Comment by a_c | Hacker News Reader

Comment by a_c | Hacker News Reader

The blurring and then sharpening in spatial domain is equivalent to applying a band-pass filter, or rather a band-stop in the article's case, in the frequency domain. Blurring requires a lot of parameter tuning, in particular the size of the mask and the weight of each the mask element. The choice of parameter can be mapped to particular filter in the frequency domain, e.g. a gaussian filter or an ideal filter. Same for sharpening. Operating in the frequency domain allows you to skip all the trial and error of parameters, by eye-balling the frequency of interest, and suppress its presence.

> what you did is still the best out there

It still is.

I worked with a printing company before. The company impart anti-forgery pattern to packaging material by tuning the half tone-ing angle of the printer. The printing company then offer a seemingly transparent film, with special pattern printed on it, to company requiring brand protection. By overlaying the film on top of packaging material, a specifically designed moire pattern would occur. If you squint your mind enough, it is like public-private key encryption in the physical world. Whenever the brand receive dispute over authenticity of purchased items, a special team, a team having that transparent film, will be summoned to verify the authenticity of item in question. It is one of the many ways the brand in question protect their goods. The printing company was looking for a mobile solution instead of delivery the transparent film, that's where I get to learn more about the printing industry.

> It still is.

It is a relatively well defined problem - removing periodic pattern due to physical arrangement of printing device. This is where algorithmic approach shines over ML approach. I think nowadays a lot of ML is an attempt to outsource the problem understanding part. These are hot dogs, these are not hot dogs. I don't know what defines a hot dog, but is this picture a hot dog?

Hyperbole, of course.

On second thought, I think the author shouldn't remove the periodic noise at all. He was preserving "a copy of history", not the first occurrence of history. It is a property worth preserving. It is a beauty of itself imo.

The moire is an artifact of the scanning or scaling process, and is by no means historic. Removing it is certainly legitimate. That is particularly true since scaling the image can make it worse.

If you are talking about preserving the halftone in an original scan, then yes I agree that it should be preserved. Yet you must also accept that the periodic pattern will have to be removed prior to publishing the image since it will be scaled at some point.

It would be cool if the photo could be printed in a similar way to re-create the effect from the recovered image.

The halftone pattern itself might look lovely and “historical” on you screen, but the book would not present the image “as-is” (i.e. with all halftone detail from the original photo losslessly captured on the page), but rather through the lens of another halftone printing process, one whose DPI is low-enough that it would arbitrarily interfere with the original photo’s to create a garbled mess. The final product wouldn’t be halftone dots encoding a picture, but rather halftone dots encoding moire stripes from an attempt to replicate other halftone dots from a “sample grid” that doesn’t line up with them.

It’d be a bit like taking one of those game emulators that displays the game with a “scanline”-effect filter… and then hooking the computer running the emulator up to an old TV that has a natural built-in scanline effect. You won’t get the pretty scanlines from the emulator, nor will you get the pretty scanlines from the TV; you’ll just get a mess.

(To put it in technical terms: to accurately show the halftone “information” from the original photo in the book, the print resolution of the book would need to be double the Nyquist frequency of said halftone information. But it very likely won’t be.)

It would be possible to print the already halftoned image as it is without running it through another halftone process. Much like how text is not halftoned, but can include very small accurate shapes. I’m sure this makes the preproduction more complicated, but it’s essentially a software problem, not a hardware problem.

But text is super sharp. Halftone is used to blend colors/shades.

The image is already halftoned. There are no shades left.

If you can extract the original halftone dots from the image, sure, you can create new mask layers using those.

But it's unlikely that you can do that, as, unless you're printing in a four-layer pure-CMYK process (and why would you?), even the best photo scanners will be unlikely to be able to capture enough information to tease apart the original offset-layers in their original colors, where halftone dots have overlapped to produce new mixed colors.

Also, you'll be unlikely to reverse the color-bleed effect of the paper to get out the original halftone dot pitch, in order to print new dots that bleed out to the same size the original's did on new, different paper with new, different inks.

Basically, this is as hard as any art forgery project (i.e. needing to match historical materials and processes and environmental conditions), but before you can even start the forging process, you need to destroy even more information by passing the image through the grid-sampling process of a scanner, and only then try to reconstruct layered vector circles from the resulting sample grid.

“I have had my results for a long time: but I do not yet know how I am to arrive at them” - C.F Gauss. Nice job a_c

are you at liberty to share the printing company here? I'm looking for something like this. If not, my email is in my profile and would appreciate info so I can reach out and maybe give them business!

Thing is a bit complicated. I actually worked with a middleman, this middleman act as the "certificate authority" - signing up brands and dispatching orders to printing company(ies). So the printing companies don't know what the matching pattern is. It was five years ago and I have since moved on.

20 years ago you could just type in the rotation angle you wanted for each color separation into your film plotter.

If you ask your printer, they should be able tell you what lpi/dpi they can resolve on the media you're printing onto (you may be limited due to dot gain on uncoated stock).

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