Page 3 of 4 FirstFirst 1234 LastLast
Results 21 to 30 of 39
  1. #21
    Alith7's Avatar
    Alith7 is offline Senior Member
    Join Date
    Oct 2007
    Location
    Wisconsin
    Posts
    345

    Default

    Ok, that makes sense.
    In poking through my settings I see that.
    and I am working with pre-screened 1-bit tif files. at least, that is what the Xenith sends to my plate burner.

    so, if I'm getting all this right, you're saying that a round dot with standard screen angles would be better than the Euclidean with the modified screen angles?

  2. #22
    gordo's Avatar
    gordo is offline Senior Member
    Join Date
    Aug 2007
    Location
    Victoria, BC, Canada
    Posts
    3,168

    Default

    Quote Originally Posted by Alith7 View Post
    Ok,and I am working with pre-screened 1-bit tif files. at least, that is what the Xenith sends to my plate burner.
    so, if I'm getting all this right, you're saying that a round dot with standard screen angles would be better than the Euclidean with the modified screen angles?
    Just to be clear, when I said: "The difference between 2540 to 2400 (2540dpi/100dpmm, 2400 dpi/96dpmm/) should not be significant unless you are working with prescreened (1 bit) files." what I was referring to was your customers sending you prescreened files. The plate burner will always receive 1 bit screened files from your RIP. However, if you receive a file from your customer that is 1 bit (already screened) then you will have artifacts if the prescreened file's resolution is not an even divisor of your output device's resolution.
    Here's a comparison:

    The upper screen is Euclidean.
    The lower screen is Round dot.
    They are very similar in that both use round dots for the majority of the tone scale.
    With a Euclidean the 51%-->99% dots are just a negative of the 1%-->49%. At 50% the dots convert to a square. That's why it is sometimes called a "transforming" or "composed" dot.
    With a Round the dots from 1%-->99% are the same, they just grow in size.
    As a general rule Round dots are most appropriate for a CTP work flow - and for reasons already listed are the best AM screen dot shape. There is much less, if any, need to offset the screen angles by 7 or 7.5 degrees to eliminate single channel moiré.
    If your RIP allows for a Round dot, you should try it. You may (or may not) need to put a curve in the plate if you lose some shadow detail.

    hope this helps, gordo

    BTW, don't examine my Round dot gradient to critically - it is for illustration purposes only. I don't have access to a RIP so I had to create the round dot threshold array and use PShop as a RIP - so the dots are not as perfect as they would be in real life.
    Last edited by gordo; 11-10-2009 at 05:05 PM.

  3. #23
    kev234 is offline Junior Member
    Join Date
    Mar 2008
    Posts
    18

    Default

    We were having problems with single color gradients, along with gradients that had up to 4 process colors in them using a round dot. Just this past week we switched to a euclidean dot, and they all came out beautiful. The round dot was fine for us as long as we didn't have any gradients on the job, but when we did there always seemed to be a problem with banding/stepping. The euclidean took care of that, and gave a very soft transition from top to bottom at 175 line screen.

  4. #24
    gordo's Avatar
    gordo is offline Senior Member
    Join Date
    Aug 2007
    Location
    Victoria, BC, Canada
    Posts
    3,168

    Default

    Quote Originally Posted by kev234 View Post
    We were having problems with single color gradients, along with gradients that had up to 4 process colors in them using a round dot. Just this past week we switched to a euclidean dot, and they all came out beautiful. The round dot was fine for us as long as we didn't have any gradients on the job, but when we did there always seemed to be a problem with banding/stepping. The euclidean took care of that, and gave a very soft transition from top to bottom at 175 line screen.
    Interesting.
    There is nothing about a round dot screen that would cause banding that wouldn't also appear with euclidean. Round dot screens also excel at vignettes. Effect does not always determine cause.

    best, gordo
    Last edited by gordo; 11-10-2009 at 05:05 PM.

  5. #25
    otherthoughts's Avatar
    otherthoughts is offline Senior Member
    Join Date
    Dec 2008
    Posts
    148

    Default

    Quote Originally Posted by kev234 View Post
    We were having problems with single color gradients, along with gradients that had up to 4 process colors in them using a round dot. Just this past week we switched to a euclidean dot, and they all came out beautiful. The round dot was fine for us as long as we didn't have any gradients on the job, but when we did there always seemed to be a problem with banding/stepping. The euclidean took care of that, and gave a very soft transition from top to bottom at 175 line screen.
    Perhaps these results are related to the tone bump characteristics for each dot shape that Gordo listed in his first post? I am assuming that nothing other than dot shape has changed for the exact same job printed with both dot shapes.

    4) The tone bump that occurs when dots touch occurs in the shadows at 75% so is much less visible in presswork (e.g. vignettes/skin tones) compared to Euclidean (50% bump where dots form a checkerboard) or Elliptical 40% & 60% bump.
    The tone bump associated with a particular dot shape should be incorporated into the calibration curve applied to the plate exposure. Data that is typically derived from some manner of press fingerprinting or profiling.

    If you change the dot shape you are in effect invalidating your press fingerprint or profile data.

    In essence, by changing your dot shape from Round to Euclidean, you likely opened up your 3/4 tone range by several percent, thereby increasing the shadow contrast. You likely also increased the contrast from the highlights to 1/4 tone range. Lastly the mid to 3/4 tone range would likely lose some contrast. Is this what you observed?

    What I am suggesting is that any time you make a significant change to your press, you should re-profile/fingerprint/calibrate it. The tone bump changes associated with a dot shape will change the gradient curve in ways that other press variables often can't. In short, dot shape is part and parcel with the plate-setter calibration curve itself.

    I personally always wanted to know precisely why something worked. This is the best I can think of as to why the dot shape change to Euclidean worked for you. It also implies that the calibration/profile that you were using to image your round dot plates were likely less than ideal by the same token.

    Best Regards
    otherthoughts
    Last edited by otherthoughts; 01-23-2009 at 11:37 PM.

  6. #26
    Slammer is offline Member
    Join Date
    Jan 2009
    Posts
    80

    Default

    Sorry all there is no such thing as a "best" dot shape, all work pretty well for that what they where designed for, Round, Euclidian, Elliptical, AM, FM.
    Not only that different companies use different spot functions to create their halftone dots. That means that AGFA's round or euclidian dot is not going to be the same as that from Harlequin, It all boils down to what you want to print. For instance something with a lot of pastel tones or skin tones would be a nightmare to print in Euclidian because of the somewhat abrupt cutoff in the mid-tones, elliptical would be better. A round dot has it's problems in the area of 70% to 80% tones, if your prints are dark above the halftones try round the jump is in the dark area anyway, A limitation would be printing with a heavy ink coating (screenprinting) of over 300% the image will be swamped over the three quarter tones. At the moment I am in love with mixing CMK in AM round dot and Y in FM coarse (or the least color in FM) this way I have a nice reach over the entire tonal range, it also keeps artefacts from forming.

  7. #27
    clark@colorpress.com is offline Junior Member
    Join Date
    Oct 2007
    Posts
    10

    Default Sorry if slightly OT but 2400 vs 2540 and screening

    Interesting discussion on dot shape and someone brought up the issue of 2540 vs 2400. My question is what happens when the front end RIPs at 2400 but the native resolution of the device is 2540? In our instance ApogeeX going to a Magnus 400. What method(s) are used to change resolution of the device to image at the lower resolution?

    We are having a lot of variability in dot size and I'm wondering if there is any connection. All my previous experience with CTP was with violet/silver plates, but I was told that thermal was much more stable. This has not been my experience with thermal so far.

    Clark

  8. #28
    gordo's Avatar
    gordo is offline Senior Member
    Join Date
    Aug 2007
    Location
    Victoria, BC, Canada
    Posts
    3,168

    Default

    Slammer wrote: "Sorry all there is no such thing as a "best" dot shape, all work pretty well for that what they where designed for, Round, Euclidian, Elliptical, AM, FM."

    GP: As as far as AM and CTP for offset is concerned - round dot is the best - see my original post for the reasons.

    Slammer wrote: "Not only that different companies use different spot functions to create their halftone dots. That means that AGFA's round or euclidian dot is not going to be the same as that from Harlequin"

    GP: Because the round dot is just that - unlike other spot functions - there is no significant difference between the different vendor's implementations - another benefit of the round dot.

    Slammer wrote: "For instance something with a lot of pastel tones or skin tones would be a nightmare to print in Euclidian because of the somewhat abrupt cutoff in the mid-tones, elliptical would be better. A round dot has it's problems in the area of 70% to 80% tones, if your prints are dark above the halftones try round the jump is in the dark area anyway"

    GP: Precisely why you shouldn't bother with Euclidean or Elliptical as they offer no value over the round dot and they have issues in important tone areas, i.e. 1%-75%

    Slammer wrote: "At the moment I am in love with mixing CMK in AM round dot and Y in FM coarse (or the least color in FM) this way I have a nice reach over the entire tonal range, it also keeps artefacts from forming."

    GP: Glad you do use the round dot. Y printing in FM is a useful way to avoid inter-screen moiré. It can, of course, be used with CMK that's using any AM dot shape.

    best, gordon p
    Last edited by gordo; 11-10-2009 at 05:05 PM.

  9. #29
    gordo's Avatar
    gordo is offline Senior Member
    Join Date
    Aug 2007
    Location
    Victoria, BC, Canada
    Posts
    3,168

    Default

    Quote Originally Posted by clark@colorpress.com View Post
    Interesting discussion on dot shape and someone brought up the issue of 2540 vs 2400. My question is what happens when the front end RIPs at 2400 but the native resolution of the device is 2540? In our instance ApogeeX going to a Magnus 400. What method(s) are used to change resolution of the device to image at the lower resolution? We are having a lot of variability in dot size and I'm wondering if there is any connection. All my previous experience with CTP was with violet/silver plates, but I was told that thermal was much more stable. This has not been my experience with thermal so far.
    You may get artifacts, however, because you are going to a higher res plotter (2400 --> 2540) you probably won't have any issues. Going the other way, i.e. 2540-->2400 will cause artifacts.
    The usual strategy is to resample the bitmap in a 2540 --> 2400 dpi situation.

    All things being equal, thermal should deliver more consistent dots on plate than visible (depends on many factors though). You should contact your CTP vendor (loudly) if you feel you are not achieving what was promised or expectations were not met with performance.

    best, gordon p
    Last edited by gordo; 11-10-2009 at 05:04 PM.

  10. #30
    Vee
    Vee is offline Senior Member
    Join Date
    Aug 2007
    Posts
    221

    Default

    Quote Originally Posted by gordo View Post
    GP: Precisely why you shouldn't bother with Euclidean or Elliptical as they offer no value over the round dot and they have issues in important tone areas, i.e. 1%-75%
    In my personal experience, I ran Heidelberg's IS Classic, Elliptical dot at 2540/175 & 200 on coated stock for years and years and never had one single moire in any flesh tones or the likes. We ran the following screen angles C-165 M-45 Y-0 K-105. Maybe I was just lucky, doubt it tho.


Page 3 of 4 FirstFirst 1234 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Sponsors

Powered by vBadvanced CMPS v4.1.2