Inverted angles in Harlequin

[MICHEL_GALEGO]

Could someone tell me if it is normal what happened to me?

It looks like the Angle of Cyan and Magenta were reversed.

I set up the cyan with 15º and the magenta with 75º, but on the plate the cyan is left with 75º and the magenta with 15º.

Thank you all for helping me.

 

[Repro_Pro]

Practically these two angles are the same.
Just rotate your (Zero-180) baseline by 90 degrees .

 

[Macmann]

These 2 angles are pretty much interchangeable. You can swap if there is a moire issue.

 

[MICHEL_GALEGO]

I was reading Gordon's post "Halftone screen angles".

He showed the best angles for tritons:

Http://the-print-guide.blogspot.com.br/2009/05/halftone-screen-angles.html

"For 3 / C jobs (e.g., tritons):
Darkest color at 45
Medium color at 75
Lightest color at 15

If I am not wrong, these angles must be based on my angle of vision.

I understood that my vision is easier to see certain angles.

For this reason, it places the darker color at the most difficult angle to see (45) and places the lightest color at the easiest angle to see (15).

If my CTP reverses the 75 with 15, will not I be printing out of the concept explained by gordon?

My medium color will be at the easiest angle to see (15). Can this cause problems with my tritone job?

I understood that these angles can be changed, just wanted to understand why my CTP reverses 75 by 15?

Thank you very much for your attention.

 

[gordo]

I was reading Gordon's post "Halftone screen angles".

He showed the best angles for tritons:

Http://the-print-guide.blogspot.com.br/2009/05/halftone-screen-angles.html

"For 3 / C jobs (e.g., tritons):
Darkest color at 45
Medium color at 75
Lightest color at 15

If I am not wrong, these angles must be based on my angle of vision.

I understood that my vision is easier to see certain angles.

For this reason, it places the darker color at the most difficult angle to see (45) and places the lightest color at the easiest angle to see (15).

If my CTP reverses the 75 with 15, will not I be printing out of the concept explained by gordon?

My medium color will be at the easiest angle to see (15). Can this cause problems with my tritone job?

I understood that these angles can be changed, just wanted to understand why my CTP reverses 75 by 15?

Thank you very much for your attention.

For some reason the angles you've selected are not being honoured by the RIP. Did you try checking "Override angles in job"? Unfortunately I don't know the Harlequin RIP so I don't know why it's not honouring your request. I would contact Harlequin support on that question.

The angles I suggested for tri-tones puts the 3 screens 30° apart which minimizes inter screen moiré and results in a rosette. The sequence from dark ink to light is from least visible angle to most. If you cannot use those angles then it's possible, but not guaranteed, that the image may not appear at its best. Put another way, you may not see a practical difference in production. The more important thing is that the angles that are used are 30° apart.

Re: the posts saying that the C and M angles are interchangeable. In a 4/C process that's true - but - keep in mind that the choice of screen angles has consequences.
So, if the angles are 15C, 75M, 0Y, 45K there is the risk of C/Y moiré because they're just 15° apart.
If the angles are 75C, 15M, 0Y, 45K there is the risk of M/Y moiré because they're now just 15° apart.

 

[MICHEL_GALEGO]

Many thanks Gordo !!
So I'm not crazy, my rip is reversing the angles of C and M.
I will try to change the settings to see what happens.

 

[MICHEL_GALEGO]

I found the harlequin manual !!!
I'm trying to understand the way RIP interprets the angles.

 

[gordo]

Bravo! And even better...you now know you're not crazy

 

[Repro_Pro]

Conventional screens are based on a square grid.
Dots placed at 15 degrees are symmetrically also at the angle of 105 degrees.
Dots placed at 75 degrees are symmetrically also at 165 degrees.
Consequently, they are both 15 degrees apart from 90 degrees which is also at 180 degrees.
So, why is there be a difference in Moire risk?

 

[gordo]

Conventional screens are based on a square grid.
Dots placed at 15 degrees are symmetrically also at the angle of 105 degrees.
Dots placed at 75 degrees are symmetrically also at 165 degrees.
Consequently, they are both 15 degrees apart from 90 degrees which is also at 180 degrees.
So, why is there be a difference in Moire risk?

You're right. The M/Y and C/Y moiré risk is the same. I'll revise my blog to clarify and correct. Thanks. Generally the screen angle swapping happens where the moiré concern is greatest. So if it's skin colors you're concerned about then you might swap the M for the K - i.e. the M goes to 40° and the K goes to 75°. The darkness contrast between the K and the Y is great enough that it should mask the moiré.

That being said though, the Y is typically set by the RIP to run at about 108% of the frequency of the other inks to help reduce the visibility of the moiré by changing its frequency.

 

[MICHEL_GALEGO]

Gordo, watch this video about RIP harlequin.

https://www.youtube.com/watch?v=dfhqg34zPP8

The angles are reversed in the video too.

Guys, I'm not having problems because of the inverted angles, I just wanted to understand why I put 15º and get 75º.

 

[Repro_Pro]

Back in the old days, when films were stripped on a light table, it was evident that if you flip a screened film (to reverse emulsion side) 15º would turn into 75º...
This knowledge had a practical use:
When I ordered quality sets of ORIGINAL angled halftone screened films at guaranteed percentages.
These usually came as sets of 4 in the conventional angles.
I never ordered the full set of 4 angles, just the 45º, 75º and 90º.
I produced my 15º by contacting the 75º as soon as it arrived.

 

[Macmann]

What's contacting? Man you're old ;-)

 

[Repro_Pro]

True, can't beat the calendar :-(
I can't remember what I ate for supper yesterday, but I can certainly bore you to sleep with explanations about contact printing and such...

 

[Macmann]

I worked at a place in the late eighties that used frames from Germany that had the light source coming from below. An arm pulled an opaque black cloth over the flats and sucked them down before exposing-that took a while to get used to. Back then strippers were wizards and were paid accordingly. Now the monkey hits the switch for $12 an hour. Heavy sigh.

 

[kyle]

My RIP (Apogee Prepress version 10) matches yours. Same angles, same result.

If you measure the angle clockwise instead of widdershins you'd measure cyan at 15 and magenta at 75. You could still have your angle line move "up" if you draw it from the right end. What would be most intuitive to me would be to set the base line vertical and the measurement line rotate clockwise from the bottom end.

I don't believe the method of measurement has been standardized anywhere (please correct me if I'm wrong), so both interpretations are probably equally correct.