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johnm214



Registered: 05/31/07
Posts: 17,582
Loc: Americas
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Question about video; When 'jumping' to new section, how come you get blurry 'ghost-like' images?
#8168940 - 03/19/08 08:19 PM (16 years, 2 months ago) |
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Hello
I've got a video, this one is a .wmv, and when you jump ahead, presumably also when you jump back, in vlc, by manually moving the position bar ahead of the present position while playing the video, the image becomes blurry. You may have ghost images from the previous frames, and you will see what seems to be brighter colors that selectivly show parts of the present frames. After a few seconds the picture suddenly (within what appears to be one frame) 'snaps' to a clear picture of the present frames and continues playing like it was prior to you mumping ahead.
Why is this? I've noticed this on many videos, probably all torrents downloaded, and I don't recall all the extensions at the moment except that the most recent one was a wmv extension. All were viewed in vlc. I notice this never happens in youtube when I jump ahead.
I'm guessing part of the compression method is to only encode the information on each frame that is different than the prior frame, or that the information for the new frames is somehow dependant upon the prior frames in some other way, to save space. So when you jump ahead it takes awhile for the player, I use vlc, to read enough frames to be able to produce the present frames accuratly, as it has to rely upon the previous frames' information.
Is this correct? I couldn't find any information in a google search, but I'd imagine its out there somewhere.
If so, why does it seem like only certain videos exhibit this behavior, I believe some are wosre than others, but then some I don't notice at all. Are only certain file types affected? Is this at all realated to vlc? Is it only low quality videos that I'd notice this on, i.e. if the example I mentioned above was a higher quality wmv, would I not notice this as much or at all?
Why doesn't youtube seem to suffer from this behavior? Is it the way the compress the videos?
Any tips to minimize this behavior, given that I can't change my source video?
Thanks to anyone who cares to address this, feel free to school me...
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sherm
sherman


Registered: 10/02/03
Posts: 20,498
Loc: Euthanasia
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Re: Question about video; When 'jumping' to new section, how come you get blurry 'ghost-like' images [Re: johnm214]
#8169326 - 03/19/08 09:33 PM (16 years, 2 months ago) |
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vlc does this for me too i have some porno that will do this and others that play fine
interesting.
-------------------- shroomery. not even once.
    
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Irradiated_Feces
doomedgeneration


Registered: 07/11/03
Posts: 4,278
Loc: Great White North
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Re: Question about video; When 'jumping' to new section, how come you get blurry 'ghost-like' images [Re: sherm]
#8171276 - 03/20/08 10:48 AM (16 years, 2 months ago) |
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It's probably because wmv is a shitty format. It's meant to be viewed from start to finish, not forwarded and reversed frame by frame.
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Seuss
Error: divide byzero



Registered: 04/27/01
Posts: 23,480
Loc: Caribbean
Last seen: 3 months, 8 days
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Re: Question about video; When 'jumping' to new section, how come you get blurry 'ghost-like' images [Re: johnm214]
#8172882 - 03/20/08 05:59 PM (16 years, 2 months ago) |
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> Why is this?
Temporal compression artifacts. (Sounds smart) The way most video compression works is by storing a single frame of data (an Index frame), and then storing deltas from this initial frame as time proceeds. After a while, a new index frame is stored and the process repeats. When you fast forward into a stream, normally you jump to an index frame. However, if they drop you in the middle of the deltas, the deltas will move data from the previous index frame (before the fast forward). Unfortunately the deltas will be appropriate for an index frame that you fast forwarded past, thus you get strange artifacts on the screen until the next index frame comes along and everything "snaps" into place.
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johnm214



Registered: 05/31/07
Posts: 17,582
Loc: Americas
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Re: Question about video; When 'jumping' to new section, how come you get blurry 'ghost-like' images [Re: Seuss]
#8173112 - 03/20/08 06:52 PM (16 years, 2 months ago) |
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cool, thanks seuss
So is this why some setups, youtube, some news channels, et cet, won't let you jump to an arbitrary point in the video, but rather put you at certain spots approximating your desired place? Like if you wanted to jump at 3:32 it would put you at 3:28 or 3:35, but not 3:32?
And this is why it "snaps" back into place w/ things like wmv? When you get to a new index frame, you suddenly get a clear picture?
Thanks, google didn't help me with this question, probably cuz I didn't use temporal compression 
thanks
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Seuss
Error: divide byzero



Registered: 04/27/01
Posts: 23,480
Loc: Caribbean
Last seen: 3 months, 8 days
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Re: Question about video; When 'jumping' to new section, how come you get blurry 'ghost-like' images [Re: johnm214]
#8174895 - 03/21/08 04:33 AM (16 years, 2 months ago) |
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> So is this why some setups, youtube, some news channels, et cet, won't let you jump to an arbitrary point in the video
Most likely if it is within a second or two and the compression is very high, but I don't know for certain. The repeating pattern I described above is only around 15 frames with mpeg (0.5 seconds or so). Other compression formats stretch this out longer (more compression, less quality), so your theory makes sense, and is a good likely answer.
> And this is why it "snaps" back into place w/ things like wmv? When you get to a new index frame, you suddenly get a clear picture?
Yes. The index frames contain "all the data" providing a new starting point. However, because they contain all the data, it is a tradeoff with the compression... more index pages means higher quality, but larger data space while fewer index pages means lower quality/more artifacts with less data space. Many compression formats will allow the encoder to decide how many frames should be grouped together between index frames, thus behavior might be different between two media files of the same type.
> Thanks, google didn't help me with this question, probably cuz I didn't use temporal compression
To be fair, is is a playback issue, not a compression issue. However, if you google for "temporal compression artifacts" you will end up in the right ballpark.
Also, just to make things more complex... there are both containers and encoders and they typically share the same name. For example, mpeg as a container can hold about any type of data while mpeg encoded data can be contained in another container type, such as avi (assuming the avi container knows how to hold mpeg data).
A real life example of this is a DVD video player. The DVD video player uses mpeg program stream (container) to hold data. It also happens to use mpeg encoded video, but allows/requires non-mpeg encoded audio as well (along with sub-pictures/captions and navigation data). All of the data (mpeg and non-mpeg) is combined into a bunch of "video objects/VOBs" with an mpeg program stream holding them.
Take a look at http://www.doom9.org/ for a good starting point if interested in this type of thing.
-------------------- Just another spore in the wind.
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johnm214



Registered: 05/31/07
Posts: 17,582
Loc: Americas
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Re: Question about video; When 'jumping' to new section, how come you get blurry 'ghost-like' images [Re: Seuss]
#8179366 - 03/22/08 11:10 AM (16 years, 1 month ago) |
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thanks for the schoolin
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