From: francis@ircam.fr (Joseph Francis)
Newsgroups: comp.text.desktop,rec.photo
Subject: Re: Photo Scanning
Date: 22 Dec 92 14:06:02 GMT
Organization: IRCAM, Paris (France)

In article <1992Dec22.004028.1@cc.curtin.edu.au> zrepachol@cc.curtin.edu.au writes:
>In article <1992Dec17.161309.10151@ircam.fr>, francis@ircam.fr (Joseph Francis) writes:
>> I have some questions for desktop gurus. I have been working with
>> photographs for some time, and it is still a complete mystery to my
>> how to balance the image digitally so the halftone output is of a 
>> high quality for offset printing.
>> 
>...
>> My third try now is that I rebalanced the image for a maximum black
>> saturation of 85% and minimum of 10%. This doesn't look too bad on
>> screen, though it just seems low contrast (obviously). The image still
>> seems dark to me, but that relates to yet another problem. I'll be
>> getting the linotron transparencies this evening (2540 dpi, ~150lpi
>> screen).
>> 
>You will have to 'calibrate' your printer. The final print needs just a trace of
>ink in the highlights, and not quite 100% in the shadows. To get this you USUALY
>need a fine dot in the halftone hilights, and a small white 'dot' in the darkest
>areas. Include a gray scale and go off that. Examine the PIXELS in the data
>with a 'magnifier', or use a real loupe on the film. Then adjust for the effects
>you printer introduces. He will probably be able to tell you about the corrections
>you need for his press. Note that if you change ink, paper, press, plate type,
>Phase of moon etc you get to do it ALL again...

I think in the next print run, all of this will have to be done. I
suddenly understood why sometimes on badly cut paper there is the
little scale on one side with graduated tint squares. I wish there was
a good book in English on the subject, but I can't find one in Paris.
I found one from Adobe on 'Adobe-Accurate' screens, but that didn't
tell me anything about duotone.  I'll see what I can find in London
next week. I've had to really work from conceptually what is going on
in all the processes.

>Why are you outputing 300dpi files BTW? Why not full 2400?

1-bit 2540 dpi / 158.75 lines per inch = 16 dots per line, = 256 dots
(grey levels) per square pixel at 158.75 pixels per line.  This is
actually less information than I send at 300dpi - sending 2400dpi
8-bit images is useless. I could scan at 8-bit 158.75 dpi and do just
the same (and I'm beginning to wonder now as I see the numbers why I'm
wasting so much information...).

That's precisly why I'm trying to figure out how to do duotone nicely.
I can lay down a layer of 40% or 50% grey ink, at my 158.75 lpi,
256-level dots, and then lay down a layer of 100% black at 158.75 lpi,
256-level dots - I should be able to start approaching 300dpi with 256
grey-levels per dot. This is what is done for fine-art B&W printing,
though it all seems to be done with wet-chemistry photo methods now
instead of digitally.

What I'd love to have is a set of realistic factors for grey/black
duotone.  To get a 50% black: do I mix 75% of 40% grey with 20% black,
do I mix do I mix 100% grey with 10% black, 50% of 50% grey with 25%
black, what about halftone angles, and black-on-grey overlay...? I'm
working out a bit of the geometry and % figures now.

Why 300dpi? Everything I've read says that the eye, in normal
conditions 29.7 cm from a 21x21cm square (the correct distance for
looking at photographs, w.r.t. the image size for least optical
distortion), can't resolve more than 300dpi @ 256 grey levels per
pixel of the image. I think it has to do with the angle subtended by
the image on the retina, and the density of receptors. Color is
similar in treatment, 300dpi per color layer, but this model doesn't
account for discrepancies in color sensitivity (green, vs. red).

-- 
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| today; American - All American; doing, a bit so, and even more so.
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