4 Encoders are configured elements in FFmpeg which allow the encoding of
7 When you configure your FFmpeg build, all the supported native encoders
8 are enabled by default. Encoders requiring an external library must be enabled
9 manually via the corresponding @code{--enable-lib} option. You can list all
10 available encoders using the configure option @code{--list-encoders}.
12 You can disable all the encoders with the configure option
13 @code{--disable-encoders} and selectively enable / disable single encoders
14 with the options @code{--enable-encoder=@var{ENCODER}} /
15 @code{--disable-encoder=@var{ENCODER}}.
17 The option @code{-codecs} of the ff* tools will display the list of
22 @chapter Audio Encoders
23 @c man begin AUDIO ENCODERS
25 A description of some of the currently available audio encoders
28 @section ac3 and ac3_fixed
32 These encoders implement part of ATSC A/52:2010 and ETSI TS 102 366, as well as
33 the undocumented RealAudio 3 (a.k.a. dnet).
35 The @var{ac3} encoder uses floating-point math, while the @var{ac3_fixed}
36 encoder only uses fixed-point integer math. This does not mean that one is
37 always faster, just that one or the other may be better suited to a
38 particular system. The floating-point encoder will generally produce better
39 quality audio for a given bitrate. The @var{ac3_fixed} encoder is not the
40 default codec for any of the output formats, so it must be specified explicitly
41 using the option @code{-acodec ac3_fixed} in order to use it.
43 @subsection AC-3 Metadata
45 The AC-3 metadata options are used to set parameters that describe the audio,
46 but in most cases do not affect the audio encoding itself. Some of the options
47 do directly affect or influence the decoding and playback of the resulting
48 bitstream, while others are just for informational purposes. A few of the
49 options will add bits to the output stream that could otherwise be used for
50 audio data, and will thus affect the quality of the output. Those will be
51 indicated accordingly with a note in the option list below.
53 These parameters are described in detail in several publicly-available
56 @item @uref{http://www.atsc.org/cms/standards/a_52-2010.pdf,A/52:2010 - Digital Audio Compression (AC-3) (E-AC-3) Standard}
57 @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}
58 @item @uref{http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/18_Metadata.Guide.pdf,Dolby Metadata Guide}
59 @item @uref{http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/46_DDEncodingGuidelines.pdf,Dolby Digital Professional Encoding Guidelines}
62 @subsubsection Metadata Control Options
66 @item -per_frame_metadata @var{boolean}
67 Allow Per-Frame Metadata. Specifies if the encoder should check for changing
68 metadata for each frame.
71 The metadata values set at initialization will be used for every frame in the
74 Metadata values can be changed before encoding each frame.
79 @subsubsection Downmix Levels
83 @item -center_mixlev @var{level}
84 Center Mix Level. The amount of gain the decoder should apply to the center
85 channel when downmixing to stereo. This field will only be written to the
86 bitstream if a center channel is present. The value is specified as a scale
87 factor. There are 3 valid values:
92 Apply -4.5dB gain (default)
97 @item -surround_mixlev @var{level}
98 Surround Mix Level. The amount of gain the decoder should apply to the surround
99 channel(s) when downmixing to stereo. This field will only be written to the
100 bitstream if one or more surround channels are present. The value is specified
101 as a scale factor. There are 3 valid values:
106 Apply -6dB gain (default)
108 Silence Surround Channel(s)
113 @subsubsection Audio Production Information
114 Audio Production Information is optional information describing the mixing
115 environment. Either none or both of the fields are written to the bitstream.
119 @item -mixing_level @var{number}
120 Mixing Level. Specifies peak sound pressure level (SPL) in the production
121 environment when the mix was mastered. Valid values are 80 to 111, or -1 for
122 unknown or not indicated. The default value is -1, but that value cannot be
123 used if the Audio Production Information is written to the bitstream. Therefore,
124 if the @code{room_type} option is not the default value, the @code{mixing_level}
125 option must not be -1.
127 @item -room_type @var{type}
128 Room Type. Describes the equalization used during the final mixing session at
129 the studio or on the dubbing stage. A large room is a dubbing stage with the
130 industry standard X-curve equalization; a small room has flat equalization.
131 This field will not be written to the bitstream if both the @code{mixing_level}
132 option and the @code{room_type} option have the default values.
136 Not Indicated (default)
147 @subsubsection Other Metadata Options
151 @item -copyright @var{boolean}
152 Copyright Indicator. Specifies whether a copyright exists for this audio.
156 No Copyright Exists (default)
162 @item -dialnorm @var{value}
163 Dialogue Normalization. Indicates how far the average dialogue level of the
164 program is below digital 100% full scale (0 dBFS). This parameter determines a
165 level shift during audio reproduction that sets the average volume of the
166 dialogue to a preset level. The goal is to match volume level between program
167 sources. A value of -31dB will result in no volume level change, relative to
168 the source volume, during audio reproduction. Valid values are whole numbers in
169 the range -31 to -1, with -31 being the default.
171 @item -dsur_mode @var{mode}
172 Dolby Surround Mode. Specifies whether the stereo signal uses Dolby Surround
173 (Pro Logic). This field will only be written to the bitstream if the audio
174 stream is stereo. Using this option does @b{NOT} mean the encoder will actually
175 apply Dolby Surround processing.
179 Not Indicated (default)
182 Not Dolby Surround Encoded
185 Dolby Surround Encoded
188 @item -original @var{boolean}
189 Original Bit Stream Indicator. Specifies whether this audio is from the
190 original source and not a copy.
197 Original Source (default)
202 @subsection Extended Bitstream Information
203 The extended bitstream options are part of the Alternate Bit Stream Syntax as
204 specified in Annex D of the A/52:2010 standard. It is grouped into 2 parts.
205 If any one parameter in a group is specified, all values in that group will be
206 written to the bitstream. Default values are used for those that are written
207 but have not been specified. If the mixing levels are written, the decoder
208 will use these values instead of the ones specified in the @code{center_mixlev}
209 and @code{surround_mixlev} options if it supports the Alternate Bit Stream
212 @subsubsection Extended Bitstream Information - Part 1
216 @item -dmix_mode @var{mode}
217 Preferred Stereo Downmix Mode. Allows the user to select either Lt/Rt
218 (Dolby Surround) or Lo/Ro (normal stereo) as the preferred stereo downmix mode.
222 Not Indicated (default)
225 Lt/Rt Downmix Preferred
228 Lo/Ro Downmix Preferred
231 @item -ltrt_cmixlev @var{level}
232 Lt/Rt Center Mix Level. The amount of gain the decoder should apply to the
233 center channel when downmixing to stereo in Lt/Rt mode.
246 Apply -4.5dB gain (default)
250 Silence Center Channel
253 @item -ltrt_surmixlev @var{level}
254 Lt/Rt Surround Mix Level. The amount of gain the decoder should apply to the
255 surround channel(s) when downmixing to stereo in Lt/Rt mode.
264 Apply -6.0dB gain (default)
266 Silence Surround Channel(s)
269 @item -loro_cmixlev @var{level}
270 Lo/Ro Center Mix Level. The amount of gain the decoder should apply to the
271 center channel when downmixing to stereo in Lo/Ro mode.
284 Apply -4.5dB gain (default)
288 Silence Center Channel
291 @item -loro_surmixlev @var{level}
292 Lo/Ro Surround Mix Level. The amount of gain the decoder should apply to the
293 surround channel(s) when downmixing to stereo in Lo/Ro mode.
302 Apply -6.0dB gain (default)
304 Silence Surround Channel(s)
309 @subsubsection Extended Bitstream Information - Part 2
313 @item -dsurex_mode @var{mode}
314 Dolby Surround EX Mode. Indicates whether the stream uses Dolby Surround EX
315 (7.1 matrixed to 5.1). Using this option does @b{NOT} mean the encoder will actually
316 apply Dolby Surround EX processing.
320 Not Indicated (default)
323 Dolby Surround EX Off
329 @item -dheadphone_mode @var{mode}
330 Dolby Headphone Mode. Indicates whether the stream uses Dolby Headphone
331 encoding (multi-channel matrixed to 2.0 for use with headphones). Using this
332 option does @b{NOT} mean the encoder will actually apply Dolby Headphone
337 Not Indicated (default)
346 @item -ad_conv_type @var{type}
347 A/D Converter Type. Indicates whether the audio has passed through HDCD A/D
352 Standard A/D Converter (default)
360 @subsection Other AC-3 Encoding Options
364 @item -stereo_rematrixing @var{boolean}
365 Stereo Rematrixing. Enables/Disables use of rematrixing for stereo input. This
366 is an optional AC-3 feature that increases quality by selectively encoding
367 the left/right channels as mid/side. This option is enabled by default, and it
368 is highly recommended that it be left as enabled except for testing purposes.
372 @subsection Floating-Point-Only AC-3 Encoding Options
374 These options are only valid for the floating-point encoder and do not exist
375 for the fixed-point encoder due to the corresponding features not being
376 implemented in fixed-point.
380 @item -channel_coupling @var{boolean}
381 Enables/Disables use of channel coupling, which is an optional AC-3 feature
382 that increases quality by combining high frequency information from multiple
383 channels into a single channel. The per-channel high frequency information is
384 sent with less accuracy in both the frequency and time domains. This allows
385 more bits to be used for lower frequencies while preserving enough information
386 to reconstruct the high frequencies. This option is enabled by default for the
387 floating-point encoder and should generally be left as enabled except for
388 testing purposes or to increase encoding speed.
392 Selected by Encoder (default)
395 Disable Channel Coupling
398 Enable Channel Coupling
401 @item -cpl_start_band @var{number}
402 Coupling Start Band. Sets the channel coupling start band, from 1 to 15. If a
403 value higher than the bandwidth is used, it will be reduced to 1 less than the
404 coupling end band. If @var{auto} is used, the start band will be determined by
405 the encoder based on the bit rate, sample rate, and channel layout. This option
406 has no effect if channel coupling is disabled.
410 Selected by Encoder (default)
417 LAME (Lame Ain't an MP3 Encoder) MP3 encoder wrapper.
419 Requires the presence of the libmp3lame headers and library during
420 configuration. You need to explicitly configure the build with
421 @code{--enable-libmp3lame}.
423 @subsection Option Mapping
425 The following options are supported by the libmp3lame wrapper,
426 the LAME-equivalent options follow the FFmpeg ones.
428 @multitable @columnfractions .2 .2
429 @item FFmpeg @tab LAME
431 FFmpeg @code{b} option is expressed in bits/s, lame @code{bitrate}
434 Quality setting for VBR.
435 @item compression_level @tab q
436 Algorithm quality. Valid options are integers from 0-9.
437 @item reservoir @tab N.A.
438 Enable use of bit reservoir. LAME has this enabled by default.
439 @item joint_stereo @tab -m j
440 Enables the encoder to use (on a frame by frame basis) either L/R
441 stereo or mid/side stereo.
444 @section libopencore-amrnb
446 OpenCORE Adaptive Multi-Rate Narrowband encoder.
448 Requires the presence of the libopencore-amrnb headers and library during
449 configuration. You need to explicitly configure the build with
450 @code{--enable-libopencore-amrnb --enable-version3}.
452 This is a mono-only encoder. Officially it only supports 8000Hz sample rate,
453 but you can override it by setting @option{strict} to @samp{unofficial} or
461 Set bitrate in bits per second. Only the following bitrates are supported,
462 otherwise libavcodec will round to the nearest valid bitrate.
476 Allow discontinuous transmission (generate comfort noise) when set to 1. The
477 default value is 0 (disabled).
483 TwoLAME MP2 encoder wrapper.
485 Requires the presence of the libtwolame headers and library during
486 configuration. You need to explicitly configure the build with
487 @code{--enable-libtwolame}.
489 @subsection Options Mapping
491 The following options are supported by the libtwolame wrapper. The
492 TwoLAME-equivalent options follow the FFmpeg ones and are in
497 (b) Set bitrate in bits/s. Note that FFmpeg @code{b} option is
498 expressed in bits/s, twolame @code{b} in kilobits/s. The default
502 (V) Set quality for experimental VBR support. Maximum value range is
503 from -50 to 50, useful range is from -10 to 10.
506 (mode) Set MPEG mode. Possible values:
510 Choose mode automatically based on the input. This is the default.
522 (psyc-mode) Set psychoacoustic model to use in encoding. The argument
523 must be an integer between -1 and 4, inclusive. The higher the value,
524 the better the quality. The default value is 3.
527 (energy) Enable energy levels extensions when set to 1. The default
528 value is 0 (disabled).
530 @item error_protection
531 (protect) Enable CRC error protection when set to 1. The default value
535 (copyright) Set MPEG audio copyright flag when set to 1. The default
536 value is 0 (disabled).
539 (original) Set MPEG audio original flag when set to 1. The default
540 value is 0 (disabled).
544 @section libvo-aacenc
546 VisualOn AAC encoder.
548 Requires the presence of the libvo-aacenc headers and library during
549 configuration. You need to explicitly configure the build with
550 @code{--enable-libvo-aacenc --enable-version3}.
554 The VisualOn AAC encoder only support encoding AAC-LC and up to 2
555 channels. It is also CBR-only. It is considered to be worse than the
556 native experimental FFmpeg AAC encoder.
565 @section libvo-amrwbenc
567 VisualOn Adaptive Multi-Rate Wideband encoder.
569 Requires the presence of the libvo-amrwbenc headers and library during
570 configuration. You need to explicitly configure the build with
571 @code{--enable-libvo-amrwbenc --enable-version3}.
573 This is a mono-only encoder. Officially it only supports 16000Hz sample
574 rate, but you can override it by setting @option{strict} to
575 @samp{unofficial} or lower.
582 Set bitrate in bits/s. Only the following bitrates are supported, otherwise
583 libavcodec will round to the nearest valid bitrate.
598 Allow discontinuous transmission (generate comfort noise) when set to 1. The
599 default value is 0 (disabled).
605 A wrapper providing WavPack encoding through libwavpack.
607 Only lossless mode using 32-bit integer samples is supported currently.
608 The @option{compression_level} option can be used to control speed vs.
609 compression tradeoff, with the values mapped to libwavpack as follows:
614 Fast mode - corresponding to the wavpack @option{-f} option.
617 Normal (default) settings.
620 High quality - corresponding to the wavpack @option{-h} option.
623 Very high quality - corresponding to the wavpack @option{-hh} option.
626 Same as 3, but with extra processing enabled - corresponding to the wavpack
627 @option{-x} option. I.e. 4 is the same as @option{-x2} and 8 is the same as
632 @c man end AUDIO ENCODERS
634 @chapter Video Encoders
635 @c man begin VIDEO ENCODERS
637 A description of some of the currently available video encoders
642 Theora format supported through libtheora.
644 Requires the presence of the libtheora headers and library during
645 configuration. You need to explicitly configure the build with
646 @code{--enable-libtheora}.
650 The following global options are mapped to internal libtheora options
651 which affect the quality and the bitrate of the encoded stream.
655 Set the video bitrate, only works if the @code{qscale} flag in
656 @option{flags} is not enabled.
659 Used to enable constant quality mode encoding through the
660 @option{qscale} flag, and to enable the @code{pass1} and @code{pass2}
667 Set the global quality in lambda units, only works if the
668 @code{qscale} flag in @option{flags} is enabled. The value is clipped
669 in the [0 - 10*@code{FF_QP2LAMBDA}] range, and then multiplied for 6.3
670 to get a value in the native libtheora range [0-63]. A higher value
671 corresponds to a higher quality.
673 For example, to set maximum constant quality encoding with
676 ffmpeg -i INPUT -flags:v qscale -global_quality:v "10*QP2LAMBDA" -codec:v libtheora OUTPUT.ogg
682 VP8 format supported through libvpx.
684 Requires the presence of the libvpx headers and library during configuration.
685 You need to explicitly configure the build with @code{--enable-libvpx}.
689 Mapping from FFmpeg to libvpx options with conversion notes in parentheses.
716 @code{(bufsize * 1000 / vb)}
719 @code{(bufsize * 1000 / vb * 5 / 6)}
721 @item rc_init_occupancy, vb
723 @code{(rc_init_occupancy * 1000 / vb)}
725 @item rc_buffer_aggressivity
732 rc_2pass_vbr_bias_pct
735 rc_2pass_vbr_maxsection_pct
736 @code{(maxrate * 100 / vb)}
739 rc_2pass_vbr_minsection_pct
740 @code{(minrate * 100 / vb)}
742 @item minrate, maxrate, vb
744 @code{(minrate == maxrate == vb)}
747 @code{VPX_CQ}, @code{VP8E_SET_CQ_LEVEL}
752 @code{VPX_DL_BEST_QUALITY}
754 @code{VPX_DL_GOOD_QUALITY}
756 @code{VPX_DL_REALTIME}
760 @code{VP8E_SET_CPUUSED}
763 @code{VP8E_SET_NOISE_SENSITIVITY}
766 @code{VP8E_SET_STATIC_THRESHOLD}
769 @code{VP8E_SET_TOKEN_PARTITIONS}
772 @code{VP8E_SET_MAX_INTRA_BITRATE_PCT}
774 @item force_key_frames
775 @code{VPX_EFLAG_FORCE_KF}
777 @item Alternate reference frame related
779 @item vp8flags altref
780 @code{VP8E_SET_ENABLEAUTOALTREF}
781 @item @var{arnr_max_frames}
782 @code{VP8E_SET_ARNR_MAXFRAMES}
783 @item @var{arnr_type}
784 @code{VP8E_SET_ARNR_TYPE}
785 @item @var{arnr_strength}
786 @code{VP8E_SET_ARNR_STRENGTH}
787 @item @var{rc_lookahead}
791 @item vp8flags error_resilient
796 For more information about libvpx see:
797 @url{http://www.webmproject.org/}
801 x264 H.264/MPEG-4 AVC encoder wrapper.
803 Requires the presence of the libx264 headers and library during
804 configuration. You need to explicitly configure the build with
805 @code{--enable-libx264}.
807 x264 supports an impressive number of features, including 8x8 and 4x4 adaptive
808 spatial transform, adaptive B-frame placement, CAVLC/CABAC entropy coding,
809 interlacing (MBAFF), lossless mode, psy optimizations for detail retention
810 (adaptive quantization, psy-RD, psy-trellis).
812 The FFmpeg wrapper provides a mapping for most of them using global options
813 that match those of the encoders and provides private options for the unique
814 encoder options. Additionally an expert override is provided to directly pass
815 a list of key=value tuples as accepted by x264_param_parse.
817 @subsection Option Mapping
819 The following options are supported by the x264 wrapper, the x264-equivalent
820 options follow the FFmpeg ones.
822 @multitable @columnfractions .2 .2
824 FFmpeg @code{b} option is expressed in bits/s, x264 @code{bitrate} in kilobits/s.
825 @item bf @tab bframes
826 Maximum number of B-frames.
829 @item qmin @tab qpmin
830 @item qmax @tab qpmax
831 @item qdiff @tab qpstep
832 @item qblur @tab qblur
833 @item qcomp @tab qcomp
835 @item sc_threshold @tab scenecut
836 @item trellis @tab trellis
839 @item me_range @tab merange
840 @item me_method @tab me
841 @item subq @tab subme
842 @item b_strategy @tab b-adapt
843 @item keyint_min @tab keyint-min
844 @item coder @tab cabac
845 Set coder to @code{ac} to use CABAC.
846 @item cmp @tab chroma-me
847 Set to @code{chroma} to use chroma motion estimation.
848 @item threads @tab threads
849 @item thread_type @tab sliced_threads
850 Set to @code{slice} to use sliced threading instead of frame threading.
851 @item flags -cgop @tab open-gop
852 Set @code{-cgop} to use recovery points to close GOPs.
853 @item rc_init_occupancy @tab vbv-init
854 Initial buffer occupancy.
857 @subsection Private Options
859 @item -preset @var{string}
860 Set the encoding preset (cf. x264 --fullhelp).
861 @item -tune @var{string}
862 Tune the encoding params (cf. x264 --fullhelp).
863 @item -profile @var{string}
864 Set profile restrictions (cf. x264 --fullhelp).
865 @item -fastfirstpass @var{integer}
866 Use fast settings when encoding first pass.
867 @item -crf @var{float}
868 Select the quality for constant quality mode.
869 @item -crf_max @var{float}
870 In CRF mode, prevents VBV from lowering quality beyond this point.
871 @item -qp @var{integer}
872 Constant quantization parameter rate control method.
873 @item -aq-mode @var{integer}
881 Variance AQ (complexity mask).
883 Auto-variance AQ (experimental).
885 @item -aq-strength @var{float}
886 AQ strength, reduces blocking and blurring in flat and textured areas.
887 @item -psy @var{integer}
888 Use psychovisual optimizations.
889 @item -psy-rd @var{string}
890 Strength of psychovisual optimization, in <psy-rd>:<psy-trellis> format.
891 @item -rc-lookahead @var{integer}
892 Number of frames to look ahead for frametype and ratecontrol.
893 @item -weightb @var{integer}
894 Weighted prediction for B-frames.
895 @item -weightp @var{integer}
896 Weighted prediction analysis method.
907 @item -ssim @var{integer}
908 Calculate and print SSIM stats.
909 @item -intra-refresh @var{integer}
910 Use Periodic Intra Refresh instead of IDR frames.
911 @item -b-bias @var{integer}
912 Influences how often B-frames are used.
913 @item -b-pyramid @var{integer}
914 Keep some B-frames as references.
921 Strictly hierarchical pyramid.
923 Non-strict (not Blu-ray compatible).
925 @item -mixed-refs @var{integer}
926 One reference per partition, as opposed to one reference per macroblock.
927 @item -8x8dct @var{integer}
928 High profile 8x8 transform.
929 @item -fast-pskip @var{integer}
930 @item -aud @var{integer}
931 Use access unit delimiters.
932 @item -mbtree @var{integer}
933 Use macroblock tree ratecontrol.
934 @item -deblock @var{string}
935 Loop filter parameters, in <alpha:beta> form.
936 @item -cplxblur @var{float}
937 Reduce fluctuations in QP (before curve compression).
938 @item -partitions @var{string}
939 A comma-separated list of partitions to consider, possible values: p8x8, p4x4, b8x8, i8x8, i4x4, none, all.
940 @item -direct-pred @var{integer}
941 Direct MV prediction mode
954 @item -slice-max-size @var{integer}
955 Limit the size of each slice in bytes.
956 @item -stats @var{string}
957 Filename for 2 pass stats.
958 @item -nal-hrd @var{integer}
959 Signal HRD information (requires vbv-bufsize; cbr not allowed in .mp4).
971 @item x264opts @var{options}
972 Allow to set any x264 option, see @code{x264 --fullhelp} for a list.
974 @var{options} is a list of @var{key}=@var{value} couples separated by
975 ":". In @var{filter} and @var{psy-rd} options that use ":" as a separator
976 themselves, use "," instead. They accept it as well since long ago but this
977 is kept undocumented for some reason.
979 For example to specify libx264 encoding options with @command{ffmpeg}:
981 ffmpeg -i foo.mpg -vcodec libx264 -x264opts keyint=123:min-keyint=20 -an out.mkv
984 For more information about libx264 and the supported options see:
985 @url{http://www.videolan.org/developers/x264.html}
987 @item -x264-params @var{string}
988 Override the x264 configuration using a :-separated list of key=value parameters.
990 -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
994 Encoding avpresets for common usages are provided so they can be used with the
995 general presets system (e.g. passing the @code{-pre} option).
1001 @subsection Private options
1004 @item dpi @var{integer}
1005 Set physical density of pixels, in dots per inch, unset by default
1006 @item dpm @var{integer}
1007 Set physical density of pixels, in dots per meter, unset by default
1012 Apple ProRes encoder.
1014 FFmpeg contains 2 ProRes encoders, the prores-aw and prores-ks encoder.
1015 The used encoder can be choosen with the @code{-vcodec} option.
1017 @subsection Private Options for prores-ks
1020 @item profile @var{integer}
1021 Select the ProRes profile to encode
1030 @item quant_mat @var{integer}
1031 Select quantization matrix.
1040 If set to @var{auto}, the matrix matching the profile will be picked.
1041 If not set, the matrix providing the highest quality, @var{default}, will be
1044 @item bits_per_mb @var{integer}
1045 How many bits to allot for coding one macroblock. Different profiles use
1046 between 200 and 2400 bits per macroblock, the maximum is 8000.
1048 @item mbs_per_slice @var{integer}
1049 Number of macroblocks in each slice (1-8); the default value (8)
1050 should be good in almost all situations.
1052 @item vendor @var{string}
1053 Override the 4-byte vendor ID.
1054 A custom vendor ID like @var{apl0} would claim the stream was produced by
1057 @item alpha_bits @var{integer}
1058 Specify number of bits for alpha component.
1059 Possible values are @var{0}, @var{8} and @var{16}.
1060 Use @var{0} to disable alpha plane coding.
1064 @subsection Speed considerations
1066 In the default mode of operation the encoder has to honor frame constraints
1067 (i.e. not produc frames with size bigger than requested) while still making
1068 output picture as good as possible.
1069 A frame containing a lot of small details is harder to compress and the encoder
1070 would spend more time searching for appropriate quantizers for each slice.
1072 Setting a higher @option{bits_per_mb} limit will improve the speed.
1074 For the fastest encoding speed set the @option{qscale} parameter (4 is the
1075 recommended value) and do not set a size constraint.
1077 @c man end VIDEO ENCODERS