FFmpeg Documentation

Table of Contents

 


 

FFmpeg Documentation

1. Introduction

FFmpeg is a very fast video and audio converter. It can also grab from a live audio/video source.

The command line interface is designed to be intuitive, in the sense that FFmpeg tries to figure out all parameters that can possibly be derived automatically. You usually only have to specify the target bitrate you want.

FFmpeg can also convert from any sample rate to any other, and resize video on the fly with a high quality polyphase filter.

2. Quick Start

2.1 Video and Audio grabbing

FFmpeg can grab video and audio from devices given that you specify the input format and device.

ffmpeg -f oss -i /dev/dsp -f video4linux2 -i /dev/video0 /tmp/out.mpg

Note that you must activate the right video source and channel before launching FFmpeg with any TV viewer such as xawtv (http://linux.bytesex.org/xawtv/) by Gerd Knorr. You also have to set the audio recording levels correctly with a standard mixer.

2.2 X11 grabbing

FFmpeg can grab the X11 display.

ffmpeg -f x11grab -s cif -i :0.0 /tmp/out.mpg

0.0 is display.screen number of your X11 server, same as the DISPLAY environment variable.

ffmpeg -f x11grab -s cif -i :0.0+10,20 /tmp/out.mpg

0.0 is display.screen number of your X11 server, same as the DISPLAY environment variable. 10 is the x-offset and 20 the y-offset for the grabbing.

2.3 Video and Audio file format conversion

* FFmpeg can use any supported file format and protocol as input:

Examples:

* You can use YUV files as input:

ffmpeg -i /tmp/test%d.Y /tmp/out.mpg

It will use the files:

/tmp/test0.Y, /tmp/test0.U, /tmp/test0.V,
/tmp/test1.Y, /tmp/test1.U, /tmp/test1.V, etc...

The Y files use twice the resolution of the U and V files. They are raw files, without header. They can be generated by all decent video decoders. You must specify the size of the image with the `-s' option if FFmpeg cannot guess it.

* You can input from a raw YUV420P file:

ffmpeg -i /tmp/test.yuv /tmp/out.avi

test.yuv is a file containing raw YUV planar data. Each frame is composed of the Y plane followed by the U and V planes at half vertical and horizontal resolution.

* You can output to a raw YUV420P file:

ffmpeg -i mydivx.avi hugefile.yuv

* You can set several input files and output files:

ffmpeg -i /tmp/a.wav -s 640x480 -i /tmp/a.yuv /tmp/a.mpg

Converts the audio file a.wav and the raw YUV video file a.yuv to MPEG file a.mpg.

* You can also do audio and video conversions at the same time:

ffmpeg -i /tmp/a.wav -ar 22050 /tmp/a.mp2

Converts a.wav to MPEG audio at 22050 Hz sample rate.

* You can encode to several formats at the same time and define a mapping from input stream to output streams:

ffmpeg -i /tmp/a.wav -ab 64k /tmp/a.mp2 -ab 128k /tmp/b.mp2 -map 0:0 -map 0:0

Converts a.wav to a.mp2 at 64 kbits and to b.mp2 at 128 kbits. '-map file:index' specifies which input stream is used for each output stream, in the order of the definition of output streams.

* You can transcode decrypted VOBs:

ffmpeg -i snatch_1.vob -f avi -vcodec mpeg4 -b 800k -g 300 -bf 2 -acodec libmp3lame -ab 128k snatch.avi

This is a typical DVD ripping example; the input is a VOB file, the output an AVI file with MPEG-4 video and MP3 audio. Note that in this command we use B-frames so the MPEG-4 stream is DivX5 compatible, and GOP size is 300 which means one intra frame every 10 seconds for 29.97fps input video. Furthermore, the audio stream is MP3-encoded so you need to enable LAME support by passing --enable-libmp3lame to configure. The mapping is particularly useful for DVD transcoding to get the desired audio language.

NOTE: To see the supported input formats, use ffmpeg -formats.

* You can extract images from a video, or create a video from many images:

For extracting images from a video:

ffmpeg -i foo.avi -r 1 -s WxH -f image2 foo-%03d.jpeg

This will extract one video frame per second from the video and will output them in files named `foo-001.jpeg', `foo-002.jpeg', etc. Images will be rescaled to fit the new WxH values.

If you want to extract just a limited number of frames, you can use the above command in combination with the -vframes or -t option, or in combination with -ss to start extracting from a certain point in time.

For creating a video from many images:

ffmpeg -f image2 -i foo-%03d.jpeg -r 12 -s WxH foo.avi

The syntax foo-%03d.jpeg specifies to use a decimal number composed of three digits padded with zeroes to express the sequence number. It is the same syntax supported by the C printf function, but only formats accepting a normal integer are suitable.

* You can put many streams of the same type in the output:

ffmpeg -i test1.avi -i test2.avi -vcodec copy -acodec copy -vcodec copy -acodec copy test12.avi -newvideo -newaudio

In addition to the first video and audio streams, the resulting output file `test12.avi' will contain the second video and the second audio stream found in the input streams list.

The -newvideo, -newaudio and -newsubtitle options have to be specified immediately after the name of the output file to which you want to add them.

3. Invocation

3.1 Syntax

The generic syntax is:

ffmpeg [[infile options][`-i' infile]]... {[outfile options] outfile}...

As a general rule, options are applied to the next specified file. Therefore, order is important, and you can have the same option on the command line multiple times. Each occurrence is then applied to the next input or output file.

* To set the video bitrate of the output file to 64kbit/s:

ffmpeg -i input.avi -b 64k output.avi

* To force the frame rate of the output file to 24 fps:

ffmpeg -i input.avi -r 24 output.avi

* To force the frame rate of the input file (valid for raw formats only) to 1 fps and the frame rate of the output file to 24 fps:

ffmpeg -r 1 -i input.m2v -r 24 output.avi

The format option may be needed for raw input files.

By default, FFmpeg tries to convert as losslessly as possible: It uses the same audio and video parameters for the outputs as the one specified for the inputs.

3.2 Main options

`-L'
Show license.
`-h, -?, -help, --help'
Show help.
`-version'
Show version.
`-formats'
Show available formats. The fields preceding the format names have the following meanings:
`D'
Decoding available
`E'
Encoding available
`-codecs'
Show available codecs. The fields preceding the codec names have the following meanings:
`D'
Decoding available
`E'
Encoding available
`V/A/S'
Video/audio/subtitle codec
`S'
Codec supports slices
`D'
Codec supports direct rendering
`T'
Codec can handle input truncated at random locations instead of only at frame boundaries
`-bsfs'
Show available bitstream filters.
`-protocols'
Show available protocols.
`-filters'
Show available libavfilter filters.
`-pix_fmts'
Show available pixel formats.
`-loglevel loglevel'
Set the logging level used by the library. loglevel is a number or a string containing one of the following values:
`quiet'
 
`panic'
 
`fatal'
 
`error'
 
`warning'
 
`info'
 
`verbose'
 
`debug'
 
`-f fmt'
Force format.
`-i filename'
input file name
`-y'
Overwrite output files.
`-t duration'
Restrict the transcoded/captured video sequence to the duration specified in seconds. hh:mm:ss[.xxx] syntax is also supported.
`-fs limit_size'
Set the file size limit.
`-ss position'
Seek to given time position in seconds. hh:mm:ss[.xxx] syntax is also supported.
`-itsoffset offset'
Set the input time offset in seconds. [-]hh:mm:ss[.xxx] syntax is also supported. This option affects all the input files that follow it. The offset is added to the timestamps of the input files. Specifying a positive offset means that the corresponding streams are delayed by 'offset' seconds.
`-timestamp time'
Set the timestamp.
`-metadata key=value'
Set a metadata key/value pair. For example, for setting the title in the output file:
		ffmpeg -i in.avi -metadata title="my title" out.flv
`-v number'
Set the logging verbosity level.
`-target type'
Specify target file type ("vcd", "svcd", "dvd", "dv", "dv50", "pal-vcd", "ntsc-svcd", ... ). All the format options (bitrate, codecs, buffer sizes) are then set automatically. You can just type:
		ffmpeg -i myfile.avi -target vcd /tmp/vcd.mpg

Nevertheless you can specify additional options as long as you know they do not conflict with the standard, as in:

		ffmpeg -i myfile.avi -target vcd -bf 2 /tmp/vcd.mpg
`-dframes number'
Set the number of data frames to record.
`-scodec codec'
Force subtitle codec ('copy' to copy stream).
`-newsubtitle'
Add a new subtitle stream to the current output stream.
`-slang code'
Set the ISO 639 language code (3 letters) of the current subtitle stream.

3.3 Video Options

`-b bitrate'
Set the video bitrate in bit/s (default = 200 kb/s).
`-vframes number'
Set the number of video frames to record.
`-r fps'
Set frame rate (Hz value, fraction or abbreviation), (default = 25).
`-s size'
Set frame size. The format is `wxh' (ffserver default = 160x128, ffmpeg default = same as source). The following abbreviations are recognized:
`sqcif'
128x96
`qcif'
176x144
`cif'
352x288
`4cif'
704x576
`16cif'
1408x1152
`qqvga'
160x120
`qvga'
320x240
`vga'
640x480
`svga'
800x600
`xga'
1024x768
`uxga'
1600x1200
`qxga'
2048x1536
`sxga'
1280x1024
`qsxga'
2560x2048
`hsxga'
5120x4096
`wvga'
852x480
`wxga'
1366x768
`wsxga'
1600x1024
`wuxga'
1920x1200
`woxga'
2560x1600
`wqsxga'
3200x2048
`wquxga'
3840x2400
`whsxga'
6400x4096
`whuxga'
7680x4800
`cga'
320x200
`ega'
640x350
`hd480'
852x480
`hd720'
1280x720
`hd1080'
1920x1080
`-aspect aspect'
Set aspect ratio (4:3, 16:9 or 1.3333, 1.7777).
`-croptop size'
Set top crop band size (in pixels).
`-cropbottom size'
Set bottom crop band size (in pixels).
`-cropleft size'
Set left crop band size (in pixels).
`-cropright size'
Set right crop band size (in pixels).
`-padtop size'
Set top pad band size (in pixels).
`-padbottom size'
Set bottom pad band size (in pixels).
`-padleft size'
Set left pad band size (in pixels).
`-padright size'
Set right pad band size (in pixels).
`-padcolor hex_color'
Set color of padded bands. The value for padcolor is expressed as a six digit hexadecimal number where the first two digits represent red, the middle two digits green and last two digits blue (default = 000000 (black)).
`-vn'
Disable video recording.
`-bt tolerance'
Set video bitrate tolerance (in bits, default 4000k). Has a minimum value of: (target_bitrate/target_framerate). In 1-pass mode, bitrate tolerance specifies how far ratecontrol is willing to deviate from the target average bitrate value. This is not related to min/max bitrate. Lowering tolerance too much has an adverse effect on quality.
`-maxrate bitrate'
Set max video bitrate (in bit/s). Requires -bufsize to be set.
`-minrate bitrate'
Set min video bitrate (in bit/s). Most useful in setting up a CBR encode:
		ffmpeg -i myfile.avi -b 4000k -minrate 4000k -maxrate 4000k -bufsize 1835k out.m2v

It is of little use elsewise.

`-bufsize size'
Set video buffer verifier buffer size (in bits).
`-vcodec codec'
Force video codec to codec. Use the copy special value to tell that the raw codec data must be copied as is.
`-sameq'
Use same video quality as source (implies VBR).
`-pass n'
Select the pass number (1 or 2). It is used to do two-pass video encoding. The statistics of the video are recorded in the first pass into a log file (see also the option -passlogfile), and in the second pass that log file is used to generate the video at the exact requested bitrate. On pass 1, you may just deactivate audio and set output to null, examples for Windows and Unix:
		ffmpeg -i foo.mov -vcodec libxvid -pass 1 -an -f rawvideo -y NUL
ffmpeg -i foo.mov -vcodec libxvid -pass 1 -an -f rawvideo -y /dev/null
`-passlogfile prefix'
Set two-pass log file name prefix to prefix, the default file name prefix is "ffmpeg2pass". The complete file name will be `PREFIX-N.log', where N is a number specific to the output stream.
`-newvideo'
Add a new video stream to the current output stream.
`-vlang code'
Set the ISO 639 language code (3 letters) of the current video stream.

3.4 Advanced Video Options

`-pix_fmt format'
Set pixel format. Use 'list' as parameter to show all the supported pixel formats.
`-sws_flags flags'
Set SwScaler flags (only available when compiled with swscale support).
`-g gop_size'
Set the group of pictures size.
`-intra'
Use only intra frames.
`-vdt n'
Discard threshold.
`-qscale q'
Use fixed video quantizer scale (VBR).
`-qmin q'
minimum video quantizer scale (VBR)
`-qmax q'
maximum video quantizer scale (VBR)
`-qdiff q'
maximum difference between the quantizer scales (VBR)
`-qblur blur'
video quantizer scale blur (VBR) (range 0.0 - 1.0)
`-qcomp compression'
video quantizer scale compression (VBR) (default 0.5). Constant of ratecontrol equation. Recommended range for default rc_eq: 0.0-1.0
`-lmin lambda'
minimum video lagrange factor (VBR)
`-lmax lambda'
max video lagrange factor (VBR)
`-mblmin lambda'
minimum macroblock quantizer scale (VBR)
`-mblmax lambda'
maximum macroblock quantizer scale (VBR) These four options (lmin, lmax, mblmin, mblmax) use 'lambda' units, but you may use the QP2LAMBDA constant to easily convert from 'q' units:
		ffmpeg -i src.ext -lmax 21*QP2LAMBDA dst.ext
`-rc_init_cplx complexity'
initial complexity for single pass encoding
`-b_qfactor factor'
qp factor between P- and B-frames
`-i_qfactor factor'
qp factor between P- and I-frames
`-b_qoffset offset'
qp offset between P- and B-frames
`-i_qoffset offset'
qp offset between P- and I-frames
`-rc_eq equation'
Set rate control equation (@xref{FFmpeg formula evaluator}) (default = tex^qComp).
`-rc_override override'
rate control override for specific intervals
`-me_method method'
Set motion estimation method to method. Available methods are (from lowest to best quality):
`zero'
Try just the (0, 0) vector.
`phods'
 
`log'
 
`x1'
 
`hex'
 
`umh'
 
`epzs'
(default method)
`full'
exhaustive search (slow and marginally better than epzs)
`-dct_algo algo'
Set DCT algorithm to algo. Available values are:
`0'
FF_DCT_AUTO (default)
`1'
FF_DCT_FASTINT
`2'
FF_DCT_INT
`3'
FF_DCT_MMX
`4'
FF_DCT_MLIB
`5'
FF_DCT_ALTIVEC
`-idct_algo algo'
Set IDCT algorithm to algo. Available values are:
`0'
FF_IDCT_AUTO (default)
`1'
FF_IDCT_INT
`2'
FF_IDCT_SIMPLE
`3'
FF_IDCT_SIMPLEMMX
`4'
FF_IDCT_LIBMPEG2MMX
`5'
FF_IDCT_PS2
`6'
FF_IDCT_MLIB
`7'
FF_IDCT_ARM
`8'
FF_IDCT_ALTIVEC
`9'
FF_IDCT_SH4
`10'
FF_IDCT_SIMPLEARM
`-er n'
Set error resilience to n.
`1'
FF_ER_CAREFUL (default)
`2'
FF_ER_COMPLIANT
`3'
FF_ER_AGGRESSIVE
`4'
FF_ER_VERY_AGGRESSIVE
`-ec bit_mask'
Set error concealment to bit_mask. bit_mask is a bit mask of the following values:
`1'
FF_EC_GUESS_MVS (default = enabled)
`2'
FF_EC_DEBLOCK (default = enabled)
`-bf frames'
Use 'frames' B-frames (supported for MPEG-1, MPEG-2 and MPEG-4).
`-mbd mode'
macroblock decision
`0'
FF_MB_DECISION_SIMPLE: Use mb_cmp (cannot change it yet in FFmpeg).
`1'
FF_MB_DECISION_BITS: Choose the one which needs the fewest bits.
`2'
FF_MB_DECISION_RD: rate distortion
`-4mv'
Use four motion vector by macroblock (MPEG-4 only).
`-part'
Use data partitioning (MPEG-4 only).
`-bug param'
Work around encoder bugs that are not auto-detected.
`-strict strictness'
How strictly to follow the standards.
`-aic'
Enable Advanced intra coding (h263+).
`-umv'
Enable Unlimited Motion Vector (h263+)
`-deinterlace'
Deinterlace pictures.
`-ilme'
Force interlacing support in encoder (MPEG-2 and MPEG-4 only). Use this option if your input file is interlaced and you want to keep the interlaced format for minimum losses. The alternative is to deinterlace the input stream with `-deinterlace', but deinterlacing introduces losses.
`-psnr'
Calculate PSNR of compressed frames.
`-vstats'
Dump video coding statistics to `vstats_HHMMSS.log'.
`-vstats_file file'
Dump video coding statistics to file.
`-top n'
top=1/bottom=0/auto=-1 field first
`-dc precision'
Intra_dc_precision.
`-vtag fourcc/tag'
Force video tag/fourcc.
`-qphist'
Show QP histogram.
`-vbsf bitstream_filter'
Bitstream filters available are "dump_extra", "remove_extra", "noise", "h264_mp4toannexb", "imxdump", "mjpegadump".
		ffmpeg -i h264.mp4 -vcodec copy -vbsf h264_mp4toannexb -an out.h264

3.5 Audio Options

`-aframes number'
Set the number of audio frames to record.
`-ar freq'
Set the audio sampling frequency (default = 44100 Hz).
`-ab bitrate'
Set the audio bitrate in bit/s (default = 64k).
`-aq q'
Set the audio quality (codec-specific, VBR).
`-ac channels'
Set the number of audio channels (default = 1).
`-an'
Disable audio recording.
`-acodec codec'
Force audio codec to codec. Use the copy special value to specify that the raw codec data must be copied as is.
`-newaudio'
Add a new audio track to the output file. If you want to specify parameters, do so before -newaudio (-acodec, -ab, etc..). Mapping will be done automatically, if the number of output streams is equal to the number of input streams, else it will pick the first one that matches. You can override the mapping using -map as usual. Example:
		ffmpeg -i file.mpg -vcodec copy -acodec ac3 -ab 384k test.mpg -acodec mp2 -ab 192k -newaudio
`-alang code'
Set the ISO 639 language code (3 letters) of the current audio stream.

3.6 Advanced Audio options:

`-atag fourcc/tag'
Force audio tag/fourcc.
`-absf bitstream_filter'
Bitstream filters available are "dump_extra", "remove_extra", "noise", "mp3comp", "mp3decomp".

3.7 Subtitle options:

`-scodec codec'
Force subtitle codec ('copy' to copy stream).
`-newsubtitle'
Add a new subtitle stream to the current output stream.
`-slang code'
Set the ISO 639 language code (3 letters) of the current subtitle stream.
`-sn'
Disable subtitle recording.
`-sbsf bitstream_filter'
Bitstream filters available are "mov2textsub", "text2movsub".
		ffmpeg -i file.mov -an -vn -sbsf mov2textsub -scodec copy -f rawvideo sub.txt

3.8 Audio/Video grab options

`-vc channel'
Set video grab channel (DV1394 only).
`-tvstd standard'
Set television standard (NTSC, PAL (SECAM)).
`-isync'
Synchronize read on input.

3.9 Advanced options

`-map input_stream_id[:sync_stream_id]'
Set stream mapping from input streams to output streams. Just enumerate the input streams in the order you want them in the output. sync_stream_id if specified sets the input stream to sync against.
`-map_meta_data outfile:infile'
Set meta data information of outfile from infile.
`-debug'
Print specific debug info.
`-benchmark'
Add timings for benchmarking.
`-dump'
Dump each input packet.
`-hex'
When dumping packets, also dump the payload.
`-bitexact'
Only use bit exact algorithms (for codec testing).
`-ps size'
Set RTP payload size in bytes.
`-re'
Read input at native frame rate. Mainly used to simulate a grab device.
`-loop_input'
Loop over the input stream. Currently it works only for image streams. This option is used for automatic FFserver testing.
`-loop_output number_of_times'
Repeatedly loop output for formats that support looping such as animated GIF (0 will loop the output infinitely).
`-threads count'
Thread count.
`-vsync parameter'
Video sync method. Video will be stretched/squeezed to match the timestamps, it is done by duplicating and dropping frames. With -map you can select from which stream the timestamps should be taken. You can leave either video or audio unchanged and sync the remaining stream(s) to the unchanged one.
`-async samples_per_second'
Audio sync method. "Stretches/squeezes" the audio stream to match the timestamps, the parameter is the maximum samples per second by which the audio is changed. -async 1 is a special case where only the start of the audio stream is corrected without any later correction.
`-copyts'
Copy timestamps from input to output.
`-shortest'
Finish encoding when the shortest input stream ends.
`-dts_delta_threshold'
Timestamp discontinuity delta threshold.
`-muxdelay seconds'
Set the maximum demux-decode delay.
`-muxpreload seconds'
Set the initial demux-decode delay.

3.10 Preset files

A preset file contains a sequence of option=value pairs, one for each line, specifying a sequence of options which would be awkward to specify on the command line. Lines starting with the hash ('#') character are ignored and are used to provide comments. Check the `ffpresets' directory in the FFmpeg source tree for examples.

Preset files are specified with the vpre, apre, spre, and fpre options. The fpre option takes the filename of the preset instead of a preset name as input and can be used for any kind of codec. For the vpre, apre, and spre options, the options specified in a preset file are applied to the currently selected codec of the same type as the preset option.

The argument passed to the vpre, apre, and spre preset options identifies the preset file to use according to the following rules:

First ffmpeg searches for a file named arg.ffpreset in the directories `$HOME/.ffmpeg', and in the datadir defined at configuration time (usually `PREFIX/share/ffmpeg') in that order. For example, if the argument is libx264-max, it will search for the file `libx264-max.ffpreset'.

If no such file is found, then ffmpeg will search for a file named codec_name-arg.ffpreset in the above-mentioned directories, where codec_name is the name of the codec to which the preset file options will be applied. For example, if you select the video codec with -vcodec libx264 and use -vpre max, then it will search for the file `libx264-max.ffpreset'.

@anchor{FFmpeg formula evaluator}

3.11 FFmpeg formula evaluator

When evaluating a rate control string, FFmpeg uses an internal formula evaluator.

The following binary operators are available: +, -, *, /, ^.

The following unary operators are available: +, -, (...).

The following statements are available: ld, st, while.

The following functions are available:

sinh(x)
 
cosh(x)
 
tanh(x)
 
sin(x)
 
cos(x)
 
tan(x)
 
atan(x)
 
asin(x)
 
acos(x)
 
exp(x)
 
log(x)
 
abs(x)
 
squish(x)
 
gauss(x)
 
mod(x, y)
 
max(x, y)
 
min(x, y)
 
eq(x, y)
 
gte(x, y)
 
gt(x, y)
 
lte(x, y)
 
lt(x, y)
 
bits2qp(bits)
 
qp2bits(qp)
 

The following constants are available:

PI
 
E
 
iTex
 
pTex
 
tex
 
mv
 
fCode
 
iCount
 
mcVar
 
var
 
isI
 
isP
 
isB
 
avgQP
 
qComp
 
avgIITex
 
avgPITex
 
avgPPTex
 
avgBPTex
 
avgTex
 

3.12 Protocols

The file name can be `-' to read from standard input or to write to standard output.

FFmpeg also handles many protocols specified with an URL syntax.

Use 'ffmpeg -protocols' to see a list of the supported protocols.

The protocol http: is currently used only to communicate with FFserver (see the FFserver documentation). When FFmpeg will be a video player it will also be used for streaming :-)

4. Tips

  • For streaming at very low bitrate application, use a low frame rate and a small GOP size. This is especially true for RealVideo where the Linux player does not seem to be very fast, so it can miss frames. An example is:
    		ffmpeg -g 3 -r 3 -t 10 -b 50k -s qcif -f rv10 /tmp/b.rm
    
  • The parameter 'q' which is displayed while encoding is the current quantizer. The value 1 indicates that a very good quality could be achieved. The value 31 indicates the worst quality. If q=31 appears too often, it means that the encoder cannot compress enough to meet your bitrate. You must either increase the bitrate, decrease the frame rate or decrease the frame size.
  • If your computer is not fast enough, you can speed up the compression at the expense of the compression ratio. You can use '-me zero' to speed up motion estimation, and '-intra' to disable motion estimation completely (you have only I-frames, which means it is about as good as JPEG compression).
  • To have very low audio bitrates, reduce the sampling frequency (down to 22050 Hz for MPEG audio, 22050 or 11025 for AC-3).
  • To have a constant quality (but a variable bitrate), use the option '-qscale n' when 'n' is between 1 (excellent quality) and 31 (worst quality).
  • When converting video files, you can use the '-sameq' option which uses the same quality factor in the encoder as in the decoder. It allows almost lossless encoding.
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