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Using the FFT filter to remove photgraphic paper texture

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  #41  
Old 10-10-2005, 02:57 PM
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Cameraken Cameraken is offline
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Hi June.

2) Flatten all Layers
You may not need this but, for precaution, flatten all the layers...<alt><L>, <F>. This is because FFT doesn’t understand layers, masks, alpha channels etc. The image must be just one “Background” layer and nothing else;

Byro included this step to make sure you only have one layer as FFT will not work on Layers. This should not make your image black
Try using the mouse instead of the shortcut. Layer > Flatten Image.
If Flatten Image is greyed out then skip this step and move to step3

Ken
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  #42  
Old 10-10-2005, 09:00 PM
june` june` is offline
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Thanks

I checked that. Tell me, is there a limitation on the size of the file the filter will work on?

Thanks again,

June
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  #43  
Old 10-12-2005, 02:14 PM
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Cameraken Cameraken is offline
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There is nothing in the code to limit file size. However I suspect that FFT could be memory intensive so file size will be limited by your computers memory and disk space.

This will not affect the quality of the results. It will just slow down the conversion.

Ken
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  #44  
Old 10-17-2005, 01:56 PM
ebbtide ebbtide is offline
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Step 8

Hi,
WOW what a great time saver, thanks Ro. One question. 99% of my work has been on gray images so I'm not familiar with a lot of the color operations. In trying an RGB and getting to your Step 8 (Put the colours back in) you state "Slide, with ......". My question is Slide what? Where does this luminosity layer come from? Its not listed in the channels or layer list.

Thanks for bearing with my ignorance, ebbtide
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  #45  
Old 10-17-2005, 03:01 PM
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byRo byRo is offline
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Glad you found this useful.

The FFT process works only on the luminosity (greyscale) of the image.
So, what we want to do in this step is to mix the colour information of the original with our "clean" luminosity.

This could be done by copying (<ctrl><C>) the new luminosity image and pasting (<ctrl><V>) on top of the original.

The "slide" method is just a short-cut. Details......
- You need to have the "luminosity" image active, but also have visible some part of the original image.
- On the layers palette, select the layer of the "luminosity" image.
- You can now drag this layer from the layers palette onto the visible part of the original image.
- Now you have a new luminosity (greyscale) layer on top of the old colour information.

Only it will be in the wrong place! Which is why you'll remember to press <Shift> while dragging, which aligns the layer correctly.
Yeah, I know - the short-cut looks longer than the original. On paper, yes. In practice, no.

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  #46  
Old 10-17-2005, 04:26 PM
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Panpan Panpan is offline
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Quote:
Originally Posted by Cameraken
2) Flatten all Layers
You may not need this but, for precaution, flatten all the layers...<alt><L>, <F>. This is because FFT doesn’t understand layers, masks, alpha channels etc. The image must be just one “Background” layer and nothing else;
This is not correct. Just put a black layer behind the fft layer.

Pierre
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  #47  
Old 10-17-2005, 07:08 PM
ebbtide ebbtide is offline
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Thanks, less star painting

Ro, I followed your additional instruction and got two gray scale images but all was OK once I realized that the color didn't return until the luminosity blending was selected, then everything was ok.

Because the FFT is a symmetric math function made of cosines you only need to paint out the stars on either the left side to the vertical axis or from the right side of the image. Saving a little painting time if you have a lot of unwanted frequencies in your image. Thanks for the instant help Ro !
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  #48  
Old 10-18-2005, 05:37 AM
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Quote:
Originally Posted by ebbtide
Because the FFT is a symmetric math function made of cosines you only need to paint out the stars on either the left side to the vertical axis or from the right side of the image.....
Are you sure about that?
I can understand the logic, but it seems strange that 50% (or maybe 75%) of the information is just waste.
Also, if I paint just one side how is the IFFT function going to know which side it should consider, original or painted?

It would be nice if it was true, but I think it's unlikely.

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  #49  
Old 10-18-2005, 06:14 AM
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Cameraken Cameraken is offline
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Pierre

I went to http://www.pages.drexel.edu/~avc25/archive.htm#FFT to try to find some more information. But is sadly lacking and does not say if a picture can be on layers.

However I did do some tests.
When you run FFT on a layer then the texture from only that layer is removed and the image comes back transparent (hence the black layer required) but this gives darker results than working from a flattened picture.

I can’t understand why you would want to work on a layer as texture removal should be the first step in the workflow

My Quote was not my words but an extract of step 2 in the tutorial and I think it is a sensible precaution and should be left as it is.

Ebbtide
I also tried your method and still had texture in the picture

Ken
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  #50  
Old 10-18-2005, 05:06 PM
ebbtide ebbtide is offline
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Too Fast

Yea, I shot too fast on saying you could only do half the symmetric star pattern. I scanned a newspaper picture and did my grand experiment. Painting out half worked as well as painting all. But on retrospect I guess it was because it was such a bad image I couldn't see the difference. On trying a high quality image with interference in it, you do definitely need to paint out all stars. Sorry for the fire drill guys.
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