Interframe vs. Intraframe (Pros & Cons)

B

Blood Angel

Member
Hey guys,

I'm a long time reader but this should be one of my first post. I'm a filmstudent in Germany and I'm currently writing an article comparing the Interframe Encoding (HDV, MPEG2) and Intraframe Encoding (DV, DVCPro HD).

Interframe Pro's:
  • Filesize
  • Downwards Compability (with HDV and MPEG2)
Interframe Con's
  • Massive loss of image Infomation
  • Precice editing is a little harder because encoding requires more CPU power
  • You might get problems with fast moving objects as your motion Blur might get screwed
Intraframe Pro's
  • The HVX is using it (well, not too serious... :happy: )
  • The huge amount of available information
  • Easy editing
Intraframe Con's
  • Filesize
What is your experience? If you could get an HVX with Interframe Encoding, would you use it? Can you give me some of your experience for my article?
 
What is "Downwards Compatibility (with HDV and MPEG2)" worth when every producer of cameras uses a different version of HDV. AFAIK you cannot use a Canon tape in a SONY or JVC camera/cheap deck.
You are not compatible with anything but your own product. Just like the DVCPorHD codec.
One pro for Interframe is the tape-archive. It is good when you have time and less budget for achivation.
One pro for the Intraframe is the NON-tape workflow. It is good when you are in a hurry and want to save capturing time and use the time for archivation later.
 
alpi69 said:
One pro for Interframe is the tape-archive. It is good when you have time and less budget for achivation.
One pro for the Intraframe is the NON-tape workflow. It is good when you are in a hurry and want to save capturing time and use the time for archivation later.
Click to expand...
Well... that doesn't quite count, cause DV is Intraframe too and can be recorded to tape.
 
I think you have to decide if you are actually listing Intraframe pros and cons versus Interframe, or HDV v DVCProHD. (Or HDV v DV.) And is the bit rate the same for each (as HDV v DV), or are you doing a constant quality comparison?

In general, in non codec or hardware specific terms, for a given bit rate Inter-frame should give a higher quality than Intra, but you never get anything for nothing, and it comes at the expense of added complexity, especially during editing. Alternatively, for a given quality, Inter-frame will give a much smaller file size - but again at the expense of complexity and less easy editing, or needing a more powerful computer.

It isn't necessarily true to say that an Interframe Con is a massive loss of image information. An Intraframe system may have to compress each frame very highly, sub sample luminance, or reduce the colour space to fit within available bandwidth - or all three! An interframe system may be able to allot far more data to it's I-frames than an intraframe system of the same bandwidth, then rebuild the intervening frames to to the same standard. Put another way, given equivalent bandwidth for each, the Inter-frame system is discarding more redundant bits, the Intra-frame is actually having to discard useful bits.

(An example being that for 720p/60, HDV has a raster size of 1280x720, DVCProHD is 960x720.)
 
Blood Angel said:
Hey guys,

Interframe Pro's:
  • Filesize
  • Downwards Compability (with HDV and MPEG2)
Interframe Con's
  • Massive loss of image Infomation
  • Precice editing is a little harder because encoding requires more CPU power
  • You might get problems with fast moving objects as your motion Blur might get screwed
Intraframe Pro's
Click to expand...

Sorry, dude, but I think you're missing it. Downward compatibility is sorta meaningless. Filesize really has nothing to do with interframe vs. intraframe... you can use high bitrates or low bitrates for either. The main issue is exactly what is lost in each case.

Editing is harder because not all frames are equal; not only does this take more process power, but it causes some loss of quality as there's a lot of recompression and "guesswork" filling in of information around edit points.

And, motion blur is not the issue with interframe compression; rather it's compression artifacts that result from more motion in the frame than the B-frames can properly handle.

  • The HVX is using it (well, not too serious... :happy: )
  • The huge amount of available information
  • Easy editing
Intraframe Con's
  • Filesize
Click to expand...

Again, filesize is not an intra- vs. inter distinction; it has to do with specific codecs.


I'd say that the comparison is really something like this:

Interframe Compression
Pros:
*Can achieve similar picture quality at lower bitrate than interframe if there's not too much motion - as bitrate lowers, it affects primarily motion quality, not image quality
*Can take advantage of multipass-approaches to really optimize compression (but, in general, not during acquisition)
Cons:
* Picture quality is not consistent between frames - especially with lots of motion in the frame (or at scene changes), the picture may fall apart for a few frames.
* Editing is harder and may introduce some quality loss at edit points

Intraframe-only Compression
Pros:
* Consistent image quality from frame to frame, regardless of motion/scene changes
* Easier editing without interframe-compression quality loss issues
Cons:
* As bitrate lowers, it affects image quality of all video, regardless of motion, so bitrates tend to be higher

Or, in brief, for a given bitrate, intraframe compression prioritizes consistency and motion quality, while interframe prioritizes (still) image quality.


Issues of tapeless workflow, ability to use tapes, etc., have noting to do with intra - vs. inter-frame compression. But if you want to address DVCProHD vs. HDV, those become relevant. Glitches (associated primarily with tape) tend to be much worse in interframe compression, since frames depend on each other; they tend to take out 1/2 second or more of video rather than a single frame as in intraframe compression. Conversely, tape problems tend to affect just a small part of the tape ( a frame or GOP, say - in bad cases, a tape can be physically mended [spliced, etc]), while problems with tapeless mechanisms (drives) tend to lose entire takes, or drives'-worth of material. How often each happens is yet to be seen...
 
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The Con of HDV needing more CPU power to edit, etc, is a mute point. With the Intermediate Codecs out for HDV now, there's no image quality loss, and you get to edit just like DV and HD.

But, HDV is simply not going to hold up to HD, simply due to color space restrictions. I'd rathe do keying and compositing with DVCPRO-HD than HDV any day!

I think you also have to factor in the application of the format. What are you recording? What is your output format? Who's your audience?

If you're doing mostly weddings/events, going out to the family/client on a couple of self made DVDs, then DV and HDV are both great. Cheaper, easy enough to work with (once you get it down), archive to cheap tapes.

If you are doing a documentary and going out to film, I'd go with DVCPRO-HD for sure.

And with the price of DVD media and hard drives and tape drives today, file size, at leat to me, is a useless argument.
 
As a general concept, Intraframe will always be better than Inter-Frame, at adequate bandwidth. There's a reason that every professional format out there has always been intraframe-only, like DVCPRO, DVCAM, Digital Betacam, MPEG-IMX, DVCPRO50, DVCPRO-HD, HDCAM, HDCAM-SR, D-9/Digital-S, REDCODE, the forthcoming AVC-Intra, etc. Interframe exists for times when there is not adequate bandwidth to do Intraframe.

Intraframe is totally appropriate for tape, even at its high-bandwidth needs; look at DigiBeta on tape (90 megabits per second), DVCPRO50 and MPEG-IMX (50 megabits), DVCPRO-HD (100 megabits), HDCAM (140 megabits) and HDCAM-SR (440 megabits). Those higher-capacity tape decks are not cheap, obviously, but it's clearly possible.

Inter-frame is a concession towards limited-bandwidth or limited-space recording media. Inter-frame only really entered the picture because of the goal of broadcasting HD in a tiny-bandwidth 19-megabit broadcast signal, or putting HD onto a limited-bandwidth recording medium -- primarily being DV tape (for HDV) or 35-megabit optical discs (XDCAM-HD), or mini-DVD or SD cards (AVC-HD). Where there's adequate capacity, Intraframe has always been the method of choice. For limited-capacity media, where only a small bandwidth is possible, interframe is the only way to get acceptable quality levels at such limited bandwidth.

Interframe can usually deliver similar quality as Intraframe, but at a lower bitrate (unless the codec gets overloaded, at which point quality suffers.) It's all about the bandwidth. With adequate bandwidth both will always deliver fantastic results, but intraframe will never suffer motion compression variabilities. At the same bandwidth, Interframe will deliver better image quality than Intraframe (but then you get into issues of CPU power when editing, etc). With limited bandwidth, that's where the comparisons start coming in, and where the tradeoffs of motion artifacting vs. file sizes start coming into play. That's where the debate is.

Unfortunately, the comparisons are not so simple to nail down. How much interframe bandwidth do you need, to match intraframe quality? HDV proponents would generally say that their 25 megabits matches DVCPRO-HD's intraframe 100 megabits. And on some types of scenes that's generally accurate. On other scenes, 25 megabits is nowhere near enough, and HDV can turn to complete crapola (explosions/fire/strobelights/smoke can turn HDV to utter mush). So is 35 megabits enough? XDCAM-HD proponents certainly seem to believe so, especially with its VBR vs. CBR capability. I haven't explored XDCAM-HD enough to know how robust it is when things get dicey.

Intraframe never suffers from motion artifacting, and there are never any "surprises." It either is good enough, or it isn't. It either has enough bandwidth for every job, or it doesn't. There are not really any variables to worry about. It is always 100% reliable, 100% trustworthy, you will get the same image quality whether shooting a static interview scene or shooting a fireworks extravaganza with strobelights, smoke, fire, over rippling water, handheld. It's always the same.

But then there are other factors to consider too: you have to factor in compression efficiency, for example. MPEG-2 is nowhere near as efficient as AVC; AVC is two to three times as efficient as MPEG-2 and can deliver comparable quality at 1/2 to 1/3 the bandwidth. At adequate bandwidth MPEG-2 can look great, but at lower bandwidth AVC really pulls ahead. So 8 megabits of AVC-HD is generally comparable to 25 megabits of MPEG-2. 24 megabits of AVC-HD would be substantially superior to 25 megabits of MPEG-2 HDV, and probably toe-to-toe with 35 megabits of XDCAM-HD (as a format only, assuming comparable camera heads). But would 24 megabits of AVC-HD hold up compared to 140 megabits of intraframe HDCAM? Or 100 megabits of DVCPRO-HD? Under ideal circumstances, certainly, but under dicey circumstances? I think it's doubtful, but I look forward to finding out.

Then there's length-of-GOP. The longer the group of pictures, technically the lower bandwidth you can go and still maintain quality. But also, the worse things are when the codec gets overloaded! So CineForm, a more professional codec, uses a GOP of two frames IIRC. If it got overloaded, you'd barely even notice as at most two frames per group would be affected. HDV 720P @ 24, 25 or 30 fps uses a GOP of six frames. HDV 720p @ 50 or 60 fps, and HDV 1080 at 50i, uses a GOP of 12 frames. HDV 1080 @ 60i uses a GOP of 15 frames. If a group gets overloaded and compression quality starts to suffer in HDV 1080i, it's going to turn 15 whole frames to mush. That's half a second of footage that all gets compromised! Think about streaming net video when it goes mushy, and then a second later it "clears up" -- that's the kind of thing that happens with long-GOP interframe compression systems. And that would never happen with intraframe.

But intraframe compression efficiency varies as well. DV compression (and its attendant variants, DVCPRO, DVCPRO50, DVCAM, and DVCPRO-HD) is generally better than Motion JPEG at equivalent bandwidth even though they're all DCT, but now there's wavelet too; so intraframe includes JPEG-2000 and AVC and REDCODE, all of which are noticeably better than DV-based or MJPG.

Finally, as ullanta pointed out, interframe has another benefit for final transmission, that cannot be realized in acquisition: multi-pass compression. You can squeeze a lot more quality into a final transmission/broadcast/DVD than you can during a realtime one-pass encode in a camcorder.

So, in general:
Intraframe = better as long as you can afford the bandwidth.

Interframe = better when you have to squeeze a signal into lower bandwidth, provided you don't exceed the codec's ability to handle changes during a given GOP; if you do, Interframe can turn to poop. That never happens with intraframe.

So it's all about the bandwidth and about the cost. Interframe only entered the picture because of limited bandwidth for transmission (realplayer, HD broadcast, etc) and for distribution (DVD @ 8 megabits), and because camcorder manufacturers started recording to limited-size, limited-bandwidth recording media (25-megabit DV tape, 35-megabit optical disc, mini-DVD disc, and 2GB SD cards). In tape interframe has always been the choice used for professional formats.

The thing about acquistion bandwidth is -- with solid state, capacities and speeds are only going to increase and get cheaper. For storage, capacities are only going to increase and get cheaper. As technology improves, the argument for needing interframe at acquisition becomes less and less relevant. But for distribution, there's always going to be pressure to squeeze more and more into less and less. Networks and cable systems will compromise the 19-megabit signal to fit two or three or four HD channels into the space where there should be only one. DVD and blu-ray and HD-DVD producers will try to fit more and more special features and cram them all into one disc to keep their manufacturing costs lower. As time marches on we will have a surplus of acquisition bandwidth, and a dearth of distribution bandwidth!

So, in summary, there are no easy answers. The one mantra I would say is: Intraframe for acquisition, Interframe for distribution. You need your best quality in your source footage; compromises in source footage are not acceptable to most professionals. But you can't control the distribution media; we're likely going to be facing an interframe-only distribution system for the foreseeable future.
 
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