Jeremy Scharlack's Blog: Moiré Madness

April 28, 2003

Moiré Madness

note: Click to enlarge images

I manipulated and image for a client of mine which included a patterned shirt:

See the problem? Moiré (moire for those of you that can’t display an accented e). It was on the proof too. Moiré can occur when you create a smaller image of a pattern. Here is the original image, a 1391 pixels high image which has been converted to RGB and saved as a jpeg; it is kind of big (596k). Here is portion of it which is only 140K. The real original is CMYK tiff.

I discovered a way to get rid of moiré when resampleing. Do it stepwise. For the image above I went directly from 1391 pixels high to 400 pixels high (holding the proportions and using bicubic interpolation) in one step. For this image:

I resampled from 1391px to 1200px then to 1000px then 800px then 700px then 600px then 500px then 450px and finally 400px. No moiré in the image. Although there might be moiré in the thumbnail that movable type generated. I have found that it is benifical to decrase the step size as I approach the final value.

While there is no more moiré in the resampled image, it still contains a pattern which can moiré with the screens used to print the image. The portion of the printed piece containing the shirt was quite small, so the image needed to be reduced, although not all the way down to the 400px high image you see above; It was more like 600px. Here:

is a simulation of the black channel after being screened. The simulation comes from an ~600px high image, and it has been reduced.

So what I did was to blur the image a bit using the dust and scratches filter, which doesn’t effect edges as much as a gaussian blur, on a seperate layer. I then brought back the original edges, collar, logo, sleeves and buttons using a layer mask. The blurred layer was then reduced to ~75% opacity to let some of the pattern still be visible.

No more moiré.

Note: To create a simulated black channel I did the following in Photoshop: I took the stepwise resized CMYK file (with no moiré) and I threw out the cyan, megenta and yellow channels. I then converted the file to grayscale. I then converted the file to bitmap (black and white), tripleing the reoution from 75ppi to 300ppi and using a halftone pattern. The pattern used a round dot, 75lpi (like a newspaper), at 45 degrees. I then converted back to grayscale going back to 75ppi. Finally I resized the image to 400px high, which you can see above.

Posted by Jeremy at 06:50 PM | Link | TrackBack (0)

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