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Jim Leonard
Guest
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 Posted:
Fri Jul 29, 2005 1:38 am Post subject:
Re: scene change detection |
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Billy Joe wrote:
| Quote: | Of course it is working, but this is not optimal.
optimal for what? Quality or size?
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Both. If bitrate is fixed, quality. If "quality" (quantization level)
is fixed, size.
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Billy Joe
Guest
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| Posted:
Fri Jul 29, 2005 3:26 am Post subject:
Re: scene change detection |
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| Quote: | Billy Joe wrote:
Well, I'm gonna defer to the folks who do this for a living
(sometimes referred to as pros). I popped a commercial DVD
(which is not encrypted)
I'm not trying to be pedantic, but if it wasn't encrypted
then I wouldn't call it "pro". Please look at a
hollywood-produced movie that has encryption. While the main
24fps movie may or may not have variable-length GOPs, you'll
find that the polished video/30fps special features do.
I'm not saying that all unencrypted DVDs aren't pro (my own
DVD has no encryption, for example); all I'm saying is that
one single DVD is not a comprehensive test.
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I agree completely, and eagerly await your comprehensive analysis of
encrypted DVDs ;-0)
The one and only disc I checked, grabbed somewhat randomly from the few
unencrypted discs I have available, had both film and TV transfers. Either
type had I frames at 15 frame intervals. The Hauppauge capture devices
which I have (PCI H250, and USB2) both produce 15 frames per GOP regardless
of scene changes.
I'll gladly look at any PRO DVD sources you can cite which have variable
GOPs, all of which begin on scene changes. Yet, even if you did/could cite
one, the fact that we see different implementations in different
professional videos somewhat implies a lack of standard, no?
BJ |
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Jim Leonard
Guest
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| Posted:
Fri Jul 29, 2005 4:41 am Post subject:
Re: scene change detection |
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Billy Joe wrote:
| Quote: | The one and only disc I checked, grabbed somewhat randomly from the few
unencrypted discs I have available, had both film and TV transfers. Either
type had I frames at 15 frame intervals. The Hauppauge capture devices
which I have (PCI H250, and USB2) both produce 15 frames per GOP regardless
of scene changes.
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My ReplayTV unit, which uses an NEC µPD61051
(http://www.necel.com/digital_av/english/mpegenc/d61051_d61052.html)
produces variable bitrate content with variable-length GOPs. Like I
said, you need to look at more than just what you have in front of you.
| Quote: | I'll gladly look at any PRO DVD sources you can cite which have variable
GOPs, all of which begin on scene changes. Yet, even if you did/could cite
one, the fact that we see different implementations in different
professional videos somewhat implies a lack of standard, no?
|
I never implied there was a standard, just best practice.
It is at this juncture in the conversation that I have lost the
original point/question you were trying to make/ask. :-) Are you
trying to prove that there is no standard for variable-length GOPs?
Correct, there is no standard on how to handle scene changes and
variable-length GOPs. Are you saying it's not part of the MPEG-2
specification? Wrong, it is indeed part of the specification (ie. it's
not a "trick" or anything, and MPEG-2 files that use them are not "out
of spec"). Are you asking how scene detection is performed and how
that is taken advantage of in MPEG-2 files? See previous discussion.
Are you asking *why* this is done? Because it makes the most sense
from a compression standpoint. I-frames contain entirely new picture
information, so it makes sense to have drastic changes in the video
content synchronized with an I-frame. Otherwise, the P and B frames
will be trying to deal with 50% or more completely changed picture
information, which they were not designed to deal with -- the end
result is that the encoder has its bitrate allocation buckets thrown
out of whack and the end result, ultimately, is less bits to allocate
to the quantized DCT blocks. In other words, picture quality suffers
when scene changes are not aligned with I-frame boundaries.
Does this answer everything? If not, let me know. |
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Tobias Bergmann
Guest
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| Posted:
Tue Aug 02, 2005 8:32 pm Post subject:
Re: scene change detection |
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Jim Leonard wrote:
| Quote: | I never implied there was a standard, just best practice.
It is at this juncture in the conversation that I have lost the
original point/question you were trying to make/ask. :-) Are you
trying to prove that there is no standard for variable-length GOPs?
Correct, there is no standard on how to handle scene changes and
variable-length GOPs. Are you saying it's not part of the MPEG-2
specification? Wrong, it is indeed part of the specification (ie. it's
not a "trick" or anything, and MPEG-2 files that use them are not "out
of spec"). Are you asking how scene detection is performed and how
that is taken advantage of in MPEG-2 files? See previous discussion.
Are you asking *why* this is done? Because it makes the most sense
from a compression standpoint. I-frames contain entirely new picture
information, so it makes sense to have drastic changes in the video
content synchronized with an I-frame. Otherwise, the P and B frames
will be trying to deal with 50% or more completely changed picture
information, which they were not designed to deal with -- the end
result is that the encoder has its bitrate allocation buckets thrown
out of whack and the end result, ultimately, is less bits to allocate
to the quantized DCT blocks. In other words, picture quality suffers
when scene changes are not aligned with I-frame boundaries.
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If the next P frame after the scene change is "denaturated" to intra
blocks only and the B frames after and on the scene change use that P as
reference only there is no degradation!
But I doubt that encoders actually do that. It's most likely that the
B's contain skips which results in a frame showing both partly the old
scene and parts of the new scene. Anybody have examples?
For DVD content a GOP of 15 is way too small but most PROs don't care...
they just reuse the equipment targeted at live streams. The DVD offers
enough space/bitrate to cover for this unoptimal setting.
bis besser,
Tobias
--
Tobias Bergmann
Institut für Technische Informatik
Lehrstuhl für Rechnerarchitektur
Uni-Stuttgart e-mail: tobias.bergmann@informatik.uni-stuttgart.de
Pfaffenwaldring 47 Tel: +49-(0)711-7816-407
D-70569 Stuttgart Fax: +49-(0)711-7816-288 |
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