Dan Margulis Applied Color Theory

Prescreened B/W Images

Prescreened B&W images
Posted by: "John Lauenstein"
Mon May 7, 2007 8:06 pm (PST)

Hey group,

Although this is a color theory group this question has to do with a chapter in an earlier edition of Professional Photoshop by Dan, so perhaps he might like to field a response.

At the recent Photoshop World in Boston, I attended a number of Dan sessions, at the end of one I asked Dan if he had any advice on how to handle prescreened B&W images. He directed me to a chapter 15, “Math, Moiré, and the Artist: A New Angle on Descreening”. This chapter once existed in “printed” form, but now resides on the accompanying CD as a pdf, in case anyone wants to read it. I have read this chapter numerous times and for what ever reason (perhaps old age) I can’t make heads or tails on one thing. Dan makes it quite clear that in order to prevent moiré the optimum angle between screens should be 30 degrees, in prescreened images this relationship already exists. The solutions offered for prescreened B&W’s are, assign a new screen angle in layout programs, scan at high resolution and down sample to soften the dot pattern, scan at a 15 degree rotation to insure the 30 degree separation between screens. Dan as always offers additional techniques to take the process over the edge, but in order to go there I have to get the images through the basics. Soooooo, the question is, are the above-mentioned solutions individual ones or do you use them in combination? For example, do you scan at a 15-degree offset and also assign a new screening angle?

I guess I’ll leave it at that. Thank you for any replies in advance.

John Lauenstein
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Re: Prescreened B&W images
Posted by: Dan Margulis
Tue May 8, 2007 12:46 pm (PST)

John Lauenstein writes,

At the recent Photoshop World in Boston, I attended a number of Dan
sessions, at the end of one I asked Dan if he had any advice on how to
handle prescreened B&W images. He directed me to a chapter 15, "Math,
Moiré, and the Artist: A New Angle on Descreening". This chapter once
existed in "printed" form, but now resides on the accompanying CD as a
pdf, in case anyone wants to read it.

Yes. While some people still have to do this, the problem of how to deal with prescreened images isn't nearly as important as it was ten years ago, when I wrote the original columns on which that chapter was based. Also, with the reduction in B/W work in favor of cheap color, what little of such work we get is usually in color, in which the screens can be subdued more easily than in B/W. After Professional Photoshop 6 (2000) the publisher and I felt we could no longer justify including a full chapter on the topic, so we exiled it to the CD thereafter.

I have read this chapter numerous times and for what ever reason (perhaps old age) I can't make heads or
tails on one thing. Dan makes it quite clear that in order to prevent
moiré the optimum angle between screens should be 30 degrees, in
prescreened images this relationship already exists.

Between C,M, and K, yes; but that takes care of all the available 30-degree increments, so the yellow has to be squeezed in edgewise. Fortunately, yellow ink is so light that nobody wll see a pattern no matter what angle is used.

The solutions
offered for prescreened B&W's are, assign a new screen angle in layout
programs, scan at high resolution and down sample to soften the dot
pattern, scan at a 15 degree rotation to insure the 30 degree
separation between screens. Dan as always offers additional techniques
to take the process over the edge, but in order to go there I have to
get the images through the basics. Soooooo, the question is, are the
above-mentioned solutions individual ones or do you use them in
combination?

In combination.

For example, do you scan at a 15-degree offset and also
assign a new screening angle?

Yes. You scan at a 15-degree angle because the angle of the black screen is almost invariably 45 degrees (ignore SWOP's recommendation to the contrary; 45-degree black has been standard for at least 15 years). By scanning at 15 degrees we create a 30-degree difference. (In the case of a color original, we scan at 45 degrees, which is 30 degrees away from both the cyan and the magenta, unless the black is particularly heavy. And if we're *shooting* something that may moire, it's best to position the camera at a 30-
degree angle relative to the pattern.

As for assigning a new screen angle, this may be problematiic today. After we scan at the 15-degree angle, we have to rotate the result so that the file itself won't print at an angle. Having done this, the file again contains a 45-degree screen (or, better, somewhere between a 43 and 47-degree screen, since it's not feasible to get the angle exactly correct on scanning).

This 45-degree screen angle, which, being native to the file, can't be eliminated, will conflict with the 45-degree screen that the RIP imposes, causing moire. Historically the solution is to save the file as EPS, which supports an instruction that overrides the RIP's defaults and instructs it to screen in a different fashion. Such an EPS can tell the RIP to output the screen at a 15 or 75-degree angle, 30 degrees away from the native screen.

The problem is that many if not most RIPs no longer honor that screen override command. So, before trying this in practice, you need to have the printer demonstrate that the command won't be ignored.

Dan Margulis
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Re: Prescreened B&W images
Posted by: J Walton
Tue May 8, 2007 4:12 pm (PST)

On 5/8/07, dmargulisnj wrote:

This 45-degree screen angle, which, being native to the file, can't be
eliminated, will conflict with the 45-degree screen that the RIP imposes,
causing moire. Historically the solution is to save the file as EPS, which
supports an instruction that overrides the RIP's defaults and instructs it to
screen in a different fashion. Such an EPS can tell the RIP to output the
screen at a 15 or 75-degree angle, 30 degrees away from the native screen.

I'm surprised this is such a big issue. Why not just scan the thing as linework and be done with it? Scan it as a Bitmap TIFF at 600 or 1200 dpi and there's no moire to worry about because the RIP doesn't have anything to halftone.

You have to be happy with whatever linescreen it was originally printed at, but without question leaving it at the original linescreen will look better than trying to fake a different one.
--
J Walton
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Re: Prescreened B&W images
Posted by: "RJay Hansen"
Tue May 8, 2007 7:31 pm (PST)

On 5/8/07, J Walton wrote:

You have to be happy with whatever linescreen it was originally
printed at, but without question leaving it at the original linescreen
will look better than trying to fake a different one.

Actually, in my experience, "without question" is not necessarily the case. Depending on the quality of the original halftone, there can be problems getting the scanning threshold set so as to be able to hold a sufficient range of both highlight and shadow dots.

I've had both experiences; halftones that could best be reproduced by scanning as bitmaps and halftones that reproduced better by scanning/descreening to grayscale and rehalftoning at the RIP.

RJay
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Re: Prescreened B&W images
Posted by: "Pylant, Brian"
Wed May 9, 2007 7:45 am (PST)

I'm surprised this is such a big issue. Why not just scan the thing as
linework and be done with it? Scan it as a Bitmap TIFF at 600 or 1200
dpi and there's no moire to worry about because the RIP doesn't have
anything to halftone.

Definitely something I would try, however I would recommend 2400 or 2540 ppi; in my experience 600 is far too low for linework (I can always see the jagged edges, without a loupe), and while 1200 ppi is better I see no reason no to go all the way and scan at output resolution.

BRIAN PYLANT
Manager, Electronic Prepress
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Re: Prescreened B&W images
Posted by: "John Lauenstein"
Wed May 9, 2007 8:54 pm (PST)

Many thanks to Dan, J Walton, RJay Hansen and Brian Pylant for your informative reply's to my prescreened problem. I am a commercial photographer by profession and do a little color management consulting on the side. I was drawn into this project, a 150th anniversary history book for a large church in Minneapolis MN., to help restore and prep images for this book. I had not anticipated that a number of the images would have been screened and 100 years old or more. To make matters even more difficult the book is being separated and printed in Korea!!! Needless to say, communication is difficult.

I have one image that is giving me a head ache. I tried to scan it as line art (bitmap??) as suggested, but it comes out all blocked up. The printing of this image is really bad, so I'm fighting that as well and to add insult to injury the designer/art director wants to run this image at 200% of the original. The existing screen ruling is about 100 dpi and if we treat this image as Dan suggested can I use it larger then life? If I manage to scan it as line art is a 200% enlargement a reality?

I might just send the original to Korea and say "we need to use this at 200% of the original, make it work".

Thanks again

John Lauenstein
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Re: Prescreened B&W images
Posted by: J Walton
Thu May 10, 2007 3:46 am (PST)

On 5/9/07, John Lauenstein wrote:

I have one image that is giving me a head ache. I tried to scan it as
line art (bitmap??) as suggested, but it comes out all blocked up. The
printing of this image is really bad, so I'm fighting that as well and
to add insult to injury the designer/art director wants to run this
image at 200% of the original. The existing screen ruling is about 100
dpi and if we treat this image as Dan suggested can I use it larger
then life? If I manage to scan it as line art is a 200% enlargement a
reality?

Hmm...printing a re-scanned, 100lpi, badly-printed picture at 200% is not the best idea. But that's our job - making bad ideas look good.

I wouldn't scan the art as line art. Scan it as grayscale as high resolution as you can to give you the best possible chance. From there you can try to manipulate the image to make a clean bitmap TIFF and simultaneously make a good contone out of it.

Come to think of it you have a third option, which is to make a decent contone out of it, res it up 200%, and then make your own Bitmap TIFF at a different angle and line screen. It might sound weird but if the job is printing in Korea I don't take the chance of letting them do anything fancy.

I might just send the original to Korea and say "we need to use this at
200% of the original, make it work".

See my previous statement. ;-)

--
J Walton
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Re: Prescreened B&W images
Posted by: "gary roushkolb"
Thu May 10, 2007 4:09 am (PST)

This might be a chance to make use of some of those ©¯strange and somewhat goofy©˜ filters that are in Photoshop. Use the filter effect photo as a large screened back (30%) background with a smaller clearer photo stamped in and add body copy over the screened photo. I don©ˆt know if you have the OK to change the design but this has been used on various aircraft books I have worked on and comes out quite nice on problem photos.

Gary Roushkolb
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Pre-screened B/W Image
Posted by: "Peter Leyland"
Thu May 10, 2007 9:22 am (PST)

A little late perhaps and not having seen the image this might not work but, we often have great success by simply vectorising the original image. Always worth a try as it is a totally automated process and simply judged.

Peter Leyland
PDQ Print Services
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Re: Prescreened B&W images
Posted by: "Stuart Larson"
Thu May 10, 2007 9:23 am (PST)

I haven't seen anyone mention this yet, but LinoColor scanning software has a built-in de-screening setting that works pretty well. Determine the line screen of the original, set the line screen at which the job will be printed, and the scans turn out quite well, I think. I've had good results with some pretty bad originals.

You just need to find someone who still is running a Lino scanner using LinoColor, which was unfortunately discontinued and is no longer supported. It was never updated for OSX, but I'm sure there are many people still using it. We still run it on an old Mac using OS9.

Stuart Larson
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Re: Prescreened B&W images
Posted by: "John Lauenstein"
Fri May 11, 2007 7:27 am (PST)

On May 10, 2007, at 1:25 AM, J Walton wrote:

I wouldn't scan the art as line art. Scan it as grayscale as high
resolution as you can to give you the best possible chance. From there
you can try to manipulate the image to make a clean bitmap TIFF and
simultaneously make a good contone out of it.

I have tried some techniques that David Blatner and Bruce Fraser offered in their book "Real World Photoshop CS", such as scan at high res, 3200 ppi in my case, then down sample to printing resolution. Use either Despeckle or Dust and Scratches filters to remove any remaining screen pattern. I think that because this image is screened with 100 lpi it lacks the detail to make a good contone image. Do you have any other techniques that you think might work?

Come to think of it you have a third option, which is to make a decent
contone out of it, res it up 200%, and then make your own Bitmap TIFF
at a different angle and line screen. It might sound weird but if the
job is printing in Korea I don't take the chance of letting them do
anything fancy.

I'm not sure I follow what you are suggesting here, but I assume you are referring to Dans procedure for handling previously screened images. I hope you will be patient with me, dealing with prescreened images is a whole new "ball game" for me.

I'm surprised this is such a big issue. Why not just scan the thing as
linework and be done with it? Scan it as a Bitmap TIFF at 600 or 1200
dpi and there's no moire to worry about because the RIP doesn't have
anything to halftone.

You have to be happy with whatever linescreen it was originally
printed at, but without question leaving it at the original linescreen
will look better than trying to fake a different one.

I wish I could use this technique, but on close inspection with a loupe the dot pattern is not all black and white dots, there are gray ones as well. We are looking for other versions of this image, but no luck so far.

John Lauenstein