Digital BW, The Print

On Tuesday, February 07, 2006, at 09:37AM, Mark Savoia <mark@...> wrote:

>Why do you say LZW is not?
>Mark
>
>On Feb 7, 2006, at 11:58 AM, Roger Howard wrote:
>
>> ZIP compression within TIFF is generally well supported these days  
>> and is fairly effective with 16bit/channel files (LZW is not).

Sorry if I wasn't clear enough.

LZW works fine; it's lossless. So is ZIP. But in my experience LZW on 16bit/channel TIFFs rarely does anything but actually increase the filesize! Quick example using a 16bit RGB image of a manuscript page:

Uncompressed TIFF - 82.4MB
ZIP compressed TIFF - 78.6MB
LZW compressed TIFF - 105MB

This is consistent with my experience; depending on the nature of the image ZIP might save you 10-40% on filesize compared with uncompressed; LZW on the other hand seems to almost always INCREASE the filesize.

This may just be a bug in Photoshop's TIFF/LZW implementation for 16bit, or may be fundamental to LZW, but since Photoshop is pretty much ubiquitous I use its results as my guide. For lossless compression of 16bit grayscale or RGB from Photoshop, TIFF/ZIP is the best option weighing compatibility, etc. Of course you might decide the performance hit (opens/saves) from ZIP outweigh any minor storage capacity savings (I agree for flat TIFFs - but I do use ZIP compression on layered TIFFs).

-R