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git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@17593 b3059339-0415-0410-9bf9-f77b7e298cf2
139 lines
5.3 KiB
Plaintext
139 lines
5.3 KiB
Plaintext
Topics:
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I. Preparing to encode
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1. Identifying source material and framerate
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2. Selecting the quality you want
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3. Constraints for efficient encoding
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4. Cropping and scaling
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5. Choosing resolution and bitrate
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II. Containers and codecs
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1. Where the movie will be played
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2. Constraints of DVD, SVCD, and VCD
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3. Limitations of AVI container
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III. Basic MEncoder usage
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1. Selecting codecs & format
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2. Selecting input file or device
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3. Loading video filters
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4. Notes on A/V sync
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IV. Encoding procedures
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1. Encoding progressive video
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2. Two-pass encoding
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3. Encoding interlaced video
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4. Deinterlacing
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5. Inverse telecine
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6. Capturing TV input
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7. Dealing with mixed-source content
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8. Low-quality & damaged sources
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V. Optimizing encoding quality
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1. Noise removal
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2. Pure quality-gain options
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3. Questionable-gain options
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4. Advanced MPEG-4 features
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II. Containers and codecs
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II.1. Where the movie will be played
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Perhaps the most important factor to choosing the format in which you
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will encode your movie is where you want to be able to play it.
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Usually this involves a tradeoff between quality and features, since
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the formats supported by the widest variety of players are also the
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worst in regards to compression.
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If you want to be able to play your encode on standalone/set-top
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players, your primary choices are DVD, VCD, and SVCD. There are also
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extensions such as KVCD and XVCD which violate the standards but work
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on many players and deliver higher quality. Modern players are
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beginning to support MPEG-4 ("DivX") movies in AVI and perhaps other
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containers as well, but these are often buggy and require you to
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restrict your encodes to certain subsets of the full MPEG-4
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functionality.
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If you wish to be able to share your movies with Windows or Macintosh
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users, without them having to install additional software, your
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choices are very limited. The ancient MPEG-1 format with MP2 or PCM
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audio is probably the only choice that is universally supported.
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Interoperability with Windows/Mac also comes into play when deciding
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how to encode and whether to scale to preserve aspect, since popular
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media player applications for these systems do not honor the aspect
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ratio encoding stored in MPEG-4 avi files.
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IV.2. Two-pass encoding
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The complexity (and thus the number of bits) required to compress the
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frames of a movie can vary greatly from one scene to another. Modern
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video encoders can adjust to these needs as they go and vary the
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bitrate. However, they cannot exceed the requested average bitrate for
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long stretches of time, because they do not know the bitrate needs of
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future scenes.
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Two-pass encoding solves this problem by encoding the movie twice.
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During the first pass, statistics are generated regarding the number
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of bits used by each frame and the quantization level (quality) at
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which it was encoded. Then, when the second pass begins, the encoder
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reads these statistics and redistributes the bits from frames where
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they are in excess to frames that are suffering from low quality.
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In order for the process to work properly, the encoder should be given
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exactly the same sequence of frames during both passes. This means
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that the same filters must be used, the same encoder parameters must
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be used (with the possible exception of bitrate), and the same frame
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drops and duplications (if any) must take place.
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In theory it's possible to use -oac pcm or -oac copy during the first
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pass to avoid spending time encoding the audio. However, this can
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result in slight variations in which frames get dropped or duplicated,
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so it may be preferable to encode the audio during the first pass as
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well as the second. This also allows you to examine the final audio
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bitrate and filesize, and to adjust the audio or video bitrate
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slightly between passes if you don't meet your target size.
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Here is an example:
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Encoding from an existing AVI file
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500 kbit/sec MPEG-4 video
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96 kbit/sec average-bitrate MP3 audio
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mencoder bar.avi -vf scale=448:336 -mc 0 -oac mp3lame -lameopts \
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abr:br=96 -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=500:vpass=1
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mencoder bar.avi -vf scale=448:336 -mc 0 -oac mp3lame -lameopts \
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abr:br=96 -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=500:vpass=2
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If you do not want to overwrite the output from the first pass when
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you begin the second, you can use the -o option to choose a different
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output filename. Note the addition of the vpass option in this
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example. If vpass is not specified, single-pass encoding is performed.
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If vpass=1, a log file is written with statistics from the first pass.
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If vpass=2, the log file is read and the second pass is encoded based
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on those statistics. If you are short on disk space or don't want the
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extra disk wear from writing the file twice, you can use -o /dev/null
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during the first pass. However, sometimes it is beneficial to watch
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the first-pass file before beginning the second pass to make sure
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nothing went wrong in the encoding.
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Next, an example using XviD instead of libavcodec:
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Encoding from an existing AVI file
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500 kbit/sec MPEG-4 video
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Copying the existing audio stream unmodified
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mencoder foo.avi -vf scale=320:240 -mc 0 -oac copy -ovc xvid \
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-xvidencopts bitrate=400:pass=1
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mencoder foo.avi -vf scale=320:240 -mc 0 -oac copy -ovc xvid \
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-xvidencopts bitrate=400:pass=2
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The options used are slightly different, but the process is otherwise
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the same.
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