desccreening filter in affinity photo

[barninga]

hello all,

 

i wonder if affinity photo implements a descreening filter (useful to eliminate the moiré effect found in images that were scanned, i.e. from magazines): maybe i missed it, but i could not find anything similar in the filters menu and live filter layer menus

 

i think such a task can be done through a sequence of operations (like scanning at a resolution highr than needed, then sampling down to the desired size, then applying a gaussian blur, and finally sharpening back).

 

any suggestion will be welcomed.

 

thank you

stefano

[Jim_Campbell]

I've owned a number of scanners, and operated many more, going back over twenty years, and I think every single one incorporated a descreen option into the scanning software… I don't recall ever trying to eliminate moiré outside the scanning process.

[barninga]

I've owned a number of scanners, and operated many more, going back over twenty years, and I think every single one incorporated a descreen option into the scanning software… I don't recall ever trying to eliminate moiré outside the scanning process.

 

hello jim, thank you for your reply.

 

you are right, and my scanner does have a descreening feature.

however, you may happen to work with images that were scanned by others, who didn't know about that feature, or didn't care to activate it, or whatever.

 

i experimented a bit with the tools that AP already offers, and achieved quite satisfactory results applying the following filters:

 

1) fft filter to the original image

2) frequency separation filter to the fft-filtered image

3) gaussian blur to the high frequency resulting layer

4) unsharp mask to the low frequency resulting layer

 

i'm no expert in this field, so there may probably be a better approach, and maybe more than one.

in addition, all of the above steps require some experimenting to find the parameters that best configure the filters.

 

for example, i found that the optimal configuration of the separation filter causes all (or at least most) of the noise that the fft filter could not remove to go to the high frequency layer. this is also the maximum detail desired on it, so you can start with a 0 radius and look at the preview to find the value where the noise stops to become more and more visible and stays constant. usually the optimal radius value appears to be between 0.8 and 1.5. 

 

i guess that a new descreening filter could be implemented as a sequence of filters, with 4 controls:

- precision of the fft filter

- radius of the frequency separation

- radius of the gaussian blur

- radius of the unsharp mask

 

i feel (but i could be wrong) that there may be some correlation among the parameter values, especially among the three radius values: this could simplify the user interface of the filter, or at least allow to set some meaningful default values.

 

optionally, the filter could offer to display the fft window and manually paint over the frequency nodes.

 

i know that this is far more simple to put in words than into lines of code... but i'd really be happy to see something like this in one of the next releases.

[Raff]

On 12/7/2015 at 4:39 PM, Jim_Campbell said:

I've owned a number of scanners, and operated many more, going back over twenty years, and I think every single one incorporated a descreen option into the scanning software… I don't recall ever trying to eliminate moiré outside the scanning process.

My scanner (Epson) does not de-screen for a resolution greater than 600 pixels.
And I need a higher resolution.
So it would be very useful to have such a filter to use in Affinity.
I don't know if it has been inserted now, I tried to search but I didn't find anything, does anyone know?

[PaulEC]

Try using the FFT Denoise filter.

[Oaf]

I found it more effective to use the Noise - Frequency Separation.

Adjust the radius so that dots are visible in left hand pane with minimal signature from the photo, and the photo is on the right screen with, as far as possible, no dots.

Then Apply, and toggle off the high frequency layer.

[firstdefence]

1 hour ago, Oaf said:

I found it more effective to use the Noise - Frequency Separation.

Adjust the radius so that dots are visible in left hand pane with minimal signature from the photo, and the photo is on the right screen with, as far as possible, no dots.

Then Apply, and toggle off the high frequency layer.

Welcome to the forum @Oaf Have you got a link to any video's showing this process please?

[Oaf]

Sorry - I don't have a video, though I think it hardly needs it as so easy to do.

Also - with the benefit of hindsight, as a newbie, I may not have been using the FFT Denoise filter effectively so I may not have done a fair comparison.

[jweitzel]

On 6/20/2020 at 11:57 AM, Raff said:

My scanner (Epson) does not de-screen for a resolution greater than 600 pixels.
And I need a higher resolution.
So it would be very useful to have such a filter to use in Affinity.
I don't know if it has been inserted now, I tried to search but I didn't find anything, does anyone know?

This answer is admittedly some years late, but – as I myself – one may stumble over that post of "Raff":
Your aspect is simply without substance. It is useless to scan a rasterized picture with more than 600 dpi because the original will not hold more information than to be packed in 600 dpi. All scan resolutions over 600 of autotypical originals are "hot air" and will not increase the information density in the scan. You will have to uprezz the scan & de-rasterized result.

Johannes