Digital BW, The Print

on 8/26/01 5:09 PM, SKID Photography at skid@... wrote:

>> Very likely it is image dependent, and with high-detailed photos it
>> is worse than those with smooth areas.
> 
> Isn't that how compression works...a complex algorithm of 'averaging'?  An
> image that has more 'plain' or
> 'broad' areas of the same tone, compresses more than an image of 'unique'
> tonal ranges.

It depends very much on the compression algorithm. See a long discussion on
the Filmscanners list and/or Digital Silver a couple of months ago.

LZW compression typically does not do well on data that does not have
consistently repeating data patterns, because it relies on replacing
repeating patterns with smaller indexed markers. It works extremely well on
e.g. text because certain patterns like 'the' or 'and' recur. It does not
work well on detailed image data like 4000 dpi Tiffs for obvious reasons. I
oversimplify but you get the point.

RLE (run length encoding) works well on images that have large identically
flat areas, like bitmapped solid-color graphics. Again it is not very
helpful for information-rich images like photographic TIFFs.

JPG does well on images that have smooth tonal transitions, because the
frequency analysis it does on each 8x8 area works best with relatively
high-frequency data. It does not do well with large flat areas because the
'chunking' of the image becomes obvious, and it does not do well with *very*
high frequency random data like fields of wheat unless you crank the quality
all the ways up. It is lossy of course so out of the running here.

Genuine Fractals is an interesting case. Its lossy compression option does
seem to do well with most images. It claims the recovered file is visually
indistinguishable and I have to agree in the tests I have done it seems to
be. One thing GF is particularly good at doing is preserving sharpness and
the impression of high-frequency detail. Its lossless compression doesn't
achieve great savings but seems to do better than LZW on photo TIFFs in my
experience.

MrSID I haven't tried, but Lizardtech have just acquired Genuine Fractals so
they will either kill it off or (hopefully) incorporate the technology into
future versions of their own software.


-- 
John Brownlow

http://www.pinkheadedbug.com

ICQ: 109343205