Unwanted Mesh pattern on RIP'd file

[xspillai]

Dear All,
I am a graphic artist with a unique problem. Sometimes (this is the third time) when I output a graphic from Corel X3 for the Service Bureau, the RIP'd film shows an unwanted mesh pattern on the Black separation only.
Changing the service Bureau does not solve the problem. Check out the attachment.

 

[dmz]

That's a Moiré

That's called a moiré, and its caused by incorrect screen angles. If that's a four-colour process job, then each screen should be angled 30º away from all others, i.e, 15º, 45º, 75º for M, K and C, and 30º for yellow, which breaks the rule, but yellow being the "lightest" colour, isn't as critical. Either that or the person setting up the plates is drunk - it looks like your registration is off too...

Of course modern imagesetters can't exactly reproduce those angles, so the RIP manufacturers will have their own recommended screen angles for different resolutions. If you're over-riding the RIPs screen angles in your PostScript file, then that's the problem, and thats why changing suppliers wouldn't change the result. Either set your file to allow the "default" screens on the RIP to be produced, or enter the correct screen angles and rulings for the imagesetter you will be using to produce your file.

Of course, this whole problem is avoided when using stochastic or HiFi screening, which doesn't depend on screen angles, but that's a horse of another colour...

dmz

 

[xspillai]

The image is a PSD. I agree that it looks like a moire perhaps, I suspect, caused by incorrect screen angles at seperation. The registration looks a bit off because the only way I could get the image I scanned to highlight the Moire was to superimpose the black film on the printed image and scan.
Is there any way to prevent this happening again? Can I prepare the screen angles myself on an EPS file? If so How?
Thanks a lot for your inputs

 

[jagolden]

The image is a PSD. I agree that it looks like a moire perhaps, I suspect, caused by incorrect screen angles at seperation. The registration looks a bit off because the only way I could get the image I scanned to highlight the Moire was to superimpose the black film on the printed image and scan.
Is there any way to prevent this happening again? Can I prepare the screen angles myself on an EPS file? If so How?
Thanks a lot for your inputs

I think the answer was given above. Make sure you are not over riding the default screen angles in Photoshop.

If the image was originally scanned from a printed piece and the sep'd again, that will be some of the problem also.

Direct, on-the-spot solution and advice should be given you by your pre-press house. If you're using a service bureau, give it up and find a true pre-press house to do your work if you want to handle that work yourself. Otherwise, let the printer handle the seps. No one knows his press needs better than him. However, ask the sb and they should be happy to give you help. It's the best way to truly learn this if you're new to it. If you have a good relation with them, just hang out there and soak it up.

Understanding how the computer generated design actually makes it to a printed piece will be some of the best knowledge you'll ever gain.
Most institutions today only teach how to do nice things with software, but with no pre-press or print background. It's astounding.

In the past, working with Color-Key progressive proofs or Cromalin progressive proofs was a great learning experience. I seen great 4c process come off of 2-color presses. That's 2 passes that not only have to register correctly, but the dot gain has to be considered and controlled over two serperate passes. That's a printer that knows prepress and press.

 

[dmz]

That's another moiré!

Yes, the above poster has hit on another source of moirés. By the way, if you want to understand just exactly what a moiré is, get two pieces of screen or sheer fabric and, while looking through them, rotate one against the other. You will see a rather spectacular illustration of just what we're talking about. It's a marvellous phenomena, but not one you want to see on press! If you're at all musical, you can compare this to two instruments playing the same note without tuning to each other - you hear a "beat" between the two, i.e. if one is 440 hz, and the other is 441 Hz, you hear the volume swell and subside at 1 Hz, or once a second. Musicians tune until the beat goes away, then they know they are in tune. It's the same phenomena, only you can see it, your screens are "out-of-tune"!

The above-poster is also correct in noting that your prepress operator should know and be able to explain this to you, and if he/she can't, then... run, screaming, in the opposite direction - they don't know what they're doing. A prepress operator should know how to over-ride your files' settings, or supply you with the correct info specific to your application. If you are giving them EPS files, then yes, your screen angles are probably embedded in the file, but they are easily changed/over-ridden by any decent prepress operation.

Scanning already-screened material can lead to moiré as well, as noted above. The trick is to scan at more than twice the screen frequency, and then "blur" slightly in Photoshop, or use the "median noise" filter to even out any rough spots. I usually scan at four times the required frequency and size and then simply reduce the size in PS, which completely removes any sign of the original screen.

Reproducing highly detailed fabrics or patterns with traditional four-colour screens can also lead to moiré, which is why Sears and others don't use these types of colour seps to print their catalogs. Compare a Sears catalog, or a copy of the Enquirer, to a "normal" magazine with a loupe(magnifier) and you will clearly see the difference.

Keep going, it's called the school of hard knocks, and these hard lessons are the best training there is - you will never forget this lesson - ever.

dmz

 

[dmz]

More on Moirés

Ooops! I forgot one other kind of moiré which I have run into since the weird-wide-web started. It's a kind of reversal of the above problem, and happens when a pattern is sub-sampled, i.e., divided or scaled down. The math is a little harder to explain, so I'll just give you an example to look at. The image on the left demonstrates what a sub-moiré (I just made that up) looks like, while the image on the right shows how to deal with it. Obviously, the graphic artist found the best way to deal with this problem was simply to blur it out. The front grill of the G5 is perforated in a grid pattern, and reduction has caused a moiré (sub-moiré ;-}) pattern to emerge.

Okay, enough on this subject!

dmz

 

[xspillai]

Thanks a lot for all your inputs. So now I know where the problem lies. Question is what do I do to prevent future Moires? I run a press in a smallish town. I have a 2 colour Adast press in which I get excellent results. I enquired from thew SB as to his angle settings. They are C-15, M-75, Y-0 and Black 45. These are the the default CorelDraw settings.

The Sb was at sea about how this happened. I cannot "turn and run" as there are just two SB's here and both are clueless as to the reason.

The only alternative, as I see it, is to use the SB's defaults and prepare an EPS myself incorporating the SB' default angles. Can anyone brief me on the details on how to do this?

 

[dmz]

Yes, presuming the SBs have the correct default angles on their RIP, your safest bet is to allow it to default to the right angles. I can't believe the SB doesn't "know" their RIP a little better than that. Sigh. I don't know how much more we can help you with this, it seems your SB operator needs an education. Trust me, assuming the SB operator even has one, his RIPs manual will be able to answer these questions:

1. Which f#$%ing angles am I supposed to enter in my file?
2. How can the RIP replace user-entered values with the correct ones?

I can't remember exactly how CorelDraw sets up an EPS file, but I believe most apps allow you to set the angles to the "printer's default". If you do this, the RIP must be set to produce the default screen angle/ruling you want before submitting the file to the RIP.

If, on the other hand, there does not seem to be a way to set it to default, then enter the exact rulings and angles you get from your SB's RIP's manual.

Every RIP is different, so it's pretty specific to the make, model and vintage of the RIP. If you're dealing with a Linotype, Varityper, AGFA or Scitex RIP, then you are in luck, they are all industry-standard RIPS. However, if you are dealing with an old Harlequin or Birmy, things are not so clear-cut.
Best of luck, maybe your SB operator should visit here?

dmz