lossless compression of still images - recommendations?

[Robert Heller]

At Sun, 19 Feb 2017 16:28:16 +0000 (UTC) "Ubuntu user technical support,  not for general discussions" <ubuntu-users at lists.ubuntu.com> wrote:

> 
> On Sun, 19 Feb 2017 09:10:59 +0000, Colin Law wrote:
> > On 18 February 2017 at 22:43, Karl Auer <kauer at biplane.com.au> wrote:
> >> On Sat, 2017-02-18 at 21:11 +0000, Colin Law wrote:
> >>> On 17 February 2017 at 21:11, Robert Heller <heller at deepsoft.com>
> >>> wrote:
> >>> >
> >>> > Use tar+bzip2.
> >>> I thought that the OP meant that he wanted to compress the series of
> >>> images, making use of the fact that they are little changed from one
> >>> image to the next. Much like video is stored.
> >>
> >> Because the images are very alike, they should compress exceptionally
> >> well. That's kinda how modern compression works; it looks for recurring
> >> sequences in the data and replaces them with a index numbers
> >> representing those sequences. It stores the sequence once, and the
> >> index many times. Multipass compression looks for metasequences and
> >> does the same with them.
> >>
> >> I think the OP will get better compression out of tarring them up
> >> first, then compressing them, because that lets the compression program
> >> look for sequences across the whole set, not just within one image.
> >> However, it would be worth trying both (compress first and tar first).
> >> It would also be worth trying a few different compression programs.
> >
> > Yes of course you are right, I had not considered the fact that
> > compressing the tarred fileset will detect the repeated patterns
> > across the set. I was thinking that the intention was to compress
> > separately, which would be less likely to find the repeats, I presume.
> 
> Ya, but.  I wonder if the "unchanging" regions in the various images 
> result in identical JPEG encoding.  It would only take the slightest 
> variation in light intensity to render regions totally different -- 
> especially if very high resolution images are sought.

If the OP wants truely lossless compression, he is not going to be using JPEG. 
JPEG is *lossy* compression.  He should be using PNG instead.

> 
> However, it _is_ something to carry out an experiment on!
> 
> Jonesy
> 
> 

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