Re: File formats

Subject: Re: File formats
From: "Walker, Arlen P" <Arlen -dot- P -dot- Walker -at- JCI -dot- COM>
Date: Thu, 21 May 1998 14:14:25 -0500

As for when and why to use them, I hope someone else will pipe up
about now. Perhaps Arlen?

OK, here's my nickel (inflation, you know):

For web (or other monitor-based) work:

JPEG or GIF. Monoplatform formats, such as WMF or BMP or PICT make me
nervous. TIFF is almost cross-platform; it was intended to be, but the
implementations of it are sometimes lacking. Proprietary formats limit the
audience. There's a new format in town, PNG, but I'd wait a little while
before adopting it, as the support for it isn't in a lot of places, yet.
It has promise; in a few years it could swallow both JPEG and GIF.

The more colors in the image, the better it will look and compress with
JPEG over GIF. Use GIF if there are few colors in the image or you need
animation or transparency in the image. If you're not sure, try both and
choose the best.

Best pixel densities are in the 72-96 pixels per inch range. I generally
use 72. Mac video is based upon 72/inch, while PC video is based on 96/
inch. (This, BTW, is at the root of why web pages occasionally look
different on one platform or another. That and the screwy gamma on most PC

For printing:
Typically EPS works well if you're working with a PS-based printer. Pick a
density appropriate for your final output. No sense sending a 2400 bpi
image to a 1200bpi printer, for example.

For storage:
JPEG makes me nervous as a storage format, because it has a lossy
compression scheme, meaning it throws away information it deems
unnecessary. This means repeated edit/save cycles may result in
unacceptable degradation. If you store in JPEG, make sure you keep the
original untouched by any subsequent edits, so you can always return to
the original quality JPEG if necessary.

Otherwise, store in a lossless format, like the native format of whatever
your usual image tool is. For me that's Photoshop (bless the Knoll
brothers). I do any editing in that format, then save the finished image
to the format I need for the use I have in mind.

There are other specialized areas where different image formats excel
(despite the "standard" of IGES, DXF is the more reliable CAD interchange
format most of the time). But those areas are so specialized that if
you're in them, you probably already know better than I what formats to
use. ;{>}

I serve as image librarian here for about 1GB of presentation images. The
server folks don't like me much, because I keep high-density images out on
their(!) server, but we've had needs for images in densities from 72dpi on
up to 1600dpi and even higher, and I don't want to get caught with a low-
res image for a high-res output device. Tacky. Very tacky.

Have fun,
Chief Managing Director In Charge, Department of Redundancy Department
DNRC 224

Arlen -dot- P -dot- Walker -at- JCI -dot- Com
In God we trust; all others must provide data.
Opinions expressed are mine and mine alone.
If JCI had an opinion on this, they'd hire someone else to deliver it.

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