Not especially related to EAD, but a couple of points on your note anyway:

In general one would not expect PNG or most other lossless compression
algorithms to produce "smaller" files than lossy algorithms such as JPEG on
CONTINUOUS TONE images (typically historical photographs or maps).  It
certainly can produce better quality images.

For example on a 6.8 meg source file uncompressed Targa image in 8 bit
grayscale,  PNG produced files ranging in size from 3.8 to 4.7 megs which is
less than 2:1 compression.  In contrast, the same source file compressed
using JPEG with a moderate quality setting, produced file of 1 meg in size,
almost 7:1 compression.

Just in case anyone was wondering/considering this in regard to continuous
tone images.

The reverse is potentially true with BI-TONAL images (straight black and
white, typically documents), since JPEG is wholly unsuited from compression
of bi-tonal images (poor compression performance, and a much higher
frequency and degree of artifacts).  Neither is GIF an especially good
choice.

Many would contend that PNG is better than TIFF (especially for bi-tonal
documents) for longer term storage and maintenance, since it is a public
domain standard, and less subject to variations in implementation.

For document imaging,  its compression performance is comparable to TIFF
CCITT Grp. IV compressed images, or PDF.  Comparative sample of all three
formats at:
http://www.bcarchives.gov.bc.ca/textual/governmt/vstats/v_events.htm#sample
You can assess file size and viewer compatibility yourself.

For access to a bi-tonal image, TIFF -  CCITT Grp. IV compression or JBIG
compression (the newer bi-tonal compression algorithm) is less helpful since
not all client computers have a suitable viewer that can handle that format
- image compression mix.  On the "wintel" side, Win98 and up have the Wang
image viewer installed that can handle TIFF Grp. IV.  On Mac I believe an
end user would have to download a separate viewer program.

PDF is okay for access, but it presumes that the user also has the PDF
viewer installed. Many, including myself would contend that it is not the
best long term storage format, hence a master scan in another format would
be an appropriate step to take.

PNG is supported by a few browsers, including more current versions of
Netscape and Internet Explorer
but there was a bug with some of the IE 5.XX versions that wouldn't allow
the file to be viewed by directly opening it from disk, instead it had to be
downloaded via a hyperlink.

Another option is to simply offer more than one format, and either
pre-convert, or convert on the fly based on the user's preference.

-----Original Message-----
From: Kathlene Ferris [mailto:[log in to unmask]]
Sent: Tuesday, March 13, 2001 10:06 AM
To: [log in to unmask]
Subject: PNG files


We are experimenting with using PNG (Portable Network Graphics) files for
web delivery of digital images linked to our encoded finding aids. PNGs are
attractive because the lossless compression format produces smaller files
than JPEGs and even GIF files, saving significant server space.  The main
problem that we have identified so far:  browser support is only fair and
applications support could be better.

Has anyone else investigated or even used PNG files? Specifically, we want
to know if there are other reasons not to go with this format.


Kathlene Ferris
Online Archive of New Mexico
University of New Mexico General Library