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78 lines
2.4 KiB
Plaintext
78 lines
2.4 KiB
Plaintext
@chapter Bitstream Filters
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@c man begin BITSTREAM FILTERS
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When you configure your Libav build, all the supported bitstream
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filters are enabled by default. You can list all available ones using
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the configure option @code{--list-bsfs}.
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You can disable all the bitstream filters using the configure option
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@code{--disable-bsfs}, and selectively enable any bitstream filter using
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the option @code{--enable-bsf=BSF}, or you can disable a particular
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bitstream filter using the option @code{--disable-bsf=BSF}.
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The option @code{-bsfs} of the ff* tools will display the list of
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all the supported bitstream filters included in your build.
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Below is a description of the currently available bitstream filters.
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@section aac_adtstoasc
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@section chomp
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@section dump_extradata
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@section h264_mp4toannexb
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@section imx_dump_header
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@section mjpeg2jpeg
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Convert MJPEG/AVI1 packets to full JPEG/JFIF packets.
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MJPEG is a video codec wherein each video frame is essentially a
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JPEG image. The individual frames can be extracted without loss,
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e.g. by
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@example
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avconv -i ../some_mjpeg.avi -c:v copy frames_%d.jpg
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@end example
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Unfortunately, these chunks are incomplete JPEG images, because
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they lack the DHT segment required for decoding. Quoting from
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@url{http://www.digitalpreservation.gov/formats/fdd/fdd000063.shtml}:
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Avery Lee, writing in the rec.video.desktop newsgroup in 2001,
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commented that "MJPEG, or at least the MJPEG in AVIs having the
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MJPG fourcc, is restricted JPEG with a fixed -- and *omitted* --
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Huffman table. The JPEG must be YCbCr colorspace, it must be 4:2:2,
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and it must use basic Huffman encoding, not arithmetic or
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progressive. . . . You can indeed extract the MJPEG frames and
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decode them with a regular JPEG decoder, but you have to prepend
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the DHT segment to them, or else the decoder won't have any idea
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how to decompress the data. The exact table necessary is given in
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the OpenDML spec."
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This bitstream filter patches the header of frames extracted from an MJPEG
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stream (carrying the AVI1 header ID and lacking a DHT segment) to
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produce fully qualified JPEG images.
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@example
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avconv -i mjpeg-movie.avi -c:v copy -vbsf mjpeg2jpeg frame_%d.jpg
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exiftran -i -9 frame*.jpg
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avconv -i frame_%d.jpg -c:v copy rotated.avi
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@end example
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@section mjpega_dump_header
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@section movsub
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@section mp3_header_compress
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@section mp3_header_decompress
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@section noise
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@section remove_extradata
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@c man end BITSTREAM FILTERS
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