@chapter Encoders
@c man begin ENCODERS

Encoders are configured elements in FFmpeg which allow the encoding of
multimedia streams.

When you configure your FFmpeg build, all the supported native encoders
are enabled by default. Encoders requiring an external library must be enabled
manually via the corresponding @code{--enable-lib} option. You can list all
available encoders using the configure option @code{--list-encoders}.

You can disable all the encoders with the configure option
@code{--disable-encoders} and selectively enable / disable single encoders
with the options @code{--enable-encoder=@var{ENCODER}} /
@code{--disable-encoder=@var{ENCODER}}.

The option @code{-codecs} of the ff* tools will display the list of
enabled encoders.

@c man end ENCODERS

@chapter Audio Encoders
@c man begin AUDIO ENCODERS

A description of some of the currently available audio encoders
follows.

@section ac3 and ac3_fixed

AC-3 audio encoders.

These encoders implement part of ATSC A/52:2010 and ETSI TS 102 366, as well as
the undocumented RealAudio 3 (a.k.a. dnet).

The @var{ac3} encoder uses floating-point math, while the @var{ac3_fixed}
encoder only uses fixed-point integer math. This does not mean that one is
always faster, just that one or the other may be better suited to a
particular system. The floating-point encoder will generally produce better
quality audio for a given bitrate. The @var{ac3_fixed} encoder is not the
default codec for any of the output formats, so it must be specified explicitly
using the option @code{-acodec ac3_fixed} in order to use it.

@subsection AC-3 Metadata

The AC-3 metadata options are used to set parameters that describe the audio,
but in most cases do not affect the audio encoding itself. Some of the options
do directly affect or influence the decoding and playback of the resulting
bitstream, while others are just for informational purposes. A few of the
options will add bits to the output stream that could otherwise be used for
audio data, and will thus affect the quality of the output. Those will be
indicated accordingly with a note in the option list below.

These parameters are described in detail in several publicly-available
documents.
@itemize
@item @uref{http://www.atsc.org/cms/standards/a_52-2010.pdf,A/52:2010 - Digital Audio Compression (AC-3) (E-AC-3) Standard}
@item @uref{http://www.atsc.org/cms/standards/a_54a_with_corr_1.pdf,A/54 - Guide to the Use of the ATSC Digital Television Standard}
@item @uref{http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/18_Metadata.Guide.pdf,Dolby Metadata Guide}
@item @uref{http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/46_DDEncodingGuidelines.pdf,Dolby Digital Professional Encoding Guidelines}
@end itemize

@subsubsection Metadata Control Options

@table @option

@item -per_frame_metadata @var{boolean}
Allow Per-Frame Metadata. Specifies if the encoder should check for changing
metadata for each frame.
@table @option
@item 0
The metadata values set at initialization will be used for every frame in the
stream. (default)
@item 1
Metadata values can be changed before encoding each frame.
@end table

@end table

@subsubsection Downmix Levels

@table @option

@item -center_mixlev @var{level}
Center Mix Level. The amount of gain the decoder should apply to the center
channel when downmixing to stereo. This field will only be written to the
bitstream if a center channel is present. The value is specified as a scale
factor. There are 3 valid values:
@table @option
@item 0.707
Apply -3dB gain
@item 0.595
Apply -4.5dB gain (default)
@item 0.500
Apply -6dB gain
@end table

@item -surround_mixlev @var{level}
Surround Mix Level. The amount of gain the decoder should apply to the surround
channel(s) when downmixing to stereo. This field will only be written to the
bitstream if one or more surround channels are present. The value is specified
as a scale factor.  There are 3 valid values:
@table @option
@item 0.707
Apply -3dB gain
@item 0.500
Apply -6dB gain (default)
@item 0.000
Silence Surround Channel(s)
@end table

@end table

@subsubsection Audio Production Information
Audio Production Information is optional information describing the mixing
environment.  Either none or both of the fields are written to the bitstream.

@table @option

@item -mixing_level @var{number}
Mixing Level. Specifies peak sound pressure level (SPL) in the production
environment when the mix was mastered. Valid values are 80 to 111, or -1 for
unknown or not indicated. The default value is -1, but that value cannot be
used if the Audio Production Information is written to the bitstream. Therefore,
if the @code{room_type} option is not the default value, the @code{mixing_level}
option must not be -1.

@item -room_type @var{type}
Room Type. Describes the equalization used during the final mixing session at
the studio or on the dubbing stage. A large room is a dubbing stage with the
industry standard X-curve equalization; a small room has flat equalization.
This field will not be written to the bitstream if both the @code{mixing_level}
option and the @code{room_type} option have the default values.
@table @option
@item 0
@itemx notindicated
Not Indicated (default)
@item 1
@itemx large
Large Room
@item 2
@itemx small
Small Room
@end table

@end table

@subsubsection Other Metadata Options

@table @option

@item -copyright @var{boolean}
Copyright Indicator. Specifies whether a copyright exists for this audio.
@table @option
@item 0
@itemx off
No Copyright Exists (default)
@item 1
@itemx on
Copyright Exists
@end table

@item -dialnorm @var{value}
Dialogue Normalization. Indicates how far the average dialogue level of the
program is below digital 100% full scale (0 dBFS). This parameter determines a
level shift during audio reproduction that sets the average volume of the
dialogue to a preset level. The goal is to match volume level between program
sources. A value of -31dB will result in no volume level change, relative to
the source volume, during audio reproduction. Valid values are whole numbers in
the range -31 to -1, with -31 being the default.

@item -dsur_mode @var{mode}
Dolby Surround Mode. Specifies whether the stereo signal uses Dolby Surround
(Pro Logic). This field will only be written to the bitstream if the audio
stream is stereo. Using this option does @b{NOT} mean the encoder will actually
apply Dolby Surround processing.
@table @option
@item 0
@itemx notindicated
Not Indicated (default)
@item 1
@itemx off
Not Dolby Surround Encoded
@item 2
@itemx on
Dolby Surround Encoded
@end table

@item -original @var{boolean}
Original Bit Stream Indicator. Specifies whether this audio is from the
original source and not a copy.
@table @option
@item 0
@itemx off
Not Original Source
@item 1
@itemx on
Original Source (default)
@end table

@end table

@subsection Extended Bitstream Information
The extended bitstream options are part of the Alternate Bit Stream Syntax as
specified in Annex D of the A/52:2010 standard. It is grouped into 2 parts.
If any one parameter in a group is specified, all values in that group will be
written to the bitstream.  Default values are used for those that are written
but have not been specified.  If the mixing levels are written, the decoder
will use these values instead of the ones specified in the @code{center_mixlev}
and @code{surround_mixlev} options if it supports the Alternate Bit Stream
Syntax.

@subsubsection Extended Bitstream Information - Part 1

@table @option

@item -dmix_mode @var{mode}
Preferred Stereo Downmix Mode. Allows the user to select either Lt/Rt
(Dolby Surround) or Lo/Ro (normal stereo) as the preferred stereo downmix mode.
@table @option
@item 0
@itemx notindicated
Not Indicated (default)
@item 1
@itemx ltrt
Lt/Rt Downmix Preferred
@item 2
@itemx loro
Lo/Ro Downmix Preferred
@end table

@item -ltrt_cmixlev @var{level}
Lt/Rt Center Mix Level. The amount of gain the decoder should apply to the
center channel when downmixing to stereo in Lt/Rt mode.
@table @option
@item 1.414
Apply +3dB gain
@item 1.189
Apply +1.5dB gain
@item 1.000
Apply 0dB gain
@item 0.841
Apply -1.5dB gain
@item 0.707
Apply -3.0dB gain
@item 0.595
Apply -4.5dB gain (default)
@item 0.500
Apply -6.0dB gain
@item 0.000
Silence Center Channel
@end table

@item -ltrt_surmixlev @var{level}
Lt/Rt Surround Mix Level. The amount of gain the decoder should apply to the
surround channel(s) when downmixing to stereo in Lt/Rt mode.
@table @option
@item 0.841
Apply -1.5dB gain
@item 0.707
Apply -3.0dB gain
@item 0.595
Apply -4.5dB gain
@item 0.500
Apply -6.0dB gain (default)
@item 0.000
Silence Surround Channel(s)
@end table

@item -loro_cmixlev @var{level}
Lo/Ro Center Mix Level. The amount of gain the decoder should apply to the
center channel when downmixing to stereo in Lo/Ro mode.
@table @option
@item 1.414
Apply +3dB gain
@item 1.189
Apply +1.5dB gain
@item 1.000
Apply 0dB gain
@item 0.841
Apply -1.5dB gain
@item 0.707
Apply -3.0dB gain
@item 0.595
Apply -4.5dB gain (default)
@item 0.500
Apply -6.0dB gain
@item 0.000
Silence Center Channel
@end table

@item -loro_surmixlev @var{level}
Lo/Ro Surround Mix Level. The amount of gain the decoder should apply to the
surround channel(s) when downmixing to stereo in Lo/Ro mode.
@table @option
@item 0.841
Apply -1.5dB gain
@item 0.707
Apply -3.0dB gain
@item 0.595
Apply -4.5dB gain
@item 0.500
Apply -6.0dB gain (default)
@item 0.000
Silence Surround Channel(s)
@end table

@end table

@subsubsection Extended Bitstream Information - Part 2

@table @option

@item -dsurex_mode @var{mode}
Dolby Surround EX Mode. Indicates whether the stream uses Dolby Surround EX
(7.1 matrixed to 5.1). Using this option does @b{NOT} mean the encoder will actually
apply Dolby Surround EX processing.
@table @option
@item 0
@itemx notindicated
Not Indicated (default)
@item 1
@itemx on
Dolby Surround EX Off
@item 2
@itemx off
Dolby Surround EX On
@end table

@item -dheadphone_mode @var{mode}
Dolby Headphone Mode. Indicates whether the stream uses Dolby Headphone
encoding (multi-channel matrixed to 2.0 for use with headphones). Using this
option does @b{NOT} mean the encoder will actually apply Dolby Headphone
processing.
@table @option
@item 0
@itemx notindicated
Not Indicated (default)
@item 1
@itemx on
Dolby Headphone Off
@item 2
@itemx off
Dolby Headphone On
@end table

@item -ad_conv_type @var{type}
A/D Converter Type. Indicates whether the audio has passed through HDCD A/D
conversion.
@table @option
@item 0
@itemx standard
Standard A/D Converter (default)
@item 1
@itemx hdcd
HDCD A/D Converter
@end table

@end table

@subsection Other AC-3 Encoding Options

@table @option

@item -stereo_rematrixing @var{boolean}
Stereo Rematrixing. Enables/Disables use of rematrixing for stereo input. This
is an optional AC-3 feature that increases quality by selectively encoding
the left/right channels as mid/side. This option is enabled by default, and it
is highly recommended that it be left as enabled except for testing purposes.

@end table

@subsection Floating-Point-Only AC-3 Encoding Options

These options are only valid for the floating-point encoder and do not exist
for the fixed-point encoder due to the corresponding features not being
implemented in fixed-point.

@table @option

@item -channel_coupling @var{boolean}
Enables/Disables use of channel coupling, which is an optional AC-3 feature
that increases quality by combining high frequency information from multiple
channels into a single channel. The per-channel high frequency information is
sent with less accuracy in both the frequency and time domains. This allows
more bits to be used for lower frequencies while preserving enough information
to reconstruct the high frequencies. This option is enabled by default for the
floating-point encoder and should generally be left as enabled except for
testing purposes or to increase encoding speed.
@table @option
@item -1
@itemx auto
Selected by Encoder (default)
@item 0
@itemx off
Disable Channel Coupling
@item 1
@itemx on
Enable Channel Coupling
@end table

@item -cpl_start_band @var{number}
Coupling Start Band. Sets the channel coupling start band, from 1 to 15. If a
value higher than the bandwidth is used, it will be reduced to 1 less than the
coupling end band. If @var{auto} is used, the start band will be determined by
the encoder based on the bit rate, sample rate, and channel layout. This option
has no effect if channel coupling is disabled.
@table @option
@item -1
@itemx auto
Selected by Encoder (default)
@end table

@end table

@section libmp3lame

LAME (Lame Ain't an MP3 Encoder) MP3 encoder wrapper

Requires the presence of the libmp3lame headers and library during
configuration. You need to explicitly configure the build with
@code{--enable-libmp3lame}.

@subsection Option Mapping

The following options are supported by the libmp3lame wrapper,
the LAME-equivalent options follow the FFmpeg ones.

@multitable @columnfractions .2 .2
@item FFmpeg            @tab LAME
@item b                 @tab b
FFmpeg @code{b} option is expressed in bits/s, lame @code{bitrate}
in kilobits/s.
@item q                 @tab V
Quality setting for VBR.
@item compression_level @tab q
Algorithm quality. Valid options are integers from 0-9.
@item reservoir         @tab N.A.
Enable use of bit reservoir. LAME has this enabled by default.
@item joint_stereo      @tab -m j
Enables the the encoder to use (on a frame by frame basis) either L/R
stereo or mid/side stereo.
@end multitable

@c man end AUDIO ENCODERS

@chapter Video Encoders
@c man begin VIDEO ENCODERS

A description of some of the currently available video encoders
follows.

@section libtheora

Theora format supported through libtheora.

Requires the presence of the libtheora headers and library during
configuration. You need to explicitly configure the build with
@code{--enable-libtheora}.

@subsection Options

The following global options are mapped to internal libtheora options
which affect the quality and the bitrate of the encoded stream.

@table @option
@item b
Set the video bitrate, only works if the @code{qscale} flag in
@option{flags} is not enabled.

@item flags
Used to enable constant quality mode encoding through the
@option{qscale} flag, and to enable the @code{pass1} and @code{pass2}
modes.

@item g
Set the GOP size.

@item global_quality
Set the global quality in lambda units, only works if the
@code{qscale} flag in @option{flags} is enabled. The value is clipped
in the [0 - 10*@code{FF_QP2LAMBDA}] range, and then multiplied for 6.3
to get a value in the native libtheora range [0-63]. A higher value
corresponds to a higher quality.

For example, to set maximum constant quality encoding with
@command{ffmpeg}:
@example
ffmpeg -i INPUT -flags:v qscale -global_quality:v "10*QP2LAMBDA" -codec:v libtheora OUTPUT.ogg
@end example
@end table

@section libvpx

VP8 format supported through libvpx.

Requires the presence of the libvpx headers and library during configuration.
You need to explicitly configure the build with @code{--enable-libvpx}.

@subsection Options

Mapping from FFmpeg to libvpx options with conversion notes in parentheses.

@table @option

@item threads
g_threads

@item profile
g_profile

@item vb
rc_target_bitrate

@item g
kf_max_dist

@item keyint_min
kf_min_dist

@item qmin
rc_min_quantizer

@item qmax
rc_max_quantizer

@item bufsize, vb
rc_buf_sz
@code{(bufsize * 1000 / vb)}

rc_buf_optimal_sz
@code{(bufsize * 1000 / vb * 5 / 6)}

@item rc_init_occupancy, vb
rc_buf_initial_sz
@code{(rc_init_occupancy * 1000 / vb)}

@item rc_buffer_aggressivity
rc_undershoot_pct

@item skip_threshold
rc_dropframe_thresh

@item qcomp
rc_2pass_vbr_bias_pct

@item maxrate, vb
rc_2pass_vbr_maxsection_pct
@code{(maxrate * 100 / vb)}

@item minrate, vb
rc_2pass_vbr_minsection_pct
@code{(minrate * 100 / vb)}

@item minrate, maxrate, vb
@code{VPX_CBR}
@code{(minrate == maxrate == vb)}

@item crf
@code{VPX_CQ}, @code{VP8E_SET_CQ_LEVEL}

@item quality
@table @option
@item @var{best}
@code{VPX_DL_BEST_QUALITY}
@item @var{good}
@code{VPX_DL_GOOD_QUALITY}
@item @var{realtime}
@code{VPX_DL_REALTIME}
@end table

@item speed
@code{VP8E_SET_CPUUSED}

@item nr
@code{VP8E_SET_NOISE_SENSITIVITY}

@item mb_threshold
@code{VP8E_SET_STATIC_THRESHOLD}

@item slices
@code{VP8E_SET_TOKEN_PARTITIONS}

@item max-intra-rate
@code{VP8E_SET_MAX_INTRA_BITRATE_PCT}

@item force_key_frames
@code{VPX_EFLAG_FORCE_KF}

@item Alternate reference frame related
@table @option
@item vp8flags altref
@code{VP8E_SET_ENABLEAUTOALTREF}
@item @var{arnr_max_frames}
@code{VP8E_SET_ARNR_MAXFRAMES}
@item @var{arnr_type}
@code{VP8E_SET_ARNR_TYPE}
@item @var{arnr_strength}
@code{VP8E_SET_ARNR_STRENGTH}
@item @var{rc_lookahead}
g_lag_in_frames
@end table

@item vp8flags error_resilient
g_error_resilient

@end table

For more information about libvpx see:
@url{http://www.webmproject.org/}

@section libx264

x264 H.264/MPEG-4 AVC encoder wrapper

Requires the presence of the libx264 headers and library during
configuration. You need to explicitly configure the build with
@code{--enable-libx264}.

x264 supports an impressive number of features, including 8x8 and 4x4 adaptive
spatial transform, adaptive B-frame placement, CAVLC/CABAC entropy coding,
interlacing (MBAFF), lossless mode, psy optimizations for detail retention
(adaptive quantization, psy-RD, psy-trellis).

The FFmpeg wrapper provides a mapping for most of them using global options
that match those of the encoders and provides private options for the unique
encoder options. Additionally an expert override is provided to directly pass
a list of key=value tuples as accepted by x264_param_parse.

@subsection Option Mapping

The following options are supported by the x264 wrapper, the x264-equivalent
options follow the FFmpeg ones.

@multitable @columnfractions .2 .2
@item b                 @tab bitrate
FFmpeg @code{b} option is expressed in bits/s, x264 @code{bitrate} in kilobits/s.
@item bf                @tab bframes
Maximum number of B-frames.
@item g                 @tab keyint
Maximum GOP size.
@item qmin              @tab qpmin
@item qmax              @tab qpmax
@item qdiff             @tab qpstep
@item qblur             @tab qblur
@item qcomp             @tab qcomp
@item refs              @tab ref
@item sc_threshold      @tab scenecut
@item trellis           @tab trellis
@item nr                @tab nr
Noise reduction.
@item me_range          @tab merange
@item me_method         @tab me
@item subq              @tab subme
@item b_strategy        @tab b-adapt
@item keyint_min        @tab keyint-min
@item coder             @tab cabac
Set coder to @code{ac} to use CABAC.
@item cmp               @tab chroma-me
Set to @code{chroma} to use chroma motion estimation.
@item threads           @tab threads
@item thread_type       @tab sliced_threads
Set to @code{slice} to use sliced threading instead of frame threading.
@item flags -cgop       @tab open-gop
Set @code{-cgop} to use recovery points to close GOPs.
@item rc_init_occupancy @tab vbv-init
Initial buffer occupancy.
@end multitable

@subsection Private Options
@table @option
@item -preset @var{string}
Set the encoding preset (cf. x264 --fullhelp).
@item -tune @var{string}
Tune the encoding params (cf. x264 --fullhelp).
@item -profile @var{string}
Set profile restrictions (cf. x264 --fullhelp).
@item -fastfirstpass @var{integer}
Use fast settings when encoding first pass.
@item -crf @var{float}
Select the quality for constant quality mode.
@item -crf_max @var{float}
In CRF mode, prevents VBV from lowering quality beyond this point.
@item -qp @var{integer}
Constant quantization parameter rate control method.
@item -aq-mode @var{integer}
AQ method

Possible values:
@table @samp
@item none

@item variance
Variance AQ (complexity mask).
@item autovariance
Auto-variance AQ (experimental).
@end table
@item -aq-strength @var{float}
AQ strength, reduces blocking and blurring in flat and textured areas.
@item -psy @var{integer}
Use psychovisual optimizations.
@item -psy-rd @var{string}
Strength of psychovisual optimization, in <psy-rd>:<psy-trellis> format.
@item -rc-lookahead @var{integer}
Number of frames to look ahead for frametype and ratecontrol.
@item -weightb @var{integer}
Weighted prediction for B-frames.
@item -weightp @var{integer}
Weighted prediction analysis method.

Possible values:
@table @samp
@item none

@item simple

@item smart

@end table
@item -ssim @var{integer}
Calculate and print SSIM stats.
@item -intra-refresh @var{integer}
Use Periodic Intra Refresh instead of IDR frames.
@item -b-bias @var{integer}
Influences how often B-frames are used.
@item -b-pyramid @var{integer}
Keep some B-frames as references.

Possible values:
@table @samp
@item none

@item strict
Strictly hierarchical pyramid.
@item normal
Non-strict (not Blu-ray compatible).
@end table
@item -mixed-refs @var{integer}
One reference per partition, as opposed to one reference per macroblock.
@item -8x8dct @var{integer}
High profile 8x8 transform.
@item -fast-pskip @var{integer}
@item -aud @var{integer}
Use access unit delimiters.
@item -mbtree @var{integer}
Use macroblock tree ratecontrol.
@item -deblock @var{string}
Loop filter parameters, in <alpha:beta> form.
@item -cplxblur @var{float}
Reduce fluctuations in QP (before curve compression).
@item -partitions @var{string}
A comma-separated list of partitions to consider, possible values: p8x8, p4x4, b8x8, i8x8, i4x4, none, all.
@item -direct-pred @var{integer}
Direct MV prediction mode

Possible values:
@table @samp
@item none

@item spatial

@item temporal

@item auto

@end table
@item -slice-max-size @var{integer}
Limit the size of each slice in bytes.
@item -stats @var{string}
Filename for 2 pass stats.
@item -nal-hrd @var{integer}
Signal HRD information (requires vbv-bufsize; cbr not allowed in .mp4).

Possible values:
@table @samp
@item none

@item vbr

@item cbr

@end table

@item x264opts @var{options}
Allow to set any x264 option, see @code{x264 --fullhelp} for a list.

@var{options} is a list of @var{key}=@var{value} couples separated by
":". In @var{filter} and @var{psy-rd} options that use ":" as a separator
themselves, use "," instead. They accept it as well since long ago but this
is kept undocumented for some reason.

For example to specify libx264 encoding options with @command{ffmpeg}:
@example
ffmpeg -i foo.mpg -vcodec libx264 -x264opts keyint=123:min-keyint=20 -an out.mkv
@end example

For more information about libx264 and the supported options see:
@url{http://www.videolan.org/developers/x264.html}

@item -x264-params @var{string}
Override the x264 configuration using a :-separated list of key=value parameters.
@example
-x264-params level=30:bframes=0:weightp=0:cabac=0:ref=1:vbv-maxrate=768:vbv-bufsize=2000:analyse=all:me=umh:no-fast-pskip=1:subq=6:8x8dct=0:trellis=0
@end example
@end table

Encoding avpresets for common usages are provided so they can be used with the
general presets system (e.g. passing the @code{-pre} option).

@section png

PNG image encoder.

@subsection Private options

@table @option
@item dpi @var{integer}
Set physical density of pixels, in dots per inch, unset by default
@item dpm @var{integer}
Set physical density of pixels, in dots per meter, unset by default
@end table

@section ProRes

Apple ProRes encoder.

FFmpeg contains 2 ProRes encoders, the prores-aw and prores-ks encoder.
The used encoder can be choosen with the @code{-vcodec} option.

@subsection Private Options for prores-ks

@table @option
@item profile @var{integer}
Select the ProRes profile to encode
@table @samp
@item proxy
@item lt
@item standard
@item hq
@end table

@item quant_mat @var{integer}
Select quantization matrix.
@table @samp
@item auto
@item default
@item proxy
@item lt
@item standard
@item hq
@end table
If set to @var{auto}, the matrix matching the profile will be picked.
If not set, the matrix providing the highest quality, @var{default}, will be
picked.

@item bits_per_mb @var{integer}
How many bits to allot for coding one macroblock. Different profiles use
between 200 and 2400 bits per macroblock, the maximum is 8000.

@item mbs_per_slice @var{integer}
Number of macroblocks in each slice (1-8); the default value (8)
should be good in almost all situations.

@item vendor @var{string}
Override the 4-byte vendor ID.
A custom vendor ID like @var{apl0} would claim the stream was produced by
the Apple encoder.

@end table

@subsection Speed considerations

In the default mode of operation the encoder has to honor frame constraints
(i.e. not produc frames with size bigger than requested) while still making
output picture as good as possible.
A frame containing a lot of small details is harder to compress and the encoder
would spend more time searching for appropriate quantizers for each slice.

Setting a higher @option{bits_per_mb} limit will improve the speed.

For the fastest encoding speed set the @option{qscale} parameter (4 is the
recommended value) and do not set a size constraint.

@c man end VIDEO ENCODERS
