Message-ID: vtu2jx9pj2.fsf@cliwe.ping.de
To: Paul Kaiser <buddy@effingham.net>
Subject: Re: tiff2ps: G4 compression in PostScript]
From: fdc@cliwe.ping.de (Frank D. Cringle)
Date: 29 Jan 2000 10:48:49 +0100
Cc: tiff@olympiakos.com
In-Reply-To: Paul Kaiser's message of "Fri, 28 Jan 2000 19:15:33 -0600"
References: <v03130300b4b7ee487663@[216.43.19.88]>
Sender: owner-tiff@olympiakos.com


Paul Kaiser <buddy@effingham.net> writes:
> When I compare the image data in the PS file to the actual image data from
> the G4 compressed TIF, they are VERY similar. However:
> -- Every now and then there is extra data in the TIF file compared to the
> data in the PS file.
> 
> I cannot figure out what extra data is written in the TIF file as compared
> to the bitmap data written in the PS file. The G4 specs from ITU specify
> that there may be some padding at the End of the transmission, but that
> doesn't seem to be the problem, as the observed differences are not all at
> the end of the file.
> 
> Anyway, If I figure anything out, I'll let the list know. I hope we can
> crack this one soon; it is very important to me!

Surely the most important property is that the results are identical
*after* expansion.  I don't remember whether the ITU specifies the
encoder in such a way that there is exactly one legal encoding of a
given image.  In fact, I suspect it does not.  An encoder can decide
that part of a line can be most efficiently represented using
uncompressed mode, for instance, whereas a simple encoder just
"compresses" everything, which can turn out bigger in pathological
cases.

-- 
Frank Cringle,      fdc@cliwe.ping.de
voice: (+49 2304) 467101; fax: 943357

