How to deal with telecine and interlacing within NTSC DVDs

Index: DOCS/xml/en/mencoder.xml =================================================================== RCS file: /cvsroot/mplayer/main/DOCS/xml/en/mencoder.xml,v retrieving revision 1.17 diff -u -r1.17 mencoder.xml --- DOCS/xml/en/mencoder.xml 21 Jan 2004 19:59:33 -0000 1.17 +++ DOCS/xml/en/mencoder.xml 1 Feb 2004 09:40:14 -0000 @@ -761,4 +761,496 @@ + +How to deal with telecine and interlacing within NTSC DVDs + + +Introduction + +I suggest you visit this page if you don't understand much of what is written in +this document: +http://www.divx.com/support/guides/guide.php?gid=10 This URL links to an +understandable and reasonably comprehensive description of what telecine is. + + + +For technical reasons pertaining to the limitations of early television +hardware, all video intended to be displayed on an NTSC television set must be +59.94 fields per second. Made-for-TV movies and shows are often filmed directly +at 59.94 fields per second, but the majority of cinema is filmed at 24 or 23.976 +frames per second. When cinematic movie DVDs are mastered, the video is then +converted for television using a process called telecine. + + + +On a DVD, the video is never actually stored as 59.94 fields per second. For +video that was originally 59.94, each pair of fields is combined to form a +frame, resulting in 29.97 frames per second. Hardware DVD players then read a +flag embedded in the video stream to determine whether the odd- or +even-numbered lines should form the first field. + + + +Usually, 23.976 frames per second content stays as it is when encoded for a DVD, +and the DVD player must perform telecining on-the-fly. Sometimes, however, the +video is telecined before being stored on the DVD; even +though it was originally 23.976 frames per second, it becomes 59.94 fields per +second, and is stored on the disk as 29.97 frames per second. + + + +When looking at individual frames formed from 59.94 fields per second video, +telecined or otherwise, interlacing is clearly visible wherever there is any +motion, because one field (say, the even-numbered lines) represents a moment in +time 1/59.94th of a second later than the other. Playing interlaced video on a +computer looks ugly both because the monitor is higher resolution and because +the video is shown frame-after-frame instead of field-after-field. + + + +Notes: + + + + This section only applies to NTSC DVDs, and not PAL. + + + The example MEncoder lines throughout the document + are not intended for actual use. They are + simply the bare minimum required to encode the pertaining video category. How + to make good DVD rips or fine-tune lavc for maximum quality is not within the + scope of this document. + + + There are a couple footnotes specific to this guide, linked like this: + [1] + + + + +How to tell what type of video you have + + +Progressive + +Progressive video was originally filmed at 23.976 fps, and stored on the DVD +without alteration. + + + +When you play a progressive DVD in MPlayer, +MPlayer will print the following line as soon as the +movie begins to play: + +demux_mpg: 24fps progressive NTSC content detected, switching framerate. +From this point forward, demux_mpg should never say it finds "30fps NTSC +content." + + + +When you watch progressive video, you should never see any interlacing. Beware, +however, because sometimes there is a tiny bit of telecine mixed in, where you +wouldn't expect. I've encountered TV show DVDs that have one second of telecine +at every scene change, or at seemingly random places. I once watched a DVD that +had a progressive first half, and the second half was telecined. If you want to +be really thorough, you can scan the entire movie: +mplayer dvd://1 -nosound -vo null -benchmark +Using makes MPlayer play +the movie as quickly as it possibly can; still, depending on your hardware, it +can take a while. Every time demux_mpg reports a framerate change, the line +immediately above will show you the time at which the change occurred. + + + +Sometimes progressive video is referred to as "soft-telecine" because it is +intended to be telecined by the DVD player. + + + + +Telecined + +Telecined video was originally filmed at 23.976, 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 + + + mplayer dvd://1 -speed 0.1 + + + Seek to a part with motion. + + + 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 and MEncoder cannot losslessly + convert it to progressive. If you don't see any pattern at all, then it is + most likely interlaced. + + + + + +Sometimes telecined video is referred to as "hard-telecine". + + + + +Interlaced + +Interlaced video was originally filmed at 59.94 fields per second, and stored on +the DVD as 29.97 frames per second. The interlacing is a result of combining +pairs of fields into frames, because within each frame, each field is 1/59.94 +seconds apart. + + + +As with telecined video, MPlayer should not ever +report any framerate changes when playing interlaced content. + + + +When you view an interlaced video closely with , you +will see that every single frame is interlaced. + + + + +Mixed progressive and telecine + +All of a "mixed progressive and telecine" video was originally 23.976 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 "30fps NTSC" and "24fps +progressive NTSC". Watch the bottom of MPlayer's +output to see these messages. + + + +You should check the "30fps NTSC" sections to make sure they are +actually telecine, and not just interlaced. + + + + +Mixed progressive and interlaced + +In "mixed progressive and interlaced" content, progressive and +interlaced video have been have been spliced together. + + + +This category looks just like "mixed progressive and telecine", until +you examine the 30fps sections and see that they don't have the telecine +pattern. + + + + + + +How to encode each category + +As I mentioned in the beginning, example MEncoder +lines below are not meant to actually be used; +they only demonstrate the minimum parameters to properly encode each category. + + + +Progressive + +Progressive video requires no special filtering to encode. The only parameter +you need to be sure to use is . Otherwise, +MEncoder will try to encode at 29.97 fps and +duplicate frames. + + + +mencoder dvd://1 -nosound -ovc lavc -ofps 23.976 + + + + +Telecined + +Telecine can be reversed to retrieve the original 23.976 content, using a +process called inverse-telecine. MPlayer contains two +filters to accomplish this: and . You +can read the manual page to see their differences, but for DVDs I've never had a +problem with . Note that you should +always inverse-telecine before any rescaling; unless you really know +what you're doing, inverse-telecine before cropping, too +[1]. Again, is needed too. + + + +mencoder dvd://1 -nosound -vf ivtc=1 -ovc lavc -ofps 23.976 + + + + +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). + + + + + Encode the video in interlaced form. Normally, interlacing wreaks havoc with + the encoder's ability to compress well, but lavc has two parameters + specifically for dealing with storing interlaced video a bit better: and . Also, using + is strongly recommended [2] + because it will encode macroblocks as non-interlaced in places where + there is no motion. Note that is NOT needed here. + mencoder dvd://1 -nosound -ovc lavc -lavcopts ildct:ilme:mbd=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 to see what's 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 . + Also, deinterlacing should be done after cropping + [1] and before scaling. + mencoder dvd://1 -nosound -vf pp=lb -ovc lavc + + + Unfortunately, this option is buggy with MEncoder; + it ought to work well with MEncoder G2, but that + isn't here yet. You might experience crahes. Anyway, the purpose of is to create a full frame out of each field, which makes + the framerate 59.94. 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 you have to + specify both and to + be twice the framerate of your original source. + mencoder dvd://1 -nosound -vf tfields=2 -ovc lavc -fps 59.94 -ofps 59.94 + + + If you plan on downscaling dramatically, you can excise and encode only one + of the two fields. Of course, you'll lose half the vertical resolution, but + if you plan on downscaling to at most 1/2 of the original, the loss won't + matter much. The result will be a progressive 29.97 frames per second file. + The procedure is to use , then crop + [1] and scale + appropriately. Remember that you'll have to adjust the scale to compensate for + the vertical resolution being halved. + mencoder dvd://1 -nosound -vf field=0 -ovc lavc + + + + + +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 two filters +that accomplish this natively, but a better solution most of the time is to use +two filters in conjunction (read onward for more detail). + + + + + Currently the most reliable method to deal with this type of video 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 or + , the final result will be entirely progressive. Cropping + and scaling should be done after the inverse-telecine operations, and is needed. + + mencoder dvd://1 -nosound -vf softpulldown,ivtc=1 -ovc lavc -ofps 23.976 + + + + is designed to inverse-telecine telecined material + while leaving progressive data alone. Pullup doesn't really work well with the + current MEncoder, though, and is really intended + for use with MEncoder G2 (whenever it's ready). It + works fine without , but is + needed to prevent choppy output. With , it sometimes + fails. The problems arise from MEncoder's behavior + of dropping frames to maintain synchronization between the audio and video: it + drops frames before sending them through the filter chain, rather than after. + As a result, is sometimes deprived of the data it + needs. + + + + If MEncoder drops too many frames in a row, it + starves 's buffers and causes it to crash. + + + + Even if MEncoder only drops one frame, still doesn't get to see it, and will end up operating on an + incorrect sequence of frames. Even though this doesn't cause a crash, won't be able to make correct decisions on how to reassemble + progressive frames, and will either match fields together incorrectly or drop + several fields to compensate. + + + + + I haven't used myself, but here's what D Richard + Felker III has to say: + "It's 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 artefacts. If you're going to use it, + you at least need to spend some time tuning the options and watching the + output first to make sure it's not messing up." + + + + + +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 23.976 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 (3/59.94 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 or . + + + + 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. 29.97 frames per second interlaced video is already a bit choppy + because it really should be shown at 59.94 fields per second, so the + duplicate frames don't stand out as much. + + + + Either way, it's 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's only half progressive, you + probably want to encode it as if it's 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. + + + + + + + + +Footnotes + + + 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, + is ok but 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 won't have proper data. + + + + For interlaced (not telecined) video, you must always crop vertically by + multiples of four unless you use before cropping. + + + + + About encoding parameters and quality: + + Just because I recommend here doesn't mean it shouldn't + be used elsewhere. Along with trell, is one of the two + lavc 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 lavc that increase + encoding quality (and decrease encoding speed) but that is beyond the scope of + this document. + + + + + + + + + +