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Old 28th March 2010, 22:04   #1  |  Link
St Devious
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Does deinterlacing reduce video quality ?

I shoot video with my Sony HDR-HC3 HDV 1080i camera. I create an avisynth script before encoding it. The video is interlaced, top field first as shown by mediainfo.

So I deinterlace using Yadif and top field first order. But the deinterlaced video seems to have less quality. Am i doing something wrong or does deinterlacing reduce quality ?
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Old 28th March 2010, 22:15   #2  |  Link
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How are you viewing the interlaced/deinterlaced videos?

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Old 28th March 2010, 22:31   #3  |  Link
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Originally Posted by davidhorman View Post
How are you viewing the interlaced/deinterlaced videos?

David
MPCHC and VLC
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Old 28th March 2010, 22:44   #4  |  Link
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I should have added: is the interlaced video being deinterlaced on playback, and if so, by what method? Can you post some screenshots of each?

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Old 28th March 2010, 23:12   #5  |  Link
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A screenshot of the output would probably help too. Or even a small source clip
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Old 29th March 2010, 11:13   #6  |  Link
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Originally Posted by St Devious View Post
So I deinterlace using Yadif and top field first order. But the deinterlaced video seems to have less quality. Am i doing something wrong or does deinterlacing reduce quality ?
Deinterlacing with Yadif reduces quality when anything moves at all.

It also reduces temporal resolution if you deinterlace to 25p / 30p. You can use doing double-rate deinterlacing to get 50p / 60p to solve this problem, if your playback hardware supports it.

There are lots of deinterlacers available. The better ones are quite slow - especially on HD content.

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Old 30th March 2010, 19:59   #7  |  Link
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Think about it. Interlaced video has twice the frame rate of progressive video. Deinterlacing throws out half the temporal information by replacing alternating interleaved fields with calculated guesses of what would be in those fields.

Go to a store which has 60 Hz and 120 Hz displays next to each other. If deinterlacing was "perfect" there would not be a need for 120 Hz, would there?

FWIW, 120 Hz displays also use "educated" guesses to create the "missing" fields but they keep far more of the temporal information.
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Old 30th March 2010, 20:12   #8  |  Link
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Go to a store which has 60 Hz and 120 Hz displays next to each other. If deinterlacing was "perfect" there would not be a need for 120 Hz, would there?
I don't follow you - it seems like you're mixing up motion interpolation (which >60Hz displays may or may not use) with deinterlacing (which all flat displays do), or I'm just missing your point...

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Old 30th March 2010, 20:40   #9  |  Link
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An interlaced frame has 2 interwoven images recorded at 2 different times. Deinterlacing involves halving the temporal information. Interpolation is, in essence, a calculated guess of the value of pixels.

Motion interpolation is a method of calculating those "missing" pixels. Those are 2 different things.

Look at an interlaced frame of an object in motion. The upper and lower frames record different periods in time.

Interlaced video has twice the number of "pictures" and half the "resolution" as progressive video.

Once you've deinterlaced you cannot reconstruct the original data so you throw away half the temporal data which means the appearance of motion during playback is less.

To illustrate: use a screen capture utility set to 15 fps and record a non-linear motion (move your mouse, for example.) Then repeat at 30 fps then at 60 fps. Which is smoohter during playback?

Real world objects in motion almost never move in a linear motion in a perfect arc around a video camera.

A flat screen is a progressive display device, true. However, that does not change the result of modifying a data set. Once you reduce the accuracy of your data, you can't recover the original data.

The human eye is better at discerning motion than color, better at edge detection than shade, right?

If you double the frame rate and interpolate the "missing" pixels the result is far more pleasing to view than if you throw away half the temporal information.

Remember, video is a projection of a 3-dimensional data set onto a plane.
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Old 30th March 2010, 20:51   #10  |  Link
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Originally Posted by FredThompson View Post
FWIW, 120 Hz displays also use "educated" guesses to create the "missing" fields but they keep far more of the temporal information.
There is no video broadcasting done that has 120 Hz of temporal information at the moment. The issue of de(interlacing) and 60 versus 120 Hz is not really related.
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Old 30th March 2010, 21:01   #11  |  Link
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There is no video broadcasting done that has 120 Hz of temporal information at the moment. The issue of de(interlacing) and 60 versus 120 Hz is not really related.
Yes. I was trying to think of an easily reproduced way to explain the difference between 30 and 60. Sorry for any unintentional confusion I added.

There is (was?) a fantastic site which compared interlaced, deinterlace and double-rate to illustrate the motion and interpolation things I'm trying to describe. I remember the site but can't seem to find it right now. It might have also been a bobbing comparison.

Here are a few links I found but not the bob site.
http://renomath.org/ejolson/video/interlace/
http://www.slashcam.com/EN/info/1920...p--394672.html
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Old 30th March 2010, 21:35   #12  |  Link
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Are you by any chance thinking of www.100fps.com ? If so, it's not that great. 100fps.com is okay to give starters an idea of the general topic. But it also draws lots of conclusions of the kind "IF we had beaming technology, then we COULD go anywhere" ...
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Old 30th March 2010, 21:46   #13  |  Link
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I was just thinking yesterday how it would be nice if YouTube supported 50/60Hz.

I recently bought a new satellite receiver which can also connect via the internet to the BBC's iPlayer, and does so with resulting 50Hz interlaced video

I don't mean to rant, but it really disappoints me when some providers seem to think smooth motion just isn't important, and likewise with games... I gave up on Wipeout Pulse after 5 minutes for exactly this reason...

Quote:
So I deinterlace using Yadif and top field first order. But the deinterlaced video seems to have less quality. Am i doing something wrong or does deinterlacing reduce quality ?
Going back to the OP, does it sound like the lack of smooth motion is part of what you're perceiving as less quality?

David
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Old 31st March 2010, 01:09   #14  |  Link
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No, 100fps isn't the site I remember but it does show some good stuff. I like to point to WWE Wrestling, too. Lots of flashing lights, long swinging hair, etc. Ivo (anyone remember him?) likes to illustrate with motor sports.

Similarly, there's a scene in Heavy Metal where a robot's head is spinning. Every other frame is a duplicate for about 2 seconds. Temporal noise reduction yields lots of corruption.

Another example I've seen was artificial scan lines in part of the frame. Deinterlacing mucks that up. Well, pretty much anything with thin lines or things like highly anti-aliased text can also get corrupted. I've got a few frames showing very thin lines in window dividers which get screwed up by most deinterlacing.

Hmm...do any of the temporal analysis methods commonly used include persistance of vision? IOW, are they assuming the calculated pixel is static or do they include "fade" as the object is in motion, sort of time-based anti-aliasing, for lack of a better term.

Good question, David. The word, "quality" can be interpreted many ways.
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Old 1st April 2010, 14:50   #15  |  Link
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Once you've deinterlaced you cannot reconstruct the original data so you throw away half the temporal data which means the appearance of motion during playback is less.
I think you're assuming that when people say "deinterlacing", they don't mean bobbing.

I don't think people are so careful with the terms.

E.g. with a "PAL" signal, you can deinterlace to 25p or 50p. The process which delivers 50p could also be called bobbing, or double-rate deinterlacing - but enough people call it deinterlacing that it's fine.

EDIT: Didée just put this far better than me...
http://forum.doom9.org/showthread.ph...23#post1388023


On the more general point: if nothing moves, interlacing doesn't lose any information, and neither does deinterlacing! The same is true for horizontal-only movement.

Vertical movement, and/or fine vertical details, clearly cause a loss of information when interlaced which any deinterlacer is going to struggle to guess at.

But apart from pathological cases like these (and sometimes even including pathological cases like these!), I think you'll find tgmc (for example ) does pretty well.

So the answer to the OP remain: in general, if done very well, your camcorder footage should lose almost nothing when (double-rate) deinterlaced using a good deinterlacer (bobber!).

Cheers,
David.

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Old 1st April 2010, 15:12   #16  |  Link
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I dunno about you guys, but TempGaussMC deinterlaced DVDs look far better than they would stock. Maybe quality is reduced to a computer, but perceived quality, to me at least is way better.
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Old 1st April 2010, 16:33   #17  |  Link
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TGMC is about the only thing I use these days.
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Old 1st April 2010, 16:37   #18  |  Link
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Originally Posted by 2Bdecided View Post
On the more general point: if nothing moves, interlacing doesn't lose any information, and neither does deinterlacing! The same is true for horizontal-only movement.
Probably you kept in mind the safety of horizontal resizing. In interlaced those horizontally moving parts will be spatially shifted between the 2 fields.
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Old 1st April 2010, 16:44   #19  |  Link
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Probably you kept in mind the safety of horizontal resizing.
No, I meant what I said!

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In interlaced those horizontally moving parts will be spatially shifted between the 2 fields.
Shifted. Not lost. All the information is there. Decent motion compensated deinterlacing can stitch it back together again. Theoretically without loss.

In practice, with so little loss that it doesn't matter. e.g. see TGMC deinterlacing "Stockholm Pan".

Cheers,
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Old 1st April 2010, 17:07   #20  |  Link
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Yes, words have definitions and they're used a little too loosely here.

Yes, double frame rate and a non-destructive calculation of the resultant "missing" fields saves all the source data and looks far nicer.

However, the playback equipment must support the frame rate to get the full benefit. 30 interlaced fps converted to 60 fps progressive needs 60 fps display to see the full information. I do this a lot with interlaced source of moving machinery (lots of reflective surfaces, rotary motion, etc.) then throw away every other frame for Internet playback or keep the 60 fps for computer playback. It can save a lot of time compared to intensive motion-compensated conversion to 30 fps progressive (common usage word: deinterlace.)

IMNSHO, storage costs are so low that it's best to save the source because tech improves over time. I DO compress NTSC DV to 4:2:2 at 8000 CBR with TMPGEnc to preserve temporal information and reduce size.
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