Dots of gum, eating crow

From: Katharine Thayer ^lt;>
Date: 03/28/05-07:56:22 AM Z
Message-id: <>

I promised, a while back, to eat some crow with regard to dots of gum,
and I finally have had some time to put a web page together about it, so
herewith I chomp, starting with some background to lead up to the

This goes back to when I was puzzling about the statement that
stochastic bitmap negatives print dots of gum. I said that it didn't
seem to me that that statement made sense, because the dots are on the
negative; what prints is the irregular, negative-space around the dots.

Fast-forward to a printing session where I was reprinting some old
negatives on Arches Bright White paper and finding that I couldn't print
with my old Pictorico negatives because there's a window-screen-like
grid on the negatives that didn't show in gum prints on the old Arches
Aquarelle or on Fabriano Uno or Lana or other papers I've used, but
prints very precisely in gum on this very smooth, very crisp paper.

So then I grabbed an old stochastic bitmap negative that had been
printed on transparency a long time ago from a 300 dpi laser printer,
slapped some emulsion on a piece of the Arches bright white paper, and
printed that, to see if I could answer for myself the question of what
shape the gum prints in, when you print a stochastic bitmap image. As
you will be able to see in the enlarged images, while the shadows and
midtones are made of patches of gum interrupted only by white dots
reflecting the occasional black dots on the negative, the highlights are
in fact made of dots of gum. I include an enlarged part of the negative
to show why this is the case.

One caveat: I don't know if this is true of all stochastic bitmap
images; I can only say it's true of laser printer negatives; I show here
two prints, one made from the old laser printer bitmap neg I just
mentioned, and one from a bitmap neg I printed on my current 600 dpi
laser printer for the purpose of this demonstration Stochastic bitmaps
from laser printers is what I always printed my gum prints from, back
in the days before inkjet printers got good enough to make reasonable
negatives with.

I tried, for the purpose of this demonstration, to print a stochastic
bitmap inkjet negative on Pictorico, but it didn't print very well on my
printer and I couldn't tell where the negative ended and printer
artifacts started, and I didn't bother to print it on gum, since I felt
the results would be uninterpretable. So like I said, this may only work
for laser printer stochastic bitmap negatives. But I think it's kind of
interesting, anyway.

In that same discussion, it was suggested by several people that
printing in dots works better for achieving a simulation of continuous
tone in gum than does printing from a continuous tone negative,
since.... (I never can get this argument straight, because it doesn't
make any sense to me)...something about the gum needing sharp edges to
keep it from pulling off the gum next to it, or something of the sort.
I said I doubted that, and this page shows a comparison of the two,
printed from the same image, from which you can draw your own

It was also suggested that stochastic dots may be a better way to print
a contone-like image on difficult surfaces like glass. The information I
provide here doesn't address that question and isn't intended to provide
support or nonsupport for that particular issue. Nor is it intended to
address the question of halftone screens, halftone dots, or anything but
stochastic bitmaps.

Oh, here's the URL (and by the way, this is a huge page with lots of big
images, so it will take a bit to download, and it also won't be staying
on my site very long because it takes up too much space) :

Katharine Thayer
Received on Mon Mar 28 15:52:12 2005

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