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Atkin admin >> Tech >> [OT Tech Question] Scanned image problem
(Message started by: Secret Drinker on Today at 13:58)

Title: [OT Tech Question] Scanned image problem
Post by Secret Drinker on Today at 13:58
For the past few years I've been in the habit of scanning in photos for use on websites etc., if I can't otherwise get hold of them in digital format. Most of the time it works fine and I'm able to do what I want to the resultant image as if it were a photo I'd taken myself. (Obviously all this is subject to copyright constraints!)

However, occasionally I scan in a picture and I notice that it appears to have lines or bands across it which were absent in the original (at least, they weren't noticeable to the naked eye). I don't know the technical term for this phenomenon, and it's difficult to describe in words, but it seems like some sort of diffraction effect, so that an area which appears to be more or less uniform in colour in the original has lighter and darker bands across it after scanning.

Does anyone know (a) what this effect is called, and/or (b) how it can be avoided, and/or (c) how you can get rid of it from a digital image?

I've tried to find out on the web but as I don't know what the effect is called it makes it more difficult.

Any advice would be appreciated. (I'm hoping someone out there will immediately know what I'm talking about from my description, but if not, I'll see if I can find an image with the problem and post it somewhere on the web.)

Cheers

Paul

Title: Re: [OT Tech Question] Scanned image problem
Post by S J Birkill on Today at 14:36
I don't know the image-processing experts' term for it, but it sounds to me like the interference pattern, aliasing or spatial beat frequency, moiré without the swirls, if you like, which occurs when two sequential sets of sampling are done on the original image. The first is performed in the printing process: any image reproduced in the press will be built up essentially of a matrix of very small coloured dots; the second is in your scanner. The closer in pitch these two processes are, the coarser (lower in frequency) will be the beat pattern. The downsampling process inherent in reducing the image size can also increase the visibility of the pattern.

For best results perform the original scan at the highest possible resolution, then lose the scanning structure by applying a slight blurring -- Photoshop and Paint Shop Pro have a filter called Gaussian Blur, within which you can set a pixel radius. Experiment with values around 0.5 to 1 until you eliminate the beat without losing too much sharpness. Then downsample the image to the size you need for the final display. You should then find you can apply a little sharpening to taste, to crispen up the result, without reintroducing the pattern.

Hope this helps -- Steve

Title: Re: [OT Tech Question] Scanned image problem
Post by Secret Drinker on Today at 16:52

on 09/27/04 at 14:36:25, S J Birkill wrote :
For best results perform the original scan at the highest possible resolution, then lose the scanning structure by applying a slight blurring -- Photoshop and Paint Shop Pro have a filter called Gaussian Blur, within which you can set a pixel radius. Experiment with values around 0.5 to 1 until you eliminate the beat without losing too much sharpness. Then downsample the image to the size you need for the final display. You should then find you can apply a little sharpening to taste, to crispen up the result, without reintroducing the pattern.


It sounds very much as if you may have hit the nail on the head, Steve. I shall try your suggested solution (I always wondered what Gaussian blur did - now's my chance to find out!)

Thanks for the reply.

Cheers

Paul

Title: Re: [OT Tech Question] Scanned image problem
Post by Gerry Smith on 30.09.04 at 22:54
Hi Paul - I'm just wondering if this is a case of JPEG artefacts?  Sort of swirly patterns which vary in intensity?  If so you could try reducing compresion or, more simply, try th JPEG artefact remover in PSP or photoshop.

Cheers

Gerry




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