Native MPEG-2 mirror imaging?

[bluerted]

Is anyone aware of MPEG editing software that could perform a vertical axis transform without forcing a re-encode? (The source is a set of VOBs which I can of course de-mux.) Thanks.

(I guess this question is really about NOT converting video, but this still seemed like the best forum...)

[guns1inger]

Can't be done. You are altering every pixel in the image by doing this, so of course you will have to re-encode. This would happen with any video type.

[bluerted]

Thanks. But I'm not entirely convinced. There's no need to requantize the frequency components in a JPEG-derived image when you flip it, even though the transform is "altering every pixel in the image". With MPEG-2 of course you have to deal with storing the post-quantized matrices that represent now-lossy images as I, P and B frames, but I think that at least theoretically it should be possible to do a flip transform on these.

[guns1inger]

Prove it by programming it. It may be theoretically possible, but no software does it, so for all practical purposes, my post still stands.

[bluerted]

Funny - I was originally going to add "Not that I plan on coding it!" to my previous post. If you're saying it would be a challenge, I'm sure you're right. If you're saying no-one's done it, well, that's what I asked about in my first post - I've never worked with any of the MPEG-2 editors out there, so I don't know what capabilities are currently available. If you're standing by your "can't be done", well - assuming my argument is correct then I guess we differ on the meaning of "can't". But thanks for your input.

[guns1inger]

I remember a couple of years back reading a paper that proposed basic image adjustments to mpeg streams without the need to re-encode, but I have not been able to find it since, or any evidence that it was ever coded to any degree. I would have thought that if it was practical and feasible, someone would have done it by now.

[filmboss80]

It's a supply-and-demand issue. There don't seem to be many people with a need to flip entire MPEG-2 movies to a mirror image. There may not be enough justification for a programmer to toil over a software feature for which there is very little demand. If there was an MPEG editor with such a capability, don't you think there would be a mention of it on this site?

[bluerted]

Uhhh, that's why I asked... Just curious - does any MPEG-2 editing software provide any type of frame-based transformation without requiring conversion?

[aedipuss]

mainconcept reference encoder will take mpeg-2 as input and re-encode flipped to mpeg-2.

[Midzuki]

One might add a "Horizontal/Vertical_Flip" flag to an MPEG file,
but it would remain meaningless to the (now) "old" MPEG-decoders.
Anything more complex than this will imply the re-writing of all frames,
which is as transforming as an actual decompression+mirroring+recompression.

[bluerted]

mainconcept reference encoder will take mpeg-2 as input and re-encode flipped to mpeg-2.

The big question for me is whether the frames are decompressed, flipped and then re-compressed - that is, does a second lossy compression compression occur, or does the frame you uploaded contain precisely the same amount of information as the MPEG-2 source? I'll check out this software - thanks.

[aedipuss]

it sure says it's re-encoding and it takes about the same amount of time as going from dvavi to mpeg2 as mpeg2 to mpeg2 flipped. the mpeg2 .mpv files stay about the same size before and after the flip and there doesn't appear to be too much loss of detail. but it's an expensive encoder so that's what you'd expect.