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ocpaul20

Image with an 'overlay' on it which only shows at high magnification.

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I have come across this greyscale image which has like an overlay on it. This overlay is like looking through a bathroom window (through the kind of glass you cannot see clearly, yet it lets light through). This overlay is not enough to destroy the picture at normal magnification, yet at high magnification examining the image in detail, you can see it is there and it stops the fine detail from being inspected.

 

 

What I am wondering is if it is possible to remove the 'overlay'? Maybe if I can isolate a piece of the overlay, can I use it as a mask to remove the rest of it? Can I do a simple subtraction on the original image?

 

 

I suspect not, which might be the reason this overlay was introduced in the first place, although it is not a restricted image.

 

I have tried blurring and then sharpening again but the the result is fine detail is merged into the overlay.

 

Does anyone have any ideas please?

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This "overlay" sounds suspiciously like a "watermark". If you have the rights to use the image, try contacting the author to see about altering the original.

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Thanks for your reply.

Although it could be a watermark I suppose, in this particular case it isn't. :-)

 

I will give a couple of examples of the kind of overlay I want to remove.

 

The first is an Apollo 16 Moon image AS16-M-0491 Link here (you will need to download the print version and zoom right in to see the overlay)

 

The second is a Mars rover image which shows a plain background with an overlay. I could be wrong, but I dont think this is due to JPG compression artifacts but seems to be this overlay on top of the image.

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I see. The crosses in the Apollo 16 image are called fiducials and are a permanent part of the image. With the right skills they are easy enough to remove in Photoshop, assuming you have the rights to do so.

 

I'm not sure about the second one - just looks noisey/grainy to me.

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It is not the crosses that I want to remove but the 'noise'.

 

Maybe I can show you a screen shot of what I am talking about in this image the blacks of the craters are overlaid with spots of 'noise' as you call it, but I do not think it is noise. These Apollo images were taken by cameras with roll film in them although they have been digitised since then for internet display.

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ideally, I would like to write some php code to go through the image and remove this 'noise' and leave the underlying image intact, but I am not sure whether it is possible to mask the 'noise' out or even isolate the 'noise' at all?

 

As I have shown in the link above, there are areas of black under this noise so it should be possible to find pure black (0,0,0) with this overlay on top of it so it can be isolated. Is this the way to proceed do you think?

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Well sure, you could write something to reduce noise. You'll probably be unable to get it perfect though, and it's a destructive process. Personally I would find an existing library to use, even if it's not written in PHP.

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