Message-ID: 003d01bf3832$40c92eb0$24a6d4d1@MELANGE
To: "John Aldridge" <john.aldridge@informatix.co.uk>, "Tom Lane" <tgl@sss.pgh.pa.us>
Subject: Re: Alternative to LZW (was Re: tiffcp)
From: "Sam Leffler" <sam@errno.com>
Date: Fri, 26 Nov 1999 09:18:21 -0800
Cc: <tiff@olympiakos.com>
References: <15969.943635660@sss.pgh.pa.us>
Sender: owner-tiff@olympiakos.com


----- Original Message -----
From: "Tom Lane" <tgl@sss.pgh.pa.us>
To: "John Aldridge" <john.aldridge@informatix.co.uk>
Cc: <tiff@olympiakos.com>
Sent: Friday, November 26, 1999 9:01 AM
Subject: Re: Alternative to LZW (was Re: tiffcp)


> John Aldridge <john.aldridge@informatix.co.uk> writes:
> > How about PNGs algorithm?  I'm not suggesting the interleaving stuff;
> > but I thought the spatial filtering bits were important in order to
> > get the best out of the subsequent ZIP style compression.
>
> That's pretty much what I said to Sam when I heard he was putting in
> plain ZIP.  His reply was that in the context of TIFF, PNG-style
> compression shouldn't be considered a monolithic Compression type,
> but rather a combination of Compression = ZIP and some appropriate
> differencing predictor (cf. section 14 of TIFF 6.0).  The existing
> Predictor = 1 tag is equivalent to PNG's "Sub" filter.  I don't think
> Sam got around to inventing more Predictor values to match the other
> PNG filter types, but it would be a straightforward extension.
>

It's been a long time (and many lost neurons ago), but I think the rationale
was that adding the zlib-based compression scheme was simple and fit in well
within the existing TIFF (and libtiff) framework.  I figured that if people
really wanted all that was in PNG they should just use the PNG format.  Of
course if we'd been able to promote the zlib-based compression stuff better
(e.g. publish a spec) then it might've been good enough to fill the void
that existed.  I haven't kept up with the current state of the image format
wars so can't comment on whether it's worth doing anything new today.  At
one point Tom had convinced me that PNG was going to take over the world and
that I should go do something else--so I did :-).

    Sam



