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Is there a way to posterize and limit an image to 3 spots?

Mar 22, 2013 10:43 PM

Tags: #spot #screen_printing #tritone

Hi There,


I need to turn a B&W photo into screen print art without using a line screen (just block colours) - essentially I'm creating a 3 colour black and white images using:

- PANTONE 4625 C

- PANTONE Warm Gray 6 C

- PANTONE Warm Gray 1 C


Using a bunch of techniques (unsharp mask, tritone and posterize) I have line art that looks  great with a posterize level of 4. But I've noticed that Photoshop is using my Tritone colours to mix itself an extra tone - and if I switch to a level of 3, Photoshop chooses to remove the level that PANTONE Warm Gray 1 C uses which deletes all the highlight detail.


Is there a way I can specify colours that Posterize  uses?


Or is there a different approch I can use to split the tones into the ones in the Tritone?


I've tried playing with the curves in the Tritone window to try and influence the colours photoshop uses (so no colour mixing) but maybe I need to somehow save the Tritone layers into the channels pallate?


Just not sure how best to approach this - any help would be much apprciated.





  • Currently Being Moderated
    Mar 23, 2013 12:09 AM   in reply to macopp

    but maybe I need to somehow save the Tritone layers into the channels pallate?


    Yes, exactly. If it's for print, you need separations. Copy&paste the image to a new Spot color channel, tweak it there. Otherwise you may wish to look into gradient maps and avoid that postirize thing alltogether...



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  • Currently Being Moderated
    Mar 23, 2013 9:42 AM   in reply to macopp

    IMHO you should use Adobe Illustrator Live Trace. Any raster image can be converted

    into a true vector graphic, based on a palette with an arbitrary number of arbitrary colors

    (I had used Lab, probably it works as well for Spots like Pantones - which have Lab



    The result depends widely on the settings of various parameters. Here comes an example:


    The folks in the Illustrator forum can probably explain the whole procedure better than I.


    By the way:

    Photoshop posterization doesn't create vector graphics. The essential difference is this:

    In Illustrator the color areas can be forced to be 'less fragmented ' by varying the

    parameters (Photoshops limited substitute: blurr in advance to posterize).


    Best regards -- Gernot Hoffmann

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