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@ -37,303 +37,6 @@ V. Optimizing encoding quality
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I. Preparing to encode
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Before you even think about encoding a movie, you need to take several
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preliminary steps to
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I.1. Identifying source material and framerate
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The first and most important step before you encode should be
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determining what type of content you're dealing with. If your source
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material comes from DVD or broadcast/cable/satellite TV, it will be
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stored in one of two formats: NTSC for North America and Japan, and
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PAL for Europe, etc. But it's important to realize that this is just
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the formatting for presentation on a television, and often does NOT
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correspond to the original format of the movie. In order to produce a
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suitable encode, you need to know the original format. Failure to take
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this into account will result in ugly combing (interlacing) artifacts
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in your encode, and will greatly reduce the quality/bitrate ratio of
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the encoder!
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Here is a list of common types of source material, where you're likely
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to find them, and their properties:
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Standard Film: Produced for theatrical display at 24fps.
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PAL video: Recorded with a PAL video camera at 50 fields per second. A
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field consists of just the even or odd numbered lines of a frame.
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Television was designed to refresh these in alternation as a cheap
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form of analog compression. The human eye supposedly compensates for
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this, but once you understand interlacing you'll learn to see it on TV
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too and never enjoy TV again. Two fields do NOT make a complete frame,
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because they are captured 1/50 of a second apart in time, and thus
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they do not line up unless there is no motion.
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NTSC Video: Recorded with an NTSC video camera at 59.94 fields per
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second, or 60 fields per second in the pre-color era. Otherwise
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similar to PAL.
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Animation: Usually drawn at 24fps, but animation also comes in
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mixed-framerate varieties.
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Computer Graphics (CG): Can be any framerate, but 24 and 30 fps are
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the most frequently encountered in NTSC regions, and 25 fps in PAL
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regions.
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Old Film: Various lower framerates.
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Movies consisting of frames are referred to as progressive, while
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those consisting of independent fields are called interlaced, or
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sometimes video, although this latter term is ambiguous.
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To further complicate matters, some movies will be a mix of several of
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the above.
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The most important distinction to make between all of these formats is
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that some are frame-based, while others are field-based. WHENEVER a
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movie is prepared for display on television (including DVD), it is
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converted to a field-based format. The various methods by which this
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can be done are collectively referred to as "pulldown", of which the
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infamous NTSC "3:2 telecine" is one variety. Unless the original
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material was also field-based (and the same fieldrate), you are
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getting the movie in a format other than the original.
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There are several common types of pulldown:
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PAL 2:2 pulldown: The nicest of them all. Each frame is shown for two
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fields duration, by extracting the even and odd lines and showing them
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in alternation. If the original material is 24fps, this process speeds
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up the movie by 4%.
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PAL 2:2:2:2:2:2:2:2:2:2:2:3 pulldown: Every 12th frame is shown for
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three fields duration, instead of just two. This avoids the 4% speedup
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issue, but makes the process much more difficult to reverse. It is
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usually seen in musical productions where adjusting the speed by 4%
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would seriously damage the musical score.
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NTSC 3:2 telecine: Frames are shown alternatively for 3 fields or 2
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fields duration. This gives a fieldrate 5/2 times the original
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framerate. The result is also slowed down very slightly from 60 fields
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per second to 59.94 fields per second to maintain NTSC fieldrate.
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NTSC 2:2 pulldown: Used for showing 30fps material on NTSC. Nice, just
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like 2:2 PAL pulldown.
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There are also methods for converting between NTSC and PAL video. Such
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topics are beyond the scope of this guide. If you encounter such a
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movie and want to encode it, your best bet is to find a copy in the
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original format. NTSC/PAL conversion is highly destructive and cannot
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be reversed cleanly, so your encode will greatly suffer if it is made
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from a converted source.
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When video is stored on DVD, consecutive pairs of fields are grouped
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as a frame, even though they are not intended to be shown at the same
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moment in time. The MPEG2 standard used on DVD and digital TV provides
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a way to encode the original progressive frames, and store the number
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of fields for which each should be shown in the frame headers. If this
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method has been used, the term "soft telecine" will often be used to
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describe the movie, since the process only directs the DVD player to
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apply pulldown to the movie rather than altering the movie itself.
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This case is highly preferable since it can easily be reversed
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(actually ignored) by the encoder, and since it preserves maximal
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quality. However, many DVD and broadcast production studios do not use
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proper encoding techniques, and instead produce movies with "hard
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telecine", where fields are actually duplicated in the encoded MPEG2.
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The procedures for dealing with these cases will be covered later in
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this guide. For now, we leave you with some guides to identifying
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which type of material you're dealing with:
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NTSC regions:
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- If MPlayer prints that the framerate has changed to 23.976 when
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watching your movie, and never changes back, it's almost certainly
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24fps content that has been "soft telecined".
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- If MPlayer shows the framerate switching back and forth between
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23.976 and 29.97, and you see "combing" at times, then there are
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several possibilities. The 23.976 fps segments are almost certainly
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24fps progressive content, "soft telecined", but the 29.97 fps parts
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could be either hard-telecined 24fps content or NTSC video content.
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Use the same guidelines as the following two cases to determine
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which.
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- If MPlayer never shows the framerate change, and every single frame
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with motion appears combed, your movie is NTSC video at 59.94 fields
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per second.
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- If MPlayer never shows the framerate change, and two frames out of
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every five appear combed, your movie is "hard telecined" 24fps
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content.
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PAL regions:
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- If you never see any combing, your movie is 2:2 pulldown.
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- If you see combing alternating in and out every half second, then
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your movie is 2:2:2:2:2:2:2:2:2:2:2:3 pulldown.
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- If you always seem combing during motion, then your movie is PAL
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video at 50 fields per second.
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Hint: MPlayer can slow down movie playback with the -speed option. Try
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using -speed 0.2 to watch the movie very slowly and identify the
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pattern, if you can't see it at full speed.
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I.2. Selecting the quality you want
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It's possible to encode your movie at a wide range of qualities. With
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modern video encoders and a bit of pre-codec compression (downscaling
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and denoising), it's possible to achieve very good quality at 700 MB,
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for a 90-110 minute widescreen movie. And all but the longest movies
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can be encoded with near-perfect quality at 1400 MB.
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If you do not plan to store your movies on CD or other size-limited
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media, and you want maximal quality at all costs, you can encode in
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constant quantizer mode, which will not aim to meet a specific target
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bitrate or filesize but instead use the maximal accuracy encoding for
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all frames. This is not recommended in most cases, because you can
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achieve significantly smaller file sizes without noticeable loss.
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However, it may be desirable for the hardcore archivists out there.
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I.4. Cropping and scaling
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Recall from the previous section that the final picture size you
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encode should be a multiple of 16 (in both width and height). This can
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be achieved by cropping, scaling, or a combination of both.
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When cropping, there are a few guidelines that must be followed to
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avoid damaging your movie. The normal YUV format, 4:2:0, stores chroma
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(color) information subsampled, i.e. chroma is only sampled half as
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often in each direction as luma (intensity) information. Observe this
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diagram, where L indicates luma sampling points and C chroma.
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L L L L L L L L
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C C C C
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L L L L L L L L
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L L L L L L L L
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C C C C
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L L L L L L L L
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As you can see, rows and columns of the image naturally come in pairs.
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Thus your crop offsets and dimensions MUST be even numbers. If they
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are not, the chroma will no longer line up correctly with the luma. In
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theory, it's possible to crop with odd offsets, but it requires
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resampling the chroma which is potentially a lossy operation and not
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supported by the crop filter.
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Further, interlaced video is sampled as follows:
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TOP FIELD BOTTOM FIELD
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L L L L L L L L
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C C C C
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L L L L L L L L
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L L L L L L L L
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C C C C
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L L L L L L L L
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L L L L L L L L
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C C C C
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L L L L L L L L
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L L L L L L L L
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C C C C
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L L L L L L L L
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As you can see, the pattern does not repeat until after 4 lines. So
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for interlaced video, your y-offset and height for cropping must be
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multiples of 4.
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So how do you determine a crop rectangle to begin with? Sometimes you
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can guess, but the cropdetect filter in MPlayer can make it easy. Run
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MPlayer with -vf cropdetect and it will print out the crop settings to
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remove the borders. You should let the movie run long enough that the
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whole picture area is used, in order to get accurate crop values.
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Then, test the values you get with MPlayer, using the command line
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cropdetect printed, and adjust the rectangle as needed. The rectangle
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filter can help by allowing you to interactively position the crop
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rectangle over your movie. Remember to follow the above divisibility
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guidelines so that you do not misalign the chroma planes.
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If you will be scaling your movie, it's usually best to crop only the
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black borders and noise, then scale so that the resulting dimensions
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are multiples of 16. This can slightly distort the aspect ratio of
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your movie, but in practice the error cannot be seen. It's certainly
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much less visible than the MPEG artifacts you will see from failing to
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crop & scale well.
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In certain cases, scaling may be undesirable. Scaling in the vertical
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direction is difficult with interlaced video, and if you wish to
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preserve the interlacing, you should usually refrain from scaling. If
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you will not be scaling but you still want to use multiple-of-16
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dimensions, you will have to overcrop. Do not undercrop, since black
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borders are very bad for encoding!
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I.5. Choosing resolution and bitrate
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If you will not be encoding in constant quantizer mode, you need to
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select a bitrate. The concept of bitrate is quite simple. It's the
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(average) number of bits that will be consumed to store your movie,
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per second. Normally bitrate is measured in kilobits (1000 bits) per
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second. The size of your movie on disk is the bitrate times the length
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of the movie in time, plus a small amount of "overhead" (see the
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section on codecs and containers). Other parameters such as scaling,
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cropping, etc. will NOT alter the file size unless you change the
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bitrate as well!
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Bitrate does NOT scale proportional to resolution. That is to say, a
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320x240 file at 200 kbit/sec will not be the same quality as the same
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movie at 640x480 and 800 kbit/sec! There are two reasons for this:
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1. Perceptual: You notice MPEG artifacts more if they're scaled up
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bigger! Artifacts appear on the scale of blocks (8x8). Your eye
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will not see errors in 4800 small blocks as easily as it sees
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errors in 1200 large blocks (assuming you'll be scaling both to
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fullscreen).
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2. Theoretical: When you scale down an image but still use the same
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size (8x8) blocks for the frequency space transform, you move more
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data to the high frequency bands. Roughly speaking, each pixel
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contains more of the detail than it did before. So even though your
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scaled-down picture contains 1/4 the information in the spacial
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directions, it could still contain a large portion of the
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information in the frequency domain (assuming that the high
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frequencies were underutilized in the original 640x480 image).
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Past guides have recommended choosing a bitrate and resolution based
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on a "bits per pixel" approach, but this is usually not valid due to
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the above reasons. A better estimate seems to be that bitrates scale
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proportional to the square root of resolution, so that 320x240 and 400
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kbit/sec would be comparable to 640x480 at 800 kbit/sec. However this
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has not been verified with theoretical or empirical rigor. Further,
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given that movies vary greatly with regard to noise, detail, degree of
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motion, etc., it's futile to make general recommendations for bits per
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length-of-diagonal (the analogue of bits per pixel, using the square
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root).
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So far we have discussed the difficulty of choosing a bitrate and
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resolution.
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.................
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II. Containers and codecs
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@ -364,353 +67,6 @@ 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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II.2. Constraints of DVD, SVCD, and VCD
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Unfortunately, the DVD, SVCD, and VCD formats are subject to heavy
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constraints. Only a small selection of encoded picture sizes & aspect
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ratios are available. If your movie does not meet one of these, you
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must scale and crop or add black borders (which are bad for quality!)
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to make it compliant.
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Format Resolution V.Codec A.Codec FPS Aspect
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NTSC DVD 720x480 MPEG-2 AC3,PCM 24,30 4:3,16:9
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NTSC DVD 352x240 * MPEG-1 AC3,PCM 24,30 4:3
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NTSC SVCD 480x480 MPEG-2 MP2 30 4:3
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NTSC VCD 352x240 MPEG-1 MP2 24,30 4:3
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PAL DVD 720x576 MPEG-2 MP2,AC3,PCM 25 4:3,16:9
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PAL DVD 352x288 * MPEG-1 MP2,AC3,PCM 25 4:3
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PAL SVCD 480x576 MPEG-2 MP2 25 4:3
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PAL VCD 352x288 MPEG-1 MP2 25 4:3
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* These resolutions are rarely used in DVD because they are fairly low
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quality.
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DVD, VCD, and SVCD also constrain you to relatively low GOP sizes. 18 is
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supposed to be the largest allowed GOP size for 30 fps NTSCP material;
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for 25 or 24 fps, the GOP size should be 15.
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VCD video is required to be CBR at 1152 kbps. This highly limiting
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constraint also comes along with an extremly low vbv buffer size of
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327 kilobits. SVCD allows varying video bitrates up to 2500 kbps, and
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a somewhat less insane vbv buffer size of 917 kilobits is allowed. DVD
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video bitrates may range anywhere up to 9800 kbps (though typical bitrates
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are about half that), and the vbv buffer size is 1835 kilobits.
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Here is a list of fields in lavcopts that you may be required to change
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in order to make usable video for VCD, SVCD, or DVD:
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acodec: mp2 for VCD, SVCD, or PAL DVD; ac3 is most commonly used for DVD.
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PCM audio may also be used for DVD, but this is mostly a big
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waste of space. Note that mp3 audio isn't spec-compliant for
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any of these formats, but players often have no problem playing
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it anyway.
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abitrate: 224 for VCD; user-selectable for DVD and SVCD, but commonly used
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values range from 192 to 384 kbps.
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vcodec: mpeg1video for VCD; mpeg2video for SVCD; mpeg2video is usually
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used for DVD but you may also use mpeg1video for CIF resolutions.
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keyint: 18 for 30fps material, or 15 for 25/24 fps material. Commercial
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producers seem to prefer keyframe intervals of 12.
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vrc_buf_size: 327 for VCD, 917 for SVCD, and 1835 for DVD.
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vrc_minrate: 1152, for VCD. May be left alone for SVCD and DVD.
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vrc_maxrate: 1152 for VCD; 2500 for SVCD; 9800 for DVD. For SVCD and DVD,
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you might wish to use lower values depending on your own
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personal preferences and requirements.
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vbitrate: 1152 for vcd; up to 2500 for SVCD; up to 9800 for DVD. For the
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latter two formats, vbitrate should be set based on personal
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preference. For instance, if you insist on fitting 20 or so
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hours on a DVD, you could use vbitrate=400. The resulting
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video quality would probably be quite bad. If you are trying
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to squeeze out the maximum possible quality on a DVD, use
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vbitrate=9800, but be warned that this could constrain you to
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less than an hour of video on a single-layer DVD.
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Here is a typical minimum set of lavcopts for encoding video for a VCD:
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-lavcopts vcodec=mpeg1video:vrc_buf_size=327:vrc_minrate=1152:\
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vrc_maxrate=1152:vbitrate=1152:keyint=15:acodec=mp2
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SVCD:
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-lavcopts vcodec=mpeg2video:vrc_buf_size=917:vrc_maxrate=2500:vbitrate=1800:\
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keyint=15:acodec=mp2
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DVD:
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-lavcopts vcodec=mpeg2video:vrc_buf_size=1835:vrc_maxrate=9800:\
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vbitrate=5000:keyint=15:acodec=ac3
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For higher quality encoding, you may also wish to add quality-enhancing
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options to lavcopts, such as trell, mbd=2, and others. Note that qpel
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and v4mv, while often useful with MPEG-4, are not usable in MPEG-1 or
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MPEG-2. Also, if you are trying to make a very high quality DVD encode,
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it may be useful to add dc=10 to lavcopts. Doing so may help reduce the
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appearance of blocks in flat-colored areas. Putting it all together,
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here is an example of a set of lavcopts for a higher quality DVD:
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-lavcopts vcodec=mpeg2video:vrc_buf_size=1835:vrc_maxrate=9800:\
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vbitrate=8000:keyint=15:trell:mbd=2:precmp=2:subcmp=2:cmp=2:dia=-10:\
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predia=-10:cbp:mv0:vqmin=1:lmin=1:dc=10
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If your movie has 2.35:1 aspect (most recent action movies), you will
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have to add black borders or crop the movie down to 16:9 to make a DVD
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or VCD. If you add black borders, try to align them at 16-pixel
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boundaries in order to minimize the impact on encoding performance.
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Thankfully DVD has sufficiently excessive bitrate that you do not have
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to worry too much about encoding efficiency, but SVCD and VCD are
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highly bitrate-starved and require effort to obtain acceptable
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quality.
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II.3. Limitations of the AVI container
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Although it's the most widely-supported format after MPEG-1, AVI also
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has some major drawbacks. Perhaps the most obvious is the overhead.
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For each chunk of the AVI file, 24 bytes are wasted on headers and
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index. This translates into a little over 5 MB per hour, or 1-2.5%
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overhead for a 700 MB movie. This may not seem like much, but it could
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mean the difference between being able to use 700 kbit/sec video or
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714 kbit/sec, and every bit of quality counts.
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||||
|
||||
In addition to gross inefficiency, AVI also has the following major
|
||||
limitations:
|
||||
|
||||
1. Only fixed-fps content can be stored. This is particularly limiting
|
||||
if the original material you want to encode is mixed content, for
|
||||
example a mix of NTSC video and film material. Actually there are
|
||||
hacks that can be used to store mixed-framerate content in AVI, but
|
||||
they increase the (already huge) overhead fivefold or more so they
|
||||
are not practical.
|
||||
|
||||
2. Audio in AVI files must be either constant-bitrate (CBR) or
|
||||
constant-framesize (i.e. all frames decode to the same number of
|
||||
samples). Unfortunately, the most efficient codec, Vorbis, does not
|
||||
meet either of these requirements. Therefore, if you plan to store
|
||||
your movie in AVI, you'll have to use a less efficient codec such
|
||||
as MP3 or AC3.
|
||||
|
||||
With all of that said, MEncoder does not support variable-fps output
|
||||
or Vorbis encoding. Therefore, you may not see these as limitations if
|
||||
MEncoder is the only tool you will be using to produce your encodes.
|
||||
However, it is possible to use MEncoder only for the video encoding,
|
||||
and then use external tools to encode the audio and mux it into
|
||||
another container format.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
III. Basic MEncoder usage
|
||||
|
||||
III.1. Selecting codecs & format
|
||||
|
||||
Audio and video codecs for encoding are selected with the -oac and
|
||||
-ovc options, respectively. The following choices are available,
|
||||
although some may not have been enabled at compiletime:
|
||||
|
||||
Audio Codecs
|
||||
mp3lame Encode VBR or CBR MP3 with LAME
|
||||
lavc Use one of libavcodec's audio encoders
|
||||
pcm Uncompressed PCM audio
|
||||
copy Do not reencode, just copy compressed frames
|
||||
|
||||
Video codecs
|
||||
lavc Use one of libavcodec's video encoders
|
||||
xvid XviD
|
||||
raw Uncompressed video frames
|
||||
copy Do not reencode, just copy compressed frames
|
||||
frameno Used for 3-pass encoding (not recommended)
|
||||
|
||||
Several other video codecs are available, but not recommended. The
|
||||
lavc audio and video encoders have additional suboptions to select
|
||||
which codec to use within lavc. The syntax is:
|
||||
|
||||
-lavcopts acodec=audio_codec_name
|
||||
-lavcopts vcodec=video_codec_name
|
||||
|
||||
Your choices for lavc audio are mp2, ac3, and various adpcm formats
|
||||
(low efficiency). For lavc video, you have many more choices:
|
||||
|
||||
mpeg1video MPEG-1 video
|
||||
mpeg2video MPEG-2 video
|
||||
mpeg4 MPEG-4 video, standards-compliant
|
||||
msmpeg4 Pre-standard MPEG-4 used by MS (aka DivX3)
|
||||
msmpeg4v2 Pre-standard MPEG-4 used by MS (low quality)
|
||||
msmpeg4v1 Pre-standard MPEG-4 used by MS (low quality)
|
||||
wmv1 Windows Media Video, V1 (aka WMV7)
|
||||
wmv2 Windows Media Video, V2 (aka WMV8)
|
||||
dvvideo DV video (used by DV cameras)
|
||||
mjpeg Motion JPEG
|
||||
ljpeg Lossless JPEG
|
||||
ffv1 Lossless FFmpeg video codec #1 (slow)
|
||||
huffyuv A standard lossless codec
|
||||
|
||||
...and lots more that aren't worth mentioning for most people.
|
||||
|
||||
|
||||
|
||||
III.2. Selecting input file or device
|
||||
|
||||
MEncoder can encode from files or directly from a DVD or VCD disc.
|
||||
Simply include the filename on the command line to encode from a file,
|
||||
or dvd://titlenumber or vcd://tracknumber to encode from a DVD title
|
||||
or VCD track. If you have already copied a DVD to your hard drive and
|
||||
wish to encode from the copy, you should still use the dvd:// syntax,
|
||||
along with -dvd-device followed by the path to the copied DVD root.
|
||||
The -dvd-device and -cdrom-device options can also be used to override
|
||||
the paths to the device nodes for reading directly from disc, if the
|
||||
defaults of /dev/dvd and /dev/cdrom do not work on your system.
|
||||
|
||||
When encoding from DVD, it is often desirable to select a chapter or
|
||||
range of chapters to encode. You can use the -chapter option for this
|
||||
purpose. For example, -chapter 1-4 will only encode chapters 1 through
|
||||
4 from the DVD. This is especially useful if you will be making a 1400
|
||||
MB encode targetted for two CDs, since you can ensure the split occurs
|
||||
exactly at a chapter boundary rather than in the middle of a scene.
|
||||
|
||||
If you have a supported TV capture card, you can also encode from the
|
||||
TV-in device. Use tv://channelnumber as the filename, and -tv to
|
||||
configure various capture settings. DVB input works similarly.
|
||||
|
||||
|
||||
III.3. Loading video filters
|
||||
|
||||
Learning how to use MEncoder's video filters is essential to producing
|
||||
good encodes. All video processing is performed through the filters --
|
||||
cropping, scaling, color adjustment, noise removal, sharpening,
|
||||
deinterlacing, telecine, inverse telecine, and deblocking, just to
|
||||
name a few. Along with the vast number of supported input formats, the
|
||||
variety of filters available in MEncoder is one of its main advantages
|
||||
over other similar programs.
|
||||
|
||||
Filters are loaded in a chain using the -vf option:
|
||||
|
||||
-vf filter1=options,filter2=options,...
|
||||
|
||||
Most filters take several numeric options separated by colons, but the
|
||||
syntax for options varies from filter to filter, so read the man page
|
||||
for details on the filters you wish to use.
|
||||
|
||||
Filters operate on the video in the order they are loaded. For
|
||||
example, the following chain:
|
||||
|
||||
-vf crop=688:464:12:4,scale=640:464
|
||||
|
||||
will first crop the 688x464 region of the picture with upper-left
|
||||
corner at (12,4), and then scale the result down to 640x464.
|
||||
|
||||
Certain filters need to be loaded at or near the beginning of the
|
||||
filter chain, in order to take advantage of information from the video
|
||||
decoder that will be lost or invalidated by other filters. The
|
||||
principal examples are pp (postprocessing, only when it is performing
|
||||
deblock or dering operations), spp (another postprocessor to remove
|
||||
MPEG artifacts), pullup (inverse telecine), and softpulldown (for
|
||||
converting soft telecine to hard telecine).
|
||||
|
||||
Advanced topics in filter chains and usage information for specific
|
||||
filters will follow in chapters IV and V, as they are needed for the
|
||||
topics covered.
|
||||
|
||||
|
||||
|
||||
III.4. Notes on A/V sync
|
||||
|
||||
MEncoder's audio/video synchronization algorithms were designed with
|
||||
the intention of recovering files with broken sync. However they seem
|
||||
to cause unnecessary skipping and duplication of frames, and possibly
|
||||
slight A/V desync, when used with proper input. It is therefore
|
||||
recommended that you switch to basic A/V sync with the -mc 0 option,
|
||||
or put this in your ~/.mplayer/mencoder config file, as long as you
|
||||
are only working with good sources (DVD, TV capture, high quality
|
||||
MPEG-4 rips, etc) and not broken ASF/RM/MOV files.
|
||||
|
||||
If you want to further guard against strange frame skips and
|
||||
duplication, you can use both -mc 0 and -noskip. This will prevent ALL
|
||||
A/V sync, and copy frames one-to-one, so you cannot use it if you will
|
||||
be using any filters that unpredictably add or drop frames, or if your
|
||||
input file has variable framerate! Therefore, using -noskip is not in
|
||||
general recommended.
|
||||
|
||||
The so-called "three-pass" encoding which MEncoder supports has been
|
||||
reported to cause A/V desync. This will definitely happen if it is
|
||||
used in conjunction with certain filters, therefore, it is now
|
||||
recommended NOT to use three-pass mode. This feature is only left for
|
||||
compatibility purposes and for expert users who understand when it is
|
||||
safe to use and when it is not. If you have never heard of three-pass
|
||||
mode before, forget that we even mentioned it!
|
||||
|
||||
There have also been reports of A/V desync when encoding from stdin
|
||||
with MEncoder. Do not do this! Always use a file or CD/DVD/etc device
|
||||
as input.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
IV.1. Encoding progressive video
|
||||
|
||||
As long as your input video is progressive (see section I.1),
|
||||
|
||||
|
||||
Let's finally see a few examples:
|
||||
|
||||
Encoding from 2:2 pulldown PAL DVD, title 1
|
||||
2.35:1 picture aspect
|
||||
1200 kbit/sec MPEG-4 video
|
||||
128 kbit/sec average-bitrate MP3 audio
|
||||
|
||||
mencoder dvd://1 -vf crop=712:432,scale=640:288 -mc 0 -oac mp3lame\
|
||||
-lameopts abr:br=128 -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1200
|
||||
|
||||
The crop size was presumably obtained by using the cropdetect filter
|
||||
in MPlayer, or experimenting first with crop rectangles in MPlayer.
|
||||
The output framerate will be 25 fps, the same as the original DVD. It
|
||||
would be preferable to adjust the playback speed to match the original
|
||||
24 fps theatrical rate, but this is not yet possible with MEncoder.
|
||||
The options we pass to libavcodec are the bare minimum, and will yield
|
||||
relatively poor quality. We will refine then in subsequent sections.
|
||||
|
||||
Now, a second example:
|
||||
|
||||
Encoding from soft-telecined NTSC DVD, title 3
|
||||
2.35:1 picture aspect
|
||||
900 kbit/sec MPEG-4 video
|
||||
Keeping the original AC3 audio
|
||||
|
||||
mencoder dvd://1 -vf crop=708:360,scale=640:288 -mc 0 -oac copy \
|
||||
-ovc lavc -lavcopts vcodec=mpeg4:vbitrate=900 -ofps 23.976023976
|
||||
|
||||
This example is very similar to the first example, except for the
|
||||
-ofps option to adjust the output framerate. Unless you tell it
|
||||
otherwise, MEncoder takes its output framerate from the input
|
||||
framerate. This is reported as 29.97 fps (actually 30000/1001), or
|
||||
rather, 29.97 pairs of fields per second. But since the DVD is
|
||||
soft-telecined, 1/5 of these fields are not actually present, but
|
||||
intended to be added by the player when it telecines the movie in
|
||||
realtime. There are actually only 23.976 (24000/1001) frames per
|
||||
second. If you leave the framerate at the default, 29.97, it will
|
||||
still work, but every 4th frame will get encoded in duplicate, making
|
||||
the motion appear choppy.
|
||||
|
||||
Finally, a comment on the number 23.976023976. You'll often see
|
||||
recommendations to use -ofps 23.976, but this is wrong. MEncoder will
|
||||
reduce 23.976 to 2997/125, which is not the same as 24000/1001. So in
|
||||
order to get the right framerate written in the output file's header,
|
||||
always use plenty of precision.
|
||||
|
||||
|
||||
|
||||
|
||||
IV.2. Two-pass encoding
|
||||
|
||||
@ -780,40 +136,3 @@ Next, an example using XviD instead of libavcodec:
|
||||
|
||||
The options used are slightly different, but the process is otherwise
|
||||
the same.
|
||||
|
||||
|
||||
|
||||
|
||||
IV.3. Encoding interlaced video
|
||||
|
||||
If the movie you want to encode is interlaced (NTSC video or PAL
|
||||
video), you will need to choose whether you want to deinterlace or
|
||||
not. While deinterlacing will make your movie usable on progressive
|
||||
scan displays such a computer monitors and projectors, it comes at a
|
||||
cost: the field rate of 50 or 59.94 fields per second is halved to 25
|
||||
or 29.97 frames per second, and roughly half the information in your
|
||||
movie will be lost during scenes with significant motion.
|
||||
|
||||
Therefore, if you are encoding for high quality archival purposes, it
|
||||
is recommended not to deinterlace. You can always deinterlace the
|
||||
movie at playback time when displaying it on progressive scan devices,
|
||||
and future players will be able to deinterlace to full fieldrate,
|
||||
interpolating 50 or 59.94 entire frames per second from the interlaced
|
||||
video.
|
||||
|
||||
Special care must be taken when working with interlaced video:
|
||||
|
||||
1. Crop height and y-offset must be multiples of 4.
|
||||
|
||||
2. Any vertical scaling must be performed in interlaced mode.
|
||||
|
||||
3. Postprocessing and denoising filters may not work as expected
|
||||
unless you take special care to operate them a field at a time, and
|
||||
they may damage the video if used incorrectly.
|
||||
|
||||
With these things in mind, here is our first example:
|
||||
|
||||
mencoder capture.avi -mc 0 -oac lavc -ovc lavc -lavcopts \
|
||||
vcodec=mpeg2video:vbitrate=6000:ilme:ildct:acodec=mp2:abitrate=224
|
||||
|
||||
Note the ilme and ildct options.
|
||||
|
Loading…
Reference in New Issue
Block a user