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This chapter is from the book

Minimizing Offset Moiré

Sooner rather than later, a client will have only a printed image clipped from a magazine or brochure for you to work with. The entire image in Figure 5.31 is covered with moiré rosette patterns caused by four-color separation offset printing. The following retouching technique, developed by the late Carl Volk, works well for removing moiré problems. In this process, you are throwing the image out of focus a little, then trying to sharpen the contrasty areas without sharpening the dot pattern. Figure 5.32 shows the improved image after this resizing and blurring process.

Before you start trying to repair the damage, ask the client politely if she might have a photographic original. If not, swallow hard and follow these recommendations to minimize the horrid moiré effect.

Figure 5.31Figure 5.31 Before


 

Figure 5.32Figure 5.32 After


NOTE

I am not condoning scanning images from magazines, books, or stock catalogs, as that would be breaking U.S. copyright law. Rather, I am recognizing that sometimes you or a client has only a magazine or brochure image of her factory (for example) for you to work with. Or in the worst case scenario, the original negative or film has been lost or damaged and the only image available is a prescreened one.

ch5_postcard.jpg

  1. Scan the printed piece at three to four times higher resolution than you will need for your final print.

  2. Start with the following values, which you might need to adjust for your own moiré problem images. Apply a .5-pixel Gaussian Blur to the Red channel, .7 pixels to the Green, and 1 pixel to the Blue.

  3. TIP

    Use keyboard shortcuts to quickly access the individual channels in your image. Use (Cmd + 1)[Ctrl + 1] for Red, (Cmd + 2)[Ctrl + 2] for Green, and (Cmd + 3)[Ctrl + 3] for Blue. (Cmd + ~)[Ctrl + ~] will return you to the composite image. When one of the separated channels is active, pressing ~ will keep the color channel active but preview the full-color image in the Photoshop window.

  4. To resample the file down 25%, select Image > Image Size (see Figure 5.33). Change the Width unit of measurement to percent and enter 75. (The other values will change automatically when Constrain Proportions is checked, as it should be.) Be sure to check Resample Image.

  5. Repeat the Gaussian Blur procedure on each channel with approximately 25% lower Gaussian Blur amounts. The .7 amount can be dropped down to .5 and the .5 to .3. The 25% lower values are used because the image size has been reduced by 25%.

  6. If the moiré pattern is still visible, resample the image down 25% again.

  7. Figure 5.33Figure 5.33 Resampling the image down by 25%.


  8. After it starts looking better, and as you are approaching your final resolution and size, choose Filter > Sharpen > Unsharp Mask. For a 300-dpi file or close, use these settings: Amount, 100; Radius, 1.7; Threshold, 12 to 16. After using the Unsharp Mask filter to offset the softness added with the Gaussian Blur and image resizing, the image details should look acceptably sharp. As described in Chapter 8, "Refining and Polishing the Image," you should use the Unsharp Mask filter carefully as you don't want to create ugly, white-halo artifacts around image edges.

  9. Resample the image down to final size. After resizing the image, you might need to run the Unsharp Mask filter again to create a pleasingly crisp image. Use a smaller Radius setting on this final pass of the Unsharp Mask filter.

As you can imagine—going through all these steps is a last resort technique to remove moiré—it would be best to start from the original film.

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