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Re: filmscanners: (anti)compression?



on 8/6/01 8:44 PM, Tomasz Zakrzewski at tomzakrz@ka.onet.pl wrote:

> I ibserved a strange phenomenon while applying compression to my tiff files
> in PS 6.0.
> 
> I scanned a photograph on my flatbed with 16bit color, RGB and at  1200dpi
> so that the resulting file was 100MB .TIF
> When I applied LZW compression the resulting file was... 125MB!
> What to think about it?
> When using ZIP compression the file was 95MB-that doesn't surprise me as I
> know .tif files don't compress well. But 125MB! :-) I didn't expect that.
> 

That is because LZW works by substituting colors with variables. If you have
an image with very few colors and shades, LZW will compact it to a tiny
fraction of its original self. On the other hand, a very diverse image with
lots of colors and shades will require tons of substitutions, and the size
becomes larger.

ZIP works by actually packing the data into "empty space". As a result, the
size does not vary as much, but is lossy. Since LZW is lossless, try
compacting with LZW first, then if it does not give you good compression and
you can stand a little bit of image quality loss, use ZIP. Actually, I did
not know you could use ZIP for images, and assumed that you were referring
to JPEG...

;-)

Jim Snyder




 




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