Hello All,

I'd like to send a lot of jpegs across a modest bandwidth line without
using up a lot of its available capacity. The images are sequential
snaps of a landscape where there is very little change from one frame
to the next. I'd like ideally to send the first frame in each hour
then subsequently send only the frame-to-frame diffs, which ought to
be small.


I'm sure this is a solved problem somewhere but I can't see it.
Certainly zipping or gzipping don't work in any obvious way (or not
obvious enough, anyway!) Suggestions?


Dave

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Countervailing Power



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On 02/16/2012 08:06 PM, Dave Stevens wrote:
> I'm sure this is a solved problem somewhere but I can't see it.
> Certainly zipping or gzipping don't work in any obvious way (or not
> obvious enough, anyway!) Suggestions?

Lossless coding like zip/bz2/xz et al won't do much on top of a lossy
media codec. The jpeg formats also don't have a notion of inter-frame
coding (frame-to-frame diffs). Unfortunately the binary coding isn't
especially amenable to diffing without special tools. A friend of mine
wrote some tools like this for a college project (not available online
sadly) - it's not that difficult but it does require a certain amount of
work.

If you want to do this with off-the-shelf parts the best option might be
to use e.g. mplayer/mencoder which will take a stream of jpeg images and
convert them into a video codec that does support predicted frames:

http://www.mplayerhq.hu/DOCS/HTML/en/menc-feat-enc-images.html

You don't have to use mpeg4 although it will probably give pretty good
compression.

Once you have the images encoded as video you can split them out to
individual frames again with something like this:

mplayer -frames 1 -vo jpeg outdir=/tmp:quality=75 path/to/video.file

Depending on your quality needs this might work - passing images through
two rounds of lossy coding is generally a bad idea as you'll get
additional artefacts in the movie as the codec tries to code the
artefacts in the original jpeg image but for many uses it's tolerable.

I think ImageMagick can also split the movie to images if it was
compiled with support for the codec you use.

Regards,
Bryn.
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On Thu, Feb 16, 2012 at 2:06 PM, Dave Stevens <geek@uniserve.com> wrote:
> Hello All,
>
> I'd like to send a lot of jpegs across a modest bandwidth line without using
> up a lot of its available capacity. The images are sequential snaps of a
> landscape where there is very little change from one frame to the next. I'd
> like ideally to send the first frame in each hour then subsequently send
> only the frame-to-frame diffs, which ought to be small.
>
> I'm sure this is a solved problem somewhere but I can't see it. Certainly
> zipping or gzipping don't work in any obvious way (or not obvious enough,
> anyway!) Suggestions?

I don't know how to get a diff, but as far as limiting your upload
speed, there's an easy solution.

If you use scp, just use the "-l" option to limit the upload speed.

Richard
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> I'm sure this is a solved problem somewhere but I can't see it.
> Certainly zipping or gzipping don't work in any obvious way (or not
> obvious enough, anyway!) Suggestions?

You are essemtially describing MPEG. In its normal form MPEG sends a key
frame about twice a second and the rest as two types of inbetween frames
of differences.

Various other codecs do similar things (eg the Ogg vorbis) codec but have
encoding technology differences.

If your input capture format is forced to be JPEG then you may well need
to use MPEG and a tool that can generate the key frames without
decoding/recoding the JPEG image. If you can capture raw and feed to the
encoder then you've got a wide variety of choices.

Alan
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On 02/16/2012 11:52 PM, Alan Cox wrote:
> If your input capture format is forced to be JPEG then you may well need
> to use MPEG and a tool that can generate the key frames without
> decoding/recoding the JPEG image. If you can capture raw and feed to the
> encoder then you've got a wide variety of choices.

Does a tool like this exist now? I was writing one for my PhD (before I
quit to join Red Hat and at the time there was nothing out there in
public. Unfortunately I lost my rights in the code when I left the position.

Regards,
Bryn.

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