Doc / menc-feat-telecine
2010-08-23 (月) 20:33:13更新
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原文:http://www.mplayerhq.hu/DOCS/HTML/en/menc-feat-telecine.html

14.2. How to deal with telecine and interlacing within NTSC DVDs

14.2.1. イントロダクション

テレシネとはなんぞや? このページが難しかったらここを読んでみよう。:http://www.divx.com/support/guides/guide.php?gid=10 ここにはテレシネの総合的で解り易い説明があります。 数字に関する注意 上で紹介したサイトもそうですが、多くの解説ではNTSCビデオのfields per secondを59.94、そしてそれに対応する frames per second は 29.97 (テレシネ&インターレースされた素材) と 23.976 (プログレッシブ素材)だとしています。解り易さのために、60, 30, 24としているサイトもあります。

厳密にはこれらは近似値です。白黒のNTSCビデオはぴったり60 fields per secondでしたが、カラー放送になってからは60000/1001です。白黒テレビでも映るようにするために少しフィールドを減したわけです。デジタル NTSC ビデオ (例えばDVD) も、60000/1001 fields per secondです。そんなわけで、テレシネ&インターレースされた映像は30000/1001 frames per second、プログレッシブ映像は24000/1001 frames per secondになっています。

古いMEncoder documentや mailing list のアーカイブでは 59.94, 29.97, 23.976を使っていましたが、MEncoder documentは全て分数表記に書き直されました。これからはあなたも分数を使って下さい。

-ofps 23.976 よりも、-ofps 24000/1001 のほうが正確です。

テレシネはどのように使われているのでしょうか。TVに映す映像は全て、60000/1001 field per secondでなくてはなりません。TV番組は元々60000/1001 field per secondで撮影される事が多いですが、大半の映画は24、または、24000/1001 frames per secondで撮影されます。従って、映画をDVDにマスタリングする際には、映像はテレシネと呼ばれる変換をかけられます。

実際のDVDの中に、映像が60000/1001 fields per secondで保存されている事は決してありません。 オリジナルが60000/1001の映像は、フィールドのペアが一枚のフレームに連結されていますので、30000/1001 frames per secondになっています。 従って、ハードウェアDVDプレイヤは、奇数フィールドと偶数フィールドのどちらを1stフィールドに使うかを決める為に、ビデストリームの中に埋め込まれたフラグを読む必要があります。

一般的には、24000/1001 frames per secondの映像は、DVD用にエンコードされた時の状態のままです。DVDプレイヤはオン・ザ・フライでテレシネ変換をする必要が有ります。しかし、中には、DVDに入れる前の段階で映像がテレシネ処理を受けている事が有ります;この場合は、例えオリジナルが24000/1001 frames per secondだったとしても、 60000/1001 fields per secondに変換されているわけです。こうしたものがDVDに入っている場合、フィールドのペアが一枚のフレームに連結された30000/1001 frames per secondになります。

60000/10001 fields per secondの個々のフレームを見ると、テレシネされていようとなかろうと、インターレースは動きのある箇所では、はっきりと視認する事が出来ます。なぜなら、フィールドは(仮に偶数ラインとすると)別のフィールドより1/(60000/1001)秒あとの瞬間の映像だからです。インターレースド映像をコンピュータで見ると汚い理由は二つあります。第一にモニタの解像度が高い事、第二に、映像がフィールド単位ではなく、フレーム単位で表示される事です。

Notes:

  • このセクションはNTSC DVDについて書かれた物であり、PALには該当しません。
  • このドキュメントで示されるMEncoderのコマンド例は実用を目的とはしていません。ここの例は映像のカテゴリ毎に必要な最低限のものを例示するに過ぎません。良いDVDリップのやり方や、libavcodecの高画質向けチューニングはこのドキュメントの目的ではありません。
  • このガイド特有の脚注は[1]のような形で示しています。

14.2.2. 映像のタイプを見分ける方法


14.2.2.1. プログレッシブ

プログレッシブ・ビデオのオリジナルの再生速度は24000/1001 fpsです。これはDVDの中でもそのままです。

プログレッシブDVDを再生する場合、MPlayerは再生開始後すぐに以下のような表示を出します。

demux_mpg: 24000/1001 fps progressive NTSC content detected, switching framerate.

この表示以降、demux_mpgの表示が"30000/1001 fps NTSC content."になることはありません。

プログレッシブ・ビデオではインターレースが見える事はありません。
しかし、稀に予期せぬ部分でテレシネミックスされた箇所がある事があります。私は場面転換の度か、ランダムに1秒のテレシネ変換が入ったTV show DVDを見た事があります。また、前半分がプログレッシブで後半はテレシネされたDVDというのも見た事があります。徹底的に行くなら素材映像を全部スキャンしておこう。

mplayer dvd://1 -nosound -vo null -benchmark

-benchmarkを使うとMPlayerは可能な限りの速さでムービーを再生します。ハードウェアによってはしばらくかかります。demux_mpgがフレームレートの変更を検出するたびに、変更時刻を直前の行に表示します。

DVD中のプログレッシブ映像は"ソフトテレシネ"と呼ばれる事もあります。これはDVDプレイヤがテレシネする事を想定しているからです。

14.2.2.2. Telecined

Telecined video was originally filmed at 24000/1001, but was telecined before it was written to the DVD.

MPlayer does not (ever) report any framerate changes when it plays telecined video.

Watching a telecined video, you will see interlacing artifacts that seem to "blink": they repeatedly appear and disappear. You can look closely at this by

  1. mplayer dvd://1
  2. Seek to a part with motion.
  3. Use the . key to step forward one frame at a time.
  4. Look at the pattern of interlaced-looking and progressive-looking frames. If the pattern you see is PPPII,PPPII,PPPII,... then the video is telecined. If you see some other pattern, then the video may have been telecined using some non-standard method; MEncoder cannot losslessly convert non-standard telecine to progressive. If you do not see any pattern at all, then it is most likely interlaced.

Sometimes telecined video on DVDs is referred to as "hard-telecine". Since hard-telecine is already 60000/1001 fields per second, the DVD player plays the video without any manipulation.

Another way to tell if your source is telecined or not is to play the source with the -vf pullup and -v command line options to see how pullup matches frames. If the source is telecined, you should see on the console a 3:2 pattern with 0+.1.+2 and 0++1 alternating. This technique has the advantage that you do not need to watch the source to identify it, which could be useful if you wish to automate the encoding procedure, or to carry out said procedure remotely via a slow connection.

14.2.2.3. Interlaced

Interlaced video was originally filmed at 60000/1001 fields per second, and stored on the DVD as 30000/1001 frames per second. The interlacing effect (often called "combing") is a result of combining pairs of fields into frames. Each field is supposed to be 1/(60000/1001) seconds apart, and when they are displayed simultaneously the difference is apparent.

As with telecined video, MPlayer should not ever report any framerate changes when playing interlaced content.

When you view an interlaced video closely by frame-stepping with the . key, you will see that every single frame is interlaced.

14.2.2.4. Mixed progressive and telecine

All of a "mixed progressive and telecine" video was originally 24000/1001 frames per second, but some parts of it ended up being telecined.

When MPlayer plays this category, it will (often repeatedly) switch back and forth between "30000/1001 fps NTSC" and "24000/1001 fps progressive NTSC". Watch the bottom of MPlayer's output to see these messages.

You should check the "30000/1001 fps NTSC" sections to make sure they are actually telecine, and not just interlaced.


14.2.2.5. Mixed progressive and interlaced

In "mixed progressive and interlaced" content, progressive and interlaced video have been spliced together.

This category looks just like "mixed progressive and telecine", until you examine the 30000/1001 fps sections and see that they do not have the telecine pattern.

14.2.3. How to encode each category

頭書の通り、以下のコマンド例は実際の使用を意図していない。
これらは各カテゴリを適切にエンコードするために必要な最低限の例に過ぎない。

14.2.3.1. Progressive

Progressive video requires no special filtering to encode. The only parameter you need to be sure to use is -ofps 24000/1001. Otherwise, MEncoder will try to encode at 30000/1001 fps and will duplicate frames.

mencoder dvd://1 -oac copy -ovc lavc -ofps 24000/1001

It is often the case, however, that a video that looks progressive actually has very short parts of telecine mixed in. Unless you are sure, it is safest to treat the video as mixed progressive and telecine. The performance loss is small [3].

14.2.3.2. Telecined

Telecine can be reversed to retrieve the original 24000/1001 content, using a process called inverse-telecine. MPlayer contains several filters to accomplish this; the best filter, pullup, is described in the mixed progressive and telecine section.


14.2.3.3. Interlaced

For most practical cases it is not possible to retrieve a complete progressive video from interlaced content. The only way to do so without losing half of the vertical resolution is to double the framerate and try to "guess" what ought to make up the corresponding lines for each field (this has drawbacks - see method 3).

  1. Encode the video in interlaced form. Normally, interlacing wreaks havoc with the encoder's ability to compress well, but libavcodec has two parameters specifically for dealing with storing interlaced video a bit better: ildct and ilme. Also, using mbd=2 is strongly recommended [2] because it will encode macroblocks as non-interlaced in places where there is no motion. Note that -ofps is NOT needed here.
    mencoder dvd://1 -oac copy -ovc lavc -lavcopts ildct:ilme:mbd=2
  2. Use a deinterlacing filter before encoding. There are several of these filters available to choose from, each with its own advantages and disadvantages. Consult mplayer -pphelp to see what is available (grep for "deint"), and search the MPlayer mailing lists to find many discussions about the various filters. Again, the framerate is not changing, so no -ofps. Also, deinterlacing should be done after cropping [1] and before scaling.
    mencoder dvd://1 -oac copy -vf pp=lb -ovc lavc
  3. Unfortunately, this option is buggy with MEncoder; it ought to work well with MEncoder G2, but that is not here yet. You might experience crahes. Anyway, the purpose of -vf tfields is to create a full frame out of each field, which makes the framerate 60000/1001. The advantage of this approach is that no data is ever lost; however, since each frame comes from only one field, the missing lines have to be interpolated somehow. There are no very good methods of generating the missing data, and so the result will look a bit similar to when using some deinterlacing filters. Generating the missing lines creates other issues, as well, simply because the amount of data doubles. So, higher encoding bitrates are required to maintain quality, and more CPU power is used for both encoding and decoding. tfields has several different options for how to create the missing lines of each frame. If you use this method, then Reference the manual, and chose whichever option looks best for your material. Note that when using tfields you have to specify both -fps and -ofps to be twice the framerate of your original source.
    mencoder dvd://1 -oac copy -vf tfields=2 -ovc lavc -fps 60000/1001 -ofps 60000/1001
  4. If you plan on downscaling dramatically, you can extract and encode only one of the two fields. Of course, you will lose half the vertical resolution, but if you plan on downscaling to at most 1/2 of the original, the loss will not matter much. The result will be a progressive 30000/1001 frames per second file. The procedure is to use -vf field, then crop [1] and scale appropriately. Remember that you will have to adjust the scale to compensate for the vertical resolution being halved.
    mencoder dvd://1 -oac copy -vf field=0 -ovc lavc
    

14.2.3.4. Mixed progressive and telecine

In order to turn mixed progressive and telecine video into entirely progressive video, the telecined parts have to be inverse-telecined. There are three ways to accomplish this, described below. Note that you should always inverse-telecine before any rescaling; unless you really know what you are doing, inverse-telecine before cropping, too [1]. -ofps 24000/1001 is needed here because the output video will be 24000/1001 frames per second.

  • vf pullup is designed to inverse-telecine telecined material while leaving progressive data alone. In order to work properly, pullup must be followed by the softskip filter or else MEncoder will crash. pullup is, however, the cleanest and most accurate method available for encoding both telecine and "mixed progressive and telecine".
    mencoder dvd://1 -oac copy -vf pullup,softskip -ovc lavc -ofps 24000/1001
  • An older method is to, rather than inverse-telecine the telecined parts, telecine the non-telecined parts and then inverse-telecine the whole video. Sound confusing? softpulldown is a filter that goes through a video and makes the entire file telecined. If we follow softpulldown with either detc or ivtc, the final result will be entirely progressive. -ofps 24000/1001 is needed.
    mencoder dvd://1 -oac copy -vf softpulldown,ivtc=1 -ovc lavc -ofps 24000/1001
  • I have not used -vf filmdint myself, but here is what D Richard Felker III has to say:

    It is OK, but IMO it tries to deinterlace rather than doing inverse telecine too often (much like settop DVD players & progressive TVs) which gives ugly flickering and other artifacts. If you are going to use it, you at least need to spend some time tuning the options and watching the output first to make sure it is not messing up.

14.2.3.5. Mixed progressive and interlaced

There are two options for dealing with this category, each of which is a compromise. You should decide based on the duration/location of each type.

  • Treat it as progressive. The interlaced parts will look interlaced, and some of the interlaced fields will have to be dropped, resulting in a bit of uneven jumpiness. You can use a postprocessing filter if you want to, but it may slightly degrade the progressive parts.

    This option should definitely not be used if you want to eventually display the video on an interlaced device (with a TV card, for example). If you have interlaced frames in a 24000/1001 frames per second video, they will be telecined along with the progressive frames. Half of the interlaced "frames" will be displayed for three fields' duration (3/(60000/1001) seconds), resulting in a flicking "jump back in time" effect that looks quite bad. If you even attempt this, you must use a deinterlacing filter like lb or l5.

    It may also be a bad idea for progressive display, too. It will drop pairs of consecutive interlaced fields, resulting in a discontinuity that can be more visible than with the second method, which shows some progressive frames twice. 30000/1001 frames per second interlaced video is already a bit choppy because it really should be shown at 60000/1001 fields per second, so the duplicate frames do not stand out as much.

    Either way, it is best to consider your content and how you intend to display it. If your video is 90% progressive and you never intend to show it on a TV, you should favor a progressive approach. If it is only half progressive, you probably want to encode it as if it is all interlaced.
  • Treat it as interlaced. Some frames of the progressive parts will need to be duplicated, resulting in uneven jumpiness. Again, deinterlacing filters may slightly degrade the progressive parts.

14.2.4. Footnotes

  • [1.]About cropping:
    Video data on DVDs are stored in a format called YUV 4:2:0. In YUV video, luma ("brightness") and chroma ("color") are stored separately. Because the human eye is somewhat less sensitive to color than it is to brightness, in a YUV 4:2:0 picture there is only one chroma pixel for every four luma pixels. In a progressive picture, each square of four luma pixels (two on each side) has one common chroma pixel. You must crop progressive YUV 4:2:0 to even resolutions, and use even offsets. For example, crop=716:380:2:26 is OK but crop=716:380:3:26 is not.

    When you are dealing with interlaced YUV 4:2:0, the situation is a bit more complicated. Instead of every four luma pixels in the frame sharing a chroma pixel, every four luma pixels in each field share a chroma pixel. When fields are interlaced to form a frame, each scanline is one pixel high. Now, instead of all four luma pixels being in a square, there are two pixels side-by-side, and the other two pixels are side-by-side two scanlines down. The two luma pixels in the intermediate scanline are from the other field, and so share a different chroma pixel with two luma pixels two scanlines away. All this confusion makes it necessary to have vertical crop dimensions and offsets be multiples of four. Horizontal can stay even.

    For telecined video, I recommend that cropping take place after inverse telecining. Once the video is progressive you only need to crop by even numbers. If you really want to gain the slight speedup that cropping first may offer, you must crop vertically by multiples of four or else the inverse-telecine filter will not have proper data.

    For interlaced (not telecined) video, you must always crop vertically by multiples of four unless you use -vf field before cropping.

  • [2.]About encoding parameters and quality:
    Just because I recommend mbd=2 here does not mean it should not be used elsewhere. Along with trell, mbd=2 is one of the two libavcodec options that increases quality the most, and you should always use at least those two unless the drop in encoding speed is prohibitive (e.g. realtime encoding). There are many other options to libavcodec that increase encoding quality (and decrease encoding speed) but that is beyond the scope of this document.
  • [3.]About the performance of pullup:
    It is safe to use pullup (along with softskip ) on progressive video, and is usually a good idea unless the source has been definitively verified to be entirely progressive. The performace loss is small for most cases. On a bare-minimum encode, pullup causes MEncoder to be 50% slower. Adding sound processing and advanced lavcopts overshadows that difference, bringing the performance decrease of using pullup down to 2%.