On 01/22/2011 01:04 PM, Chris Mohler wrote:
> On Sat, Jan 22, 2011 at 11:15 AM, Ralph Zerbonia
> <ra...@universecentral.com>  wrote:
>> My question is whether or not anyone knows of any image disadvantages, is
>> there anything about LZW that would allow a loss of any visual fidelity?
> LZW is indeed lossless.  I just did a quick test[0] to be sure that
> GIMP's implementation is lossless and it turned out OK[1].
> PNG is a lossless format as well, and may give a slightly smaller file
> size than TIFF.
> Chris
> [0] I did the following in GIMP 2.6.10:
> - Open an uncompressed TIFF file
> - Save a copy with LZW
> - Close all files
> - Open original
> - Open As Layers LZW copy
> - Change LZW layer mode to "Difference"
> - Move the cursor about in the image while looking in the 'Pointer' dialog
> - All RGB values are 0% (black) throughout the image
> (could also flatten the image at this point and use levels or curves
> to verify that there are no anomalies)
> [1] In an older version of Photoshop (5? 6?) I did the above and came
> out with a very slight "shift" or "loss".  It could have been a
> mistake on my part, but at the time we figured it was a bug in Adobe's
> LZW handling.  I can't reproduce this any more in Photoshop - so
> whether or not it was pilot error or a bug, it's no longer relevant -
> except it's the reason I remember the above procedure ;)

I have always wondered about this.  Thanks, Chris for the information.

However, a couple related questions about LZW-compressed TIFF files....

a) For images under (for example) 50 MB in size, and on systems that are 
modern, fast, and with lots of memory, is there an approximate rule of 
thumb regarding how much longer opening and saving operations will take?

b) (Though not a Gimp issue) Are there any implications using 
LZW-compressed TIFF images in other applications?  Specifically, I use 
Perl scripts and ImageMagick to create four different sizes of JPEGs 
from each TIFF file.  Do such operations care whether or not a TIFF file 
has been LZW compressed?

c) Other than saving disk space and/or reducing file transmission time 
in the case of uploading/downloading, are there any particular reasons 
to use (or not to use) LZW compression?

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