Thursday, April 23, 2009

Rosettes – everything you didn't realize you needed to know - part 1 of 2

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7 comments:

  1. Hi gordo, how do we reduce rosette without using higher line screen? i'm printing at 150 lpi the shadow on the neck looks terrible

    thank you
    Andy

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  2. To Andy:

    Without changing the screening, about the only thing I can suggest is changing how you do your RGB--->CMYK separations. The majority of separations done in PhotoShop use Medium GCR (PShop's default). This means there is quite a bit of black with C and M in skin tones and hence prominent rosettes. If you use UCR as the separation method instead, then the amount of black used in skin tone will be much reduced, and therefore the rosettes should be less prominent. Color won't be as stable on press though - but that may, or may not be an issue for you.

    Staying with an AM screen, there maybe another method available to you: Megadot 2 or Megadot Plus screening. It used to be available with Heidelberg workflows circa 2001. I don't know if any other RIP vendors have licensed it. I'm not even sure if Megadot Systems Ltd are still in business.

    best, gordon p

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  3. Update on reducing the visibility of rosettes -
    Please look at the below post prompted by Andy's question that expands on my answer above:

    http://qualityinprint.blogspot.com/2009/07/reducing-or-eliminating-objectionable.html

    best, gordon p

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  4. Why is the cyan at 105º?
    Isn't the dot pattern identical every time it's rotated 90º?
    If it is, why isn't the cyan angle simply 15º?

    I know there's an explanation to this, as sometimes the yellow rotation is also told to be 90º. So why is 90º different than 0º?

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  5. To anonymous,

    You are correct, 90° is the same as 0° (it's also the same as 180° and 270°). And Cyan at 105° is the same angle as 15°. Whether it is called 105° or 15° depends, I think on where you place 0° - on the vertical or horizontal plane and whether you count the degrees by going clock-wise or counter-clockwise.

    One should at least be consistent (I'm probably not), however, other than perhaps being initially a bit confusing, I don't think it matters very much since, in the end, they are all the same angle.

    best, gordo

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  6. The difference between 15 and 105 makes sense for elliptical screens.
    To reduce a moire pattern the elliptical screens for CMK should have angles (measured for principal axis) with a difference 120 degrees.
    A 90 degrees turn of one of its screen will result in worse moire than with round screens.

    Best regards,
    Alex

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  7. It doesn't matter whether the screens are elliptical or not. The important thing is the angle offset between the three screens and that is 30 degrees. A 30 degree offset is the same offset as the 120 degree offset that you mentioned.

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