Image REs and Line screen

[gordo]

I have never liked supercell.

if you are mixing the percentages like 10% and 12% to get a pseudo 11% this will only look goo with one or two steps available.

as you climb higher in the LPI the fewer levels of grey are available. If you start mixing 1% and 4% to get 2% or 3% there is some artifacting that occurs. The higher the worst the artifacting. I know that the layman may have a hard time distinguishing this but try telling that to the agency that is looking at a silver metallic car that is complaining about the "feel" of the tone not being right but cannot figure out the problem.

supercell does not solve all of the problems. but no one is offering a way to optimize the supercell with the proper LPI/DPI/supercell tech.

for example using 2400 DPI can I create a smooth gradient at 600LPI from 1% to 10% over a 6" area? what if this is a photo with the same type of gradient?

There are two main aspects of supercell screening. The first and primary is that it allows for near perfect irrational screen angles - i.e. screen angles other than 45° and 90°. The other aspect is the optional "dithering" of halftone dots to increase grey levels. These two reasons have enabled 2400/2540 dpi to become the standard, most used, CtP and imagesetter resolution. I would guess that virtually all AM/XM halftone screens today from all the vendors use supercell screening for those two reasons. I know for a fact that not just the layman would have a hard time distinguishing the difference between 2400 dpi Supercell and higher dpi rendered screening at the same lpi as far as visual quality is concerned - but so would the vast majority of printers and press operators.

Once you go over 175 lpi, it becomes increasingly difficult, IMHO, to justify why you would still use an AM/XM halftone screen. It makes much more sense to switch to an FM screen which does not have the angle or grey level issues of an AM/XM screen. So, in your example of using 2400 DPI to create a smooth gradient at 600LPI from 1% to 10% over a 6" area - I would say that was a foolish print production method. Instead, use an FM screen which could render your example with no trouble at all.

best, gordo

 

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Yes it makes sense - the penny has finally dropped.
Thanks very much Gordo.
BTW a Heidelberg marketing document says that Adobe PostScript level 3 RIPs have a 12 bit capability and that Heidelberg is trying to harness this. I believe this would give a potential 4096 shades of grey. Do you know of anything happening here?

 

[gordo]

Yes it makes sense - the penny has finally dropped.
Thanks very much Gordo.
BTW a Heidelberg marketing document says that Adobe PostScript level 3 RIPs have a 12 bit capability and that Heidelberg is trying to harness this. I believe this would give a potential 4096 shades of grey. Do you know of anything happening here?

Well, PostScript 3 is a 15 year old technology...

AFAIK, Heidelberg has two methods for overcoming grey level limitations.

First is the dithering of halftone dots using supercell screening to achieve a the number of grey levels they feel is required - minimum of 1000 - for quality reproduction. Their dithering technique to do this is called "Multidot" and is available in their HQS (High Quality Screening) AM screening and IS (Irrational Screening) screening.

Second is a technique to increase grey levels based on asymmetrical resolution. Basically you increase the resolution of the output device in the fast scan direction (the rotational direction of laser mirror or drum that holds the plate) by slowing down the rotation. That effectively gives you double the resolution in the fast scan direction and as a result gives you more pixels to build the dot and therefore more grey levels. Unfortunately by slowing down the imaging you reduce throughput so I doubt that this method is very popular.

best, gordo