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Combining Conventional and Stochastic Halftones to Overcome Moiré

(March 2003) posted on Mon Apr 21, 2003

Learn how to overcome naturally occurring moiré by combining conventional and stochastic halftones on the same printing screen.

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By Mark A. Coudray

2. Rename the original file as [<I>File-name</I>] <I>Wkg</I> to indicate that you will be conducting operations on a copy of the original file.


3. Duplicate each channel in the Wkg file. You will now have eight channels: CMYK plus copies of each channel.


4. Use the Selection tool in Photoshop to contain the problem area.


5. Save the selection.


6. With the selection loaded, delete the selected area from each of the CMYK channels. The deletion will leave a hole in the original CMYK channels where the problem area was located. Make sure that the background is set to white with an RGB value of 255,255,255.


7. Activate the copied channels so that they are visible (highlighted in the Channel pallet).


8. Load the selection (>Selection >Load Selection) and make sure the Invert box is checked.


9. Delete the background from the copied channels. The deletion will leave you with only the problem area in the copied channels--the opposite of what appears in the original CMYK channels.


10. Go to the Channels pallet and click on the disclosure triangle in the upper right corner of the pallet. Go to Split Channels. This will split the document into eight new, grayscale documents that can be addressed individually.


11. Apply whatever dot-gain correction you would normally add at this point to each of the copy channels (cyan copy, magenta copy, etc.). This is very important because you will not be able to accurately correct for dot gain after you apply the pseudostochastic effect that follows.


12. The first channel to work with is the cyan copy. Convert the cyan copy file under >Image>Mode> Bitmap. Choose a resolution around 240 and make sure that the method is set to Diffusion Dither. This will convert the grayscale information into a pseudostochastic bitmap file.


13. Repeat this process for each of the remaining copy channels (magenta, yellow, and black.)


14. When you have finished, select >Image>Mode>Grayscale for each of these channels. This step converts the bitmap back to a grayscale, even though there are no grayscale values in the image--only black and white.


15. Now the fun begins! You will use channel calculations to merge the two different channels into a new document. Go to >Image>Calculations. Select the original cyan channel with the problem area deleted as Source 1. Select the cyan copy channel as Source 2. Choose Blending method as "Multiply" with Opacity set at 100%. Put the result into a new document called [Filename] Fnl. This sequence marries the normal grayscale of the cyan channel with the pseudostochastic grayscale of the problem area into a new document.


16. Repeat these steps for magenta, yellow, and black, in that order. Following this specific sequence is crucial.


17. You will now convert this hybrid grayscale image into a new CMYK file using the following sequence: >Image> Mode>CMYK Color.


18. The final step is to output this document through your RIP as you normally would. No halftoning will take place in the altered areas because the pseudostochastic areas are only black and white. Conventional halftoning will occur in the grayscale areas of the original CMYK channels. The final film will show both conventional and stochastic halftones.


Making moiré go away


Combining traditional and stochastic halftones is a great technique for areas that have naturally occurring moiré. The method can also be used to greatly increase the apparent visual acuity of fine detail--images of leaves or grass, for example. When the image is printed, you will have the benefit of fine, moiré-free detail in the problem areas and smooth, continuous tones that are free from excess grain in the smooth portions of the image.





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