Bitstream Filters
*****************

When you configure your Libav build, all the supported bitstream filters are enabled by default. You can list all available ones using the configure option --list-bsfs.

You can disable all the bitstream filters using the configure option --disable-bsfs, and selectively enable any bitstream filter using the option --enable-bsf=BSF, or you can disable a particular bitstream filter using the option --disable-bsf=BSF.

The option -bsfs of the av* tools will display the list of all the supported bitstream filters included in your build.

Below is a description of the currently available bitstream filters.

aac_adtstoasc
=============

chomp
=====

dump_extradata
==============

extract_extradata
=================

Extract the in-band extradata.

Certain codecs allow the long-term headers (e.g. MPEG-2 sequence headers, or H.264/HEVC (VPS/)SPS/PPS) to be transmitted either "in-band" (i.e. as a part of the bitstream containing the coded frames) or "out of band" (e.g. on the container level). This latter form is called "extradata" in Libav terminology.

This bitstream filter detects the in-band headers and makes them available as extradata.

remove
     When this option is enabled, the long-term headers are removed from the bitstream after extraction.

h264_mp4toannexb
================

imx_dump_header
===============

mjpeg2jpeg
==========

Convert MJPEG/AVI1 packets to full JPEG/JFIF packets.

MJPEG is a video codec wherein each video frame is essentially a JPEG image. The individual frames can be extracted without loss, e.g. by

     avconv -i ../some_mjpeg.avi -c:v copy frames_%d.jpg

Unfortunately, these chunks are incomplete JPEG images, because they lack the DHT segment required for decoding. Quoting from http://www.digitalpreservation.gov/formats/fdd/fdd000063.shtml:

Avery Lee, writing in the rec.video.desktop newsgroup in 2001, commented that "MJPEG, or at least the MJPEG in AVIs having the MJPG fourcc, is restricted JPEG with a fixed -- and *omitted* -- Huffman table. The JPEG must be YCbCr colorspace, it must be 4:2:2, and it must use basic Huffman encoding, not arithmetic or progressive. . . . You can indeed extract the MJPEG frames and decode them with a regular JPEG decoder, but you have to prepend the DHT segment to them, or else the decoder won't have any idea how to decompress the data. The exact table necessary is given in the OpenDML spec."

This bitstream filter patches the header of frames extracted from an MJPEG stream (carrying the AVI1 header ID and lacking a DHT segment) to produce fully qualified JPEG images.

     avconv -i mjpeg-movie.avi -c:v copy -bsf:v mjpeg2jpeg frame_%d.jpg
     exiftran -i -9 frame*.jpg
     avconv -i frame_%d.jpg -c:v copy rotated.avi

mjpega_dump_header
==================

movsub
======

mp3_header_compress
===================

mp3_header_decompress
=====================

noise
=====

remove_extradata
================