Re: filmscanners: tiff compression

[Ivar Järnefors <ivj@jarnefors.com>]

LZW is as you say lossless, it is just a more clever way of 
describing rows of pixels using a surprisingly simple little 
algorithm.

Most GIF-primers for web graphics will tell you exactly what is done. 
However, I have found that especially in 48 bit mode for some reason 
I actually get slightly larger files using LZW that no compression at 
all (less compression would not have surprised me, but largerŠ oh 
wellŠ)

The algorithm also depends on uniform (read non grainy) color fields 
to work well, which could be a culprit in my case (very grainy 
available light pictures)

I have not delved into it further, just stopped saving 48 bit files 
with LZW so I don't know if its just a strange fluke with those files 
I actually tested saving both ways or the LZW algorithm failing 
miserably on 48 bit on the whole. Come to think of it, it was 
probably 16 bit grey scale I tested anyway...

Anyone know the details of 48/16 bit versus LZW?

Regarding cross platform and software compatibility you don't have to 
worry to much. Any implementation that can read GIFs should be able 
to read LZW compressed TIFFs as well. Same copyright protected 
algorithm. And in this web crazed world reading GIFs is more or less 
mandatory for any viewer, editor, presentation program, word editor, 
mail virus etc. But perhaps LZW makes the file more prone to 
corruption, Harvey Ferdschneider did warn that clients of his had 
experienced problems with it, so caution using it on files for 
delivery is probably wise.

The dialog you get when saving (for mac or PC format) is a relic from 
old times past when software on the different platforms was dependent 
on a certain bit order when compressing.

Best regards,

Ivar


>Hi all,
>
>what do people think about saving my raw scans as LZW tiff's?
>I am making 48 bit 6x7 scans on Nikon 8000, and they are over 500Mb 
>each, so lossless compression would save a hell of a lot of space, 
>but what are the drawbacks? can most programmes decode them if I 
>send them to people on a CD?
>
>Also, for catalogue/ magazine use what sort of RGB file size should I supply,
>something like a 12" print at 300dpi is still 75Gb uncompresses, 
>which seems big for most magazines
>
>thanks,
>
>Paul