
+
GPT data structures destroyed! You may now partition the disk using fdisk or
+
other utilities.
+
</console>
+
 
+
This output is also nothing to worry about, as the command still succeded:
+
 
+
<console>
+
***************************************************************
+
Found invalid GPT and valid MBR; converting MBR to GPT format
+
in memory.
+
***************************************************************
+
</console>
+
 
+
===== Partitioning =====
+
 
+
Now we will use <code>fdisk</code> to create the MBR partition table and partitions:
+
 
+
<console>
+
# ##i##fdisk /dev/sda
+
</console>
+
 
+
Within <code>fdisk</code>, follow these steps:
+
 
+
'''Empty the partition table''':
+
 
+
<console>
+
Command (m for help): ##i##o ↵
+
</console>
+
 
+
'''Create Partition 1''' (boot):
+
 
+
<console>
+
Command (m for help): ##i##n ↵
+
Partition type (default p): ##i##↵
+
Partition number (1-4, default 1): ##i##↵
+
First sector: ##i##↵
+
Last sector: ##i##+128M ↵
+
</console>
+
 
+
'''Create Partition 2''' (swap):
+
 
+
<console>
+
Command (m for help): ##i##n ↵
+
Partition type (default p): ##i##↵
+
Partition number (2-4, default 2): ##i##↵
+
First sector: ##i##↵
+
Last sector: ##i##+2G ↵
+
Command (m for help): ##i##t ↵
+
Partition number (1,2, default 2): ##i## ↵
+
Hex code (type L to list all codes): ##i##82 ↵
+
</console>
+
 
+
'''Create the root partition:'''
+
 
+
<console>
+
Command (m for help): ##i##n ↵
+
Partition type (default p): ##i##↵
+
Partition number (3,4, default 3): ##i##↵
+
First sector: ##i##↵
+
Last sector: ##i##↵
+
</console>
+
 
+
'''Verify the partition table:'''
+
 
+
<console>
+
Command (m for help): ##i##p
+
 
+
Disk /dev/sda: 298.1 GiB, 320072933376 bytes, 625142448 sectors
+
Units: sectors of 1 * 512 = 512 bytes
+
Sector size (logical/physical): 512 bytes / 512 bytes
+
I/O size (minimum/optimal): 512 bytes / 512 bytes
+
Disklabel type: dos
+
Disk identifier: 0x82abc9a6
+
 
+
Device    Boot    Start      End    Blocks  Id System
+
/dev/sda1          2048    264191    131072  83 Linux
+
/dev/sda2        264192  4458495  2097152  82 Linux swap / Solaris
+
/dev/sda3        4458496 625142447 310341976  83 Linux
+
</console>
+
 
+
'''Write the parition table to disk:'''
+
 
+
<console>
+
Command (m for help): ##i##w
+
</console>
+
 
+
Your new MBR partition table will now be written to your system disk.
+
 
+
{{Note|You're done with partitioning! Now, jump over to [[#Creating filesystems|Creating filesystems]].}}
+
 
+
==== New-School (UEFI/GPT) Method ====
+
 
+
{{Note|Use this method if you are booting using UEFI, and if your System Rescue CD initial boot menu was black and white. If it was light blue, this method will not work.}}
+
 
+
The <tt>gdisk</tt> commands to create a GPT partition table are as follows. Adapt sizes as necessary, although these defaults will work for most users. Start <code>gdisk</code>:
+
 
+
<console>
+
# ##i##gdisk
+
</console>
+
 
+
Within <tt>gdisk</tt>, follow these steps:
+
 
+
'''Create a new empty partition table''' (This ''will'' erase all data on the disk when saved):
+
 
+
<console>
+
Command: ##i##o ↵
+
This option deletes all partitions and creates a new protective MBR.
+
Proceed? (Y/N): ##i##y ↵
+
</console>
+
 
+
'''Create Partition 1''' (boot):
+
 
+
<console>
+
Command: ##i##n ↵
+
Partition Number: ##i##1 ↵
+
First sector: ##i##↵
+
Last sector: ##i##+500M ↵
+
Hex Code: ##i##↵
+
</console>
+
 
+
'''Create Partition 2''' (swap):
+
 
+
<console>
+
Command: ##i##n ↵
+
Partition Number: ##i##2 ↵
+
First sector: ##i##↵
+
Last sector: ##i##+4G ↵
+
Hex Code: ##i##8200 ↵
+
</console>
+
 
+
'''Create Partition 3''' (root):
+
 
+
<console>
+
Command: ##i##n ↵
+
Partition Number: ##i##3 ↵
+
First sector: ##i##↵
+
Last sector: ##i##↵##!i## (for rest of disk)
+
Hex Code: ##i##↵
+
</console>
+
 
+
Along the way, you can type "<tt>p</tt>" and hit Enter to view your current partition table. If you make a mistake, you can type "<tt>d</tt>" to delete an existing partition that you created. When you are satisfied with your partition setup, type "<tt>w</tt>" to write your configuration to disk:
+
 
+
'''Write Partition Table To Disk''':
+
 
+
<console>
+
Command: ##i##w ↵
+
Do you want to proceed? (Y/N): ##i##Y ↵
+
</console>
+
 
+
The partition table will now be written to disk and <tt>gdisk</tt> will close.
+
 
+
Now, your GPT/GUID partitions have been created, and will show up as the following ''block devices'' under Linux:
+
 
+
* <tt>/dev/sda1</tt>, which will be used to hold the <tt>/boot</tt> filesystem,
+
* <tt>/dev/sda2</tt>, which will be used for swap space, and
+
* <tt>/dev/sda3</tt>, which will hold your root filesystem.
+
 
+
==== Creating filesystems ====
+
 
+
{{Note|This section covers both BIOS ''and'' UEFI installs. Don't skip it!}}
+
 
+
Before your newly-created partitions can be used, the block devices need to be initialized with filesystem ''metadata''. This process is known as ''creating a filesystem'' on the block devices. After filesystems are created on the block devices, they can be mounted and used to store files.
+
 
+
Let's keep this simple. Are you using old-school MBR partitions? If so, let's create an ext2 filesystem on /dev/sda1:
+
 
+
<console>
+
# ##i##mkfs.ext2 /dev/sda1
+
</console>
+
 
+
If you're using new-school GPT partitions for UEFI, you'll want to create a vfat filesystem on /dev/sda1, because this is what UEFI is able to read:
+
 
+
<console>
+
# ##i##mkfs.vfat -F 32 /dev/sda1
+
</console>
+
 
+
Now, let's create a swap partition. This partition will be used as disk-based virtual memory for your Funtoo Linux system.
+
 
+
You will not create a filesystem on your swap partition, since it is not used to store files. But it is necessary to initialize it using the <code>mkswap</code> command. Then we'll run the <code>swapon</code> command to make your newly-initialized swap space immediately active within the live CD environment, in case it is needed during the rest of the install process:
+
 
+
<console>
+
# ##i##mkswap /dev/sda2
+
# ##i##swapon /dev/sda2
+
</console>
+
 
+
Now, we need to create a root filesystem. This is where Funtoo Linux will live. We generally recommend ext4 or XFS root filesystems. If you're not sure, choose ext4. Here's how to create a root ext4 filesystem:
+
 
+
<console>
+
# ##i##mkfs.ext4 /dev/sda3
+
</console>
+
 
+
...and here's how to create an XFS root filesystem, if you choose to use XFS:
+
 
+
<console>
+
# ##i##mkfs.xfs /dev/sda3
+
</console>
+
 
+
Your filesystems (and swap) have all now been initialized, so that that can be mounted (attached to your existing directory heirarchy) and used to store files. We are ready to begin installing Funtoo Linux on these brand-new filesystems.
+
 
+
{{fancywarning|1=
+
When deploying an OpenVZ host, please use ext4 exclusively. The Parallels development team tests extensively with ext4, and modern versions of <code>openvz-rhel6-stable</code> are '''not''' compatible with XFS, and you may experience kernel bugs.
+
}}
+
  
==== Mounting filesystems ====
+
If Funtoo decides to start supporting libav as an option, it may need to add USE="-libav" to profiles if Gentoo decides for libav default. Funtoo will want to progressively update forked packages to match Gentoo dependency specifications.
  
Mount the newly-created filesystems as follows, creating <code>/mnt/funtoo</code> as the installation mount point:
+
== Detailed information on FFmpeg and libav ==
 +
=== FFmpeg and Libav history ===
 +
In 2011, parts of the FFmpeg developers were unhappy about the FFmpeg leadership, and decided to take over. This didn't quite work out. Apparently Fabrice Bellard, original FFmpeg developer and owner of the ffmpeg.org domain name, decided not to hand over the domain name to the new maintainers. So they followed Plan B, and forked FFmpeg, resulting in Libav. Since then, Libav did its own development, and completely ignored whatever FFmpeg did. FFmpeg, on the other hand, started to merge literally everything Libav did.
  
<console>
+
The reason for the fork is most likely that the developers hate each other. While this formulation seems somewhat sloppy, it is most likely the truth. To this date, the #libav-devel IRC channel still has Michael Niedermayer (the FFmpeg maintainer since 2004 according to Wikipedia) on their ban list (similar misbehavior is exhibited by some FFmpeg developers). There is little to no cooperation between the two projects.
# ##i##mkdir /mnt/funtoo
+
# ##i##mount /dev/sda3 /mnt/funtoo
+
# ##i##mkdir /mnt/funtoo/boot
+
# ##i##mount /dev/sda1 /mnt/funtoo/boot
+
</console>
+
  
Optionally, if you have a separate filesystem for <code>/home</code> or anything else:
+
More about FFmpeg's history and the fork incident can be found on Wikipedia
  
<console>
+
=== Situation today ===
# ##i##mkdir /mnt/funtoo/home
+
FFmpeg has more features and slightly more active development than Libav, going by mailing list and commit volume. In particular, FFmpeg's features are a superset of Libav's features. This is because FFmpeg merges Libav's git master on a daily basis. Libav on the other hand seems to prefer to ignore FFmpeg development (with occasional cherry-picking of bug fixes and features).
# ##i##mount /dev/sda4 /mnt/funtoo/home
+
</console>
+
  
If you have <code>/tmp</code> or <code>/var/tmp</code> on a separate filesystem, be sure to change the permissions of the mount point to be globally-writeable after mounting, as follows:
+
Some Linux distributions, especially those that had Libav developers as FFmpeg package maintainers, replaced FFmpeg with Libav, while other distributions stick with FFmpeg. Application developers typically have to make sure their code works with both libraries. This can be trivial to hard, depending on the details. One larger problem is that the difference between the libraries makes it hard to keep up a consistent level of the user experience, since either library might silently or blatantly be not up to the task. It also encourages library users to implement some features themselves, rather than dealing with the library differences, or the question to which project to contribute.
  
<console>
+
FFmpeg and Libav developers also seem to have the tendency to ignore the damage their rivalry is causing. Apparently fighting out these issues on the users' backs is better than reconciling. This means everyone using these libraries either has to suffer from the differences, or reimplement functionality that is not the same between FFmpeg and Libav.
# ##i##chmod 1777 /mnt/funtoo/tmp
+
{{FLOPFooter}}
</console>
+

Revision as of 23:22, January 31, 2015

Created on
2015/01/31
Original Author(s)
{{#set:Author={{ #show: User:Oleg | ?Full name }}}}{{ #show: User:Oleg | ?Full name }},{{#set:Author={{ #show: User:Mgorny | ?Full name }}}}{{ #show: User:Mgorny | ?Full name }}
Current Maintainer(s)
Oleg Vinichenko,Michał Górny
Status
Reference Bug
FL-844

Funtoo Linux Optimization Proposal: FFmpeg

Funtoo Linux prefers FFmpeg. Some enlightenment about our choice and why we prefer this or could switch to alternative in future.

Introduction

FFmpeg and Libav are library sets for multimedia decoding (and more). Both libraries expose similar API and features.

Both project have common origins and diverged only recently. The developers share the same bad coding practices causing permanent lack of API and ABI stability, therefore requiring frequent rebuilds of reverse dependencies.

Worse than that, after the split projects use colliding SONAMEs for libraries with potentially different ABI. This means that after switching from one implementation to another, the reverse dependencies may become broken instantly (preserved-libs doesn't help) and need to be rebuilt ASAP.

Many packages for video decoding, are done via FFmpeg or Libav. Differences between FFmpeg and Libav can have a major impact on its behavior: the number of files it can decode, whether it decodes correctly, what video and audio filters are provided, network behavior, and more.

Current status

Gentoo

Gentoo supports both ffmpeg and libav, with a weak preference towards libav. The preference is caused by package order in virtual/ffmpeg — when no other circumstance affects the package choice, Portage will prefer libav. However, if ffmpeg is already installed or a package incompatible with libav is requested, Portage will use ffmpeg instead.

There are two major technical issues with this design:

  1. there is no technically correct way of forcing rebuilds on ABI changes — subslot dependencies do not work with virtuals or || () deps,
  2. there is no way of forcing rebuilds when switching from libav to ffmpeg, and the other way around.

Funtoo

Funtoo supports only FFmpeg. It is forced by Funtoo version of virtual/ffmpeg. While this provides the ability to avoid the Gentoo issues, virtual still breaks ABI rebuilds.

Decision made by Oleg, forced by #funtoo community and bugtracker reports.

Future status

Gentoo

There is a planned Gentoo change which will eventually replace virtual/ffmpeg and explicit || () deps with 'libav' USE flag. The flag will be added to all packages that support both FFmpeg and libav. When the flag is enabled, the package will use libav; otherwise it will use FFmpeg. The choice of flag name is forced by the fact that USE=ffmpeg is already used as generic ffmpeg-or-libav flag.

This change fixes both Gentoo issues:

  1. USE-conditional dependencies allow subslot dependencies to force rebuilds on ABI changes,
  2. provider change will force rebuild because of USE flag change.

The change may also eventually make it possible to install FFmpeg and libav side-by-side. Until then, the flag state would involve 'strong' preference of one implementation over the other, and user will have to change USE=libav as a global flag. Installing a package that supports only one of the two implementations will result in blocker that needs to be handled manually.

Funtoo

If Funtoo decides to keep supporting FFmpeg only, it only needs to mask libav in the profiles. Then dependencies on updated packages will unconditionally use FFmpeg. Eventually Funtoo will want to remove virtual/ffmpeg and depend on media-video/ffmpeg:0= directly in forked packages.

If Funtoo decides to start supporting libav as an option, it may need to add USE="-libav" to profiles if Gentoo decides for libav default. Funtoo will want to progressively update forked packages to match Gentoo dependency specifications.

Detailed information on FFmpeg and libav

FFmpeg and Libav history

In 2011, parts of the FFmpeg developers were unhappy about the FFmpeg leadership, and decided to take over. This didn't quite work out. Apparently Fabrice Bellard, original FFmpeg developer and owner of the ffmpeg.org domain name, decided not to hand over the domain name to the new maintainers. So they followed Plan B, and forked FFmpeg, resulting in Libav. Since then, Libav did its own development, and completely ignored whatever FFmpeg did. FFmpeg, on the other hand, started to merge literally everything Libav did.

The reason for the fork is most likely that the developers hate each other. While this formulation seems somewhat sloppy, it is most likely the truth. To this date, the #libav-devel IRC channel still has Michael Niedermayer (the FFmpeg maintainer since 2004 according to Wikipedia) on their ban list (similar misbehavior is exhibited by some FFmpeg developers). There is little to no cooperation between the two projects.

More about FFmpeg's history and the fork incident can be found on Wikipedia

Situation today

FFmpeg has more features and slightly more active development than Libav, going by mailing list and commit volume. In particular, FFmpeg's features are a superset of Libav's features. This is because FFmpeg merges Libav's git master on a daily basis. Libav on the other hand seems to prefer to ignore FFmpeg development (with occasional cherry-picking of bug fixes and features).

Some Linux distributions, especially those that had Libav developers as FFmpeg package maintainers, replaced FFmpeg with Libav, while other distributions stick with FFmpeg. Application developers typically have to make sure their code works with both libraries. This can be trivial to hard, depending on the details. One larger problem is that the difference between the libraries makes it hard to keep up a consistent level of the user experience, since either library might silently or blatantly be not up to the task. It also encourages library users to implement some features themselves, rather than dealing with the library differences, or the question to which project to contribute.

FFmpeg and Libav developers also seem to have the tendency to ignore the damage their rivalry is causing. Apparently fighting out these issues on the users' backs is better than reconciling. This means everyone using these libraries either has to suffer from the differences, or reimplement functionality that is not the same between FFmpeg and Libav.

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