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Thread: GATF color test

  1. #1
    lnivin is offline Senior Member
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    Default GATF color test

    We have a GATF chart that has a bar for each color, cyan, magenta, yellow, black, orange, green.

    These have thin horizontal and vertical lines that printed horizontal and vertical, except the orange looks different on output, The angle of the rules changes on output to be about 20 degrees angled.

    Why does it do this?

  2. #2
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    Sounds like you are describing slur targets. I can not think of a reason one spot would be different. Are you printing spot colors or cmyk builds for the spots?

  3. #3
    lnivin is offline Senior Member
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    These are the bars that run the length of the sheet at left and right edges on the full plate. We are outputting a proof that has been refined as 4-color process.

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    gordo is offline Senior Member
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    Quote Originally Posted by lnivin View Post
    These are the bars that run the length of the sheet at left and right edges on the full plate. We are outputting a proof that has been refined as 4-color process.
    Does the test form look something like this:



    gordon p

  5. #5
    lnivin is offline Senior Member
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    Yes, that's it. Except it also has orange and green bars at left and right. The orange bars are the ones that have the weird angles. The Refined PDF looks the lines should be vertical and horizontal.

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    lnivin is offline Senior Member
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    The proofs, the refined file the lines are true vertical and horizontal. On the press sheet, the lines print diagonal. I'm more interested in how the file is built to be able to do this, more than what these lines are used for on press - slur, paper, cylinders, etc.

  7. #7
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    gordo is offline Senior Member
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    Quote Originally Posted by lnivin View Post
    Yes, that's it. Except it also has orange and green bars at left and right. The orange bars are the ones that have the weird angles. The Refined PDF looks the lines should be vertical and horizontal.
    Those bars are called "Ladder Targets" They are for testing solid ink density and laydown from gripper to tail.
    The Orange and Green are because you have a test form for a 6 color press (CMYKOG) and the O and G should be printed on a 6 color press as solid single colors just like the CMY and K targets.
    The weird angles are likely caused from halftone screening the O and G targets. The halftone screen angle clashing with the lines themselves. Basically subject moiré.

    If you did not screen the O and G, then it's possible that the file has been resampled at some point. Resampling can cause anti-aliasing artifacts which in turn can appear as moiré. If that's the case then you should see the same problem occurring in the CMY and K targets.

    Another (remote) possibility is that the frequency of the lines in those bars are beating with the frequency of the output device that you are outputting to (pixel frequency or dpi). Again, subject moiré. Again, if that's the case then you should see the same problem occurring in the CMY and K targets.

    best, gordon p
    Last edited by gordo; 10-01-2010 at 05:16 PM.

  8. #8
    lnivin is offline Senior Member
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    It doesn't look like moiré. It is vector art. The lines are solid and the gaps are solid white. No screens.

    We are printing this as all process.

  9. #9
    lnivin is offline Senior Member
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    Gordo,

    You are right on the money!! I took a second look and because we are running as process, the screening is creating the illusion of diagonal bars.

    Thank you!!

  10. #10
    sfhellwig is offline Junior Member
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    It is also more visible in the orange because of the yellow content. If you are using the CMYKOG targets but printing them in CMYK, then the orange is all M and Y. Besides your screens being at angles the M and Y are only offset by 15 degrees instead of 30 degrees. In full 4C you don't tend to see it but in yellow toned pictures the natural dot pattern is much more apparent due to the disonance of the yellow screen.


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