mjpeg_howto.txt

Motion JPEG编解码器源代码

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  This file can be fed into glav or lavplay, or you can pipe it into
  mpeg2enc with lav2yuv or combine the whole stuff into one single mjpeg
  file with lavtrans or lav2yuv|yuv2lav.


  7.  Converting the stream to MPEG or DIVx videos

  First there is some general description in the encoding process and
  afterwards there is a detailed description of some commonly used
  output formats.

  If you want a one command conversation to mpeg videos try lav2mpeg in
  the scripts directory
  The encoding with the lav2mpeg script looks like this for mpeg1
  output:

  >lav2mpeg -a 160 -b 2110 -d 320x240 -m mpeg1 -o output.mpg file.eli


  o  Will create a mpeg1 with videobitrate of 2110kBit/sec and
     audiobitrate of 160 kBit/sec

  o  at a resolution of 320x240

  Or for the generation of mpeg2 output:

  lav2mpeg -o mpeg2 -O output.mpg file.eli


  o  Will create a mpeg2 with default bitrate in same resolution as the
     input resolution

  Better results can be accomplished, however, by trying various options
  and find out which ones work best for you. These are discussed below.

  The creation of MPEG1 movies is explained with more examples and in
  greater detail because most of the things that can be used for MPEG1
  also work for the other output formats

  For the creation of of VCD/SVCD Stills sequences (-f 6, -f 7 in
  mpeg2enc) you should see: http://www.mir.com/DMG/ Still sequences are
  needed for the creation of menus in VCD/SVCD. The creation of menus is
  described in the doku of vcdimager.


  7.1.  Creating sound

  MPEG-1 videos need MPEG1-layer2 sound files. For MPEG-2 videos you can
  use MPEG1-Layer2 and MPEG1-Layer3 (MP3).    Layer 3 (MP3) audio is not
  an offically valid audio format but many VCD players will recognize
  it.   MP3 audio is not valid for DVDs.  You should stick to
  MPEG1-Layer2 because most of the MPEG2 players (DVD Player for example
  usually the different Winxx Versions have great problems with this
  too) are not able to play MPEG2-Video and MPEG1-Layer3 sound.


  mp2enc is a MPEG1-layer 2 Audio encoder. The toolame encoder is also
  able to produce an layer 2 file. You can use that one as well.
  Toolame is much faster than mp2enc but toolame does not peform
  resampling (48000 to 44100 samples/second).  Many hardware players
  will play SVCDs using 48000 rate audio.  For mp3 creation I'm be sure
  you have an encoder.

  Example:

  > lav2wav stream.avi | mp2enc -o sound.mp2

  This creates a mpeg sound file out of the stream.avi with 224kBit/sec
  bitrate and a sample rate of 48kHz. If you audio file has 44.1kHz
  mp2enc resamples the audio to create a 48kHz output. If you want a
  44.1kHz output sample rate you have to add -r 44100 to the mp2enc
  command

  Example

  > cat sound.wav | mp2enc -v 2 -V -o sound.mp2

  This creates a VCD (-V bitrate=224, stereo, sampling rate:44100)
  compatible output from the wav file.
  With -v 2 mp2enc is more verbose, while encoding you see the number of
  sec of audio already encoded.

  You can test the output with:

  > plaympeg sound.mp2

  NOTE: plaympeg is a MPEG1 Player for Linux, you can use other players
  as well, for MPEG audio testing you can also use mpg123.


  7.2.  Converting video

  You can create MPEG1 and MPEG2 videos.

  Normally the first video you create is not the best. For optimal
  quality/size you need to play with the bitrate, search radius, noise
  filter .... The options of mpeg2enc are described in the manpage of
  mpeg2enc.

  Example:

  lav2yuv stream.avi stream1.avi | mpeg2enc -o video.m1v

  This creates an video file with the default constant bitrate of
  1152kBit/sec.  This is the bitrate you need if you want to create
  VCDs. You can specify more files and also use the placeholder %nd.
  Where n describes the number.

  Example:

  > lav2yuv streami%02d.avi | mpeg2enc -b 1500 -r 16 -o video.m1v


  mpeg2enc creates a video with a bitrate of 1500kBit/s uses an search
  radius of 16. That means when trying to find similar 16*16 macroblocks
  of pixels between frames the encoder looks up to 16 pixels away from
  the current position of each block. It looks twice as far when
  comparing frames 1 frame apart and so on. Reasonable values are 16 or
  24. The default is 16 so adding the option here is silly.  Lower
  values (0, 8), improve the encoding speed but you get lower quality
  (more visible artifacts) while higher values (24, 32) improve the
  quality at the cost of the speed. With the file description of
  stream%02d.avi all files are processed that match this pattern with
  00, 01....


  7.2.1.  Scaling

  Using yuvscaler one can now also scale the video before encoding it.
  This can be useful for users with a DC10 or DC10+ cards which captures
  at -d 1 768x576 or -d 2 384x288 (PAL/SECAM) or -d 1 640x480 (NTSC).

  You get a full description of all commands by reading the manpage or
  running:

  >yuvscaler -h

  Example:

  > lav2yuv stream.avi | yuvscaler -O VCD | mpeg2enc -o video.m1v

  This will scale the stream to VCD size which for PAL/SECAM is 352x288
  and for NTSC is 352x240. The scaled yuvstream is encoded to MPEG-1.


  It can also do SVCD scaling to 480x480 (NTSC) or 480x576 (PAL/SECAM):

  > lav2yuv stream.avi | yuvscaler -O SVCD -M BICUBIC | mpeg2enc -o
  video.m1v

  The mode keyword (-M) forces yuvscaler to use the higher quality
  bicubic algorithms for downscaling and not the default resample
  algorithms. Upscaling is always done using the bicubic algorithm.

  Example

  > lav2yuv stream.avi | yuvscaler -I USE_450x340+20+30 -O SIZE_320x200
  | mpeg2enc -o video.m1v

  Here we only use a part of the input and specify a nonstandard output
  resolution.

  NOTE: yuvscaler can set a active area, and set everything else to
  black using: -I ACTIVE_WidthxHeight+WidthOffset+HeightOffset

  Testing is done by:

  > plaympeg video.m1v

  NOTE:These are only examples. There are more options you can use. You
  can use most of them together to create high quality videos with the
  lowest possible bitrate.

  NOTE2:The higher you set the search radius the longer the conversion
  will take. In general the more options used the longer encoding takes.

  NOTE3:MPEG1 was not designed to be a VBR (variable bitrate stream) !!
  So if you encode with -q 15 mpeg2enc sets the maximal bitrate -b to
  1152. If you want a VBR MPEG1 you have to set -b very high (2500).

  NOTE4:Maybe you should give better names than video.mpg. A good idea
  is to use the options as part of the filename (for example:
  video_b1500_r16_41_21.m1v).  Another possibility is to call all the
  layer 2 files ".mp2" all the MPEG-1 video files ".m1v" and all MPEG-2
  video files ".m2v" Easy to see what's happening then. Reserve .mpg for
  multiplexed MPEG-1/2 streams.


  7.3.  Putting the streams together

  Example:

  > mplex sound.mp2 video.m1v -o my_video.m1v

  Puts the sound.mp2 and the video.m1v stream together to my_video.mpg

  Now you can use your preferred MPEG player and watch it. All players
  (gtv for example) based on the SMPEG library work well for MPEG-1.
  Other players (which can play MPEG-2 as well as MPEG-1 movies) are:
  xmovie, xine, and MPlayer.

  NOTE: If you have specified the -S option for mpeg2enc mplex will
  automatically split the files if there is in the output filename a %d
  (looks like: -o test%d.mpg) The files generated this way are separate
  stand-alone MPEG steams!

  NOTE2: xine might have a problem with seeking through videos.  mplayer
  has a problem with the "seek backward/forward" with variable bitrate
  streams because it goes forward in the file the amount of data for a
  constant bitrate stream. That amount might be significantly more than
  10 seconds or one minute (those are the amount mplayer seeks for each
  press of the arrow keys). So don't wonder if it seeks much more time
  forward or backward than you expect.

  Variable bit-rate multiplexing: Remember to tell mplex you're encoding
  VBR (-V option) as well as mpeg2enc (see the example scripts). It
  *could* auto-detect but it is not working yet. You should tell mplex a
  video buffer size at least as large as the one you specified to
  "mpeg2enc" Sensible numbers for MPEG-1 might be a ceiling bit-rate of
  2800Kbps, a quality ceiling (quantization floor) of 6 and a buffer
  size of 400K.

  Example:

  > mplex -V -r 1740 audio.mp2 video_vbr.m1v -o vbr_stream.mpg

  Here we multiplex a variable bitrate stream. mplex is now a single
  pass multiplexer so it can't detect the maximal bitrate and we have to
  specify it. The data rate for the output stream is: audio bitrate +
  peak videobitrate + 1-2% for mplex information. If audio (-b 224) has
  224kBit, video has 1500kBit (was encoded with -b 1500 -q 9) then we
  have 1724 * 1.01 or about 1740kBit.

  Example:

  > plaympeg my_video.mpg

  oder

  > gtv my_video.mpg

  7.4.  Creating MPEG1 Videos

  For MPEG1 you can use mpeg layer 2 Audio and mpeg1 video. A subset of
  MPEG1 movies are VCD's. You can use VBR (Variable BitRate) for the
  Video (although VCDs are almost always use CBR video) but the Audio
  has to be CBR (Constant BitRate).

  MPEG1 is recommended for picture sizes up to 352x288 for PAL and
  352x240 for NTSC for larger sizes MPEG2 is the better choice.  There
  is no exact resolution where MPEG1 is better than MPEG2.  Just to make
  soure, MPEG1 can't handle interlaced sources. If you video is
  interlaced you need MPEG2 to get it proper encoded. .


  7.4.1.  MPEG1 Audio creation Example

  > lav2wav editlist.eli | mp2enc -r 44100 -o sound.mp2

  You can save some bits by telling mp2enc to use a lower bitrate (-b
  option) like 160 or 192 kBit/s.  The -r 44100 option forces mp2enc to
  generate a 44.1kHz audio file.

  > lav2wav editlist.eli | mp2enc -b 128 -m -o sound.mp2

  This creates a mono output with an bitrate of 128kBit/sec bitrate.
  The input this time is the editlistfile (can have any name) created
  with glav so all changes you made in glav are direct processed and
  handed over to mp2enc.  You do NOT have to create an edited stream
  with lavtrans to get it converted properly.


  7.4.2.  MPEG1 Video creation Example

  > lav2yuv editlist.eli | mpeg2enc -b 2000 -r 24 -q 6 -o video.m1v


  mpeg2enc creates an video with an bitrate of 2000kBit/s (or
  2048000Bit/s) but the -q flag activates the variable bitrate and a
  quality factor of 6. It uses a search radius of 24.


  Explanation:when mpeg2enc is invoked without the 'q' flag it creates
  "constantbit-rate" MPEG streams. Where (loosely speaking) the strength
  of compression (and hence picture quality) is adjusted to ensure that
  on average each frame of video has exactly the specified number of
  bits. Such constant bit-rate streams are needed for broadcasting and
  for low-cost hardware like DVD and VCD players which use slow fixed-
  speed player hardware.

  Obviously this is fairly inefficient as it means inactive scenes use
  up bits that could better be "spent" on rapidly changing scenes.
  Setting the 'q' flag tells mpeg2enc to generate variable bit-rate
  streams. For such streams the bit-rate specified is simply the maximum
  permissible. The 'q' parameter specifies the minimum degree of
  compression to be applied by specifying how exactly picture
  information is recorded. Typically, 'q' would be set so that quiet
  scenes would use less than the specified maximum (around 6 or 8) but
  fast moving scenes would still be bit-rate limited. For archival
  purposes setting a maximum bit-rate high enough never to be reached
  (e.g. 10Mbps) and a q of 2 or 3 are reasonable choices.

  Example:

  > lav2yuv stream.avi | yuvscaler -I ACTIVE_352x240+0+24 | mpeg2enc -b
  1152 -r 16 -4 1 -2 1 -o video.m1v

  Usually there is at the top and at the bottom a nearly black border
  and a lot of bandwidth is used for something you do not like. The
  yuvscaler -I ACTIVE option sets everything that is not in the
  described area to black, but the imagesize (352x288) is not changed.
  So you have a real black border the encoder only uses a few bits for
  encoding them. You are still compatible with the VCD's format in this
  example.  To determine the active window extract one frame to the jpeg
  format:

  > lavtrans -f i -i 100 -o frame.jpg test.avi

  Than use your favorite graphic display program to determine the active
  size.  The -4 1 and -2 1 options improves the quality about 10% but
  conversion is slower.

  At the size of 352x288 (1/2 PAL size, created when using the -d 2
  option when recording) the needed bitrate is/should be between 1000 -
  1500kBit/s. For NTSC it should be about the same, because the image is
  smaller, but there are more frames per second than in PAL.



  Anyways, the major factor is quality of the original and the degree of
  filtering. Poor quality unfiltered material typically needs a higher
  rate to avoid visible artifacts.  If you want to reduce bit-rate
  without annoying artifacts when compressing broadcast material you
  should try one (or more) of the noise filters.

  Example:

  > lav2yuv stream.avi | mpeg2enc -b 1500 -n s -g 6 -G 20 -P -o
  video.m1v

  Here the stream.avi will be encoded with: