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CT to LW trapping

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  • CT to LW trapping

    I am hoping to confirm something about continuous tone to line weight trapping.

    Say you have an InDesign file (jacket cover for example). In that file you have a Photoshop link that has rasterized text. The text is no longer recognized as line weight, correct? And, therefore, can not be trapped properly?

    What I am confused about is how, if at all, rasterized text withing Photoshop gets trapped.

    Thanks in advance for any feedback.

  • #2
    Re: CT to LW trapping

    Well, it depends.

    Let's say you have an RGB document that has the type built (say black type RGB 0,0,0). You convert this to CMYK, and the type goes four-color. Then it doesn't need trapping because it has all four colors. This is what also happens if the type is RGB black and is placed into InDesign and converted upon output to CMYK. Whether converted in Photoshop or InDesign, any RGB black will become four-color.

    Let's say you have a Photoshop document that has the type, when it gets rasterized, it will all be CT, right? Let's say the type was 100% Cyan, and the background is 100% Magenta. Well, if you zoom in real close, you will see that there are some pixels that have both Cyan and Magenta. So being a CT, it's already trapped (as long as the trap required is around .003", because 300 pixels per inch (resolution) x .003 = .9, so really 1 pixel trap needed in my case, and the slight overlap of colors made by rasterizing creates enough trap for me). This is the option that happens if the file is CMYK before placing into InDesign.

    Let's say I have a trap I want to make in Photoshop (because I need more than .003" trap). I can do this manually by going to Image menu and choosing Trap... which will bring up a box to specify what amount of trap needed. Note: Say you have a spot color job and want a lighter channel to trap into a darker channel. You would select the lighter channel in Channel palette, and deselect the darker channel, then choose the trap option, which will only spread the lighter color into the darker.

    And of course there's the option a prepress person has of trapping in the rip and choosing the option to trap CTs, which would also trap this area if the rip saw it as untrapped. I usually leave this option off for a couple reasons, but do manually turn it on if I feel the job warrants it.



    • #3
      Re: CT to LW trapping

      Thanks, Don.

      I am still a little confused, but I think it has more to do with my lack of experience.

      So if you have a CMYK photoshop file and it has small color text (10% y) that is sitting on top of a artwork, wouldn't the 10y need to spread? And when and how would that be done?

      I guess I always was under the impression that any line-weight would either choke or spread, based on the color/object it needs to trap with.


      • #4
        Re: CT to LW trapping

        I've aways employed image/trap method in Photoshop that Don mentioned that way I don't have to worry about it later.

        and yes depending on your background it sounds like you would want to spread the light type into the background.

        Edited by: G_Town on Oct 16, 2007 4:00 PM


        • #5
          Re: CT to LW trapping

          You and G_Town are right.

          Having said that, if you look at where the 10% yellow touches another color, does the other color have at least 10% yellow in it? If so, then no need to trap (because the two colors share a common color: 10% yellow). If not, zoom in real close, and see if there is at least one pixel around the edge of the yellow where it has a mix of the yellow and the other color. If there is a pixel around the edge of the yellow that has a mix of the yellow and other color (even if percentages), then it's trapped (just by being rasterized). For instance, if I have a 10% yellow against a 100% cyan background, and rasterize the text, then flatten the layers and zoom in close, I might find a pixel where the yellow touches the cyan, and those pixels may be 5% yellow and 50% cyan. So a trap is created by rasterizing the text and flattening the layers. If you need more than say .003" trap, then a manual trap using Photoshop's trap (Image > Trap) is in order.



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