Reduce moire when dropping halftone comics

Question

How can I reduce the effects of moire when reducing comic book images in midtones during live zoom on iPhone or iPad?

I am writing a program for watching comics. It would be nice to provide higher resolution images and allow the user to zoom in while reading a comic strip. However, my client does not like moire effects and will not allow this function if there are noticeable moire artifacts when scaling, which, of course, is.

Changing images that are less susceptible to moire will only work if modifications are not noticeable. Blur was specifically banned, like anything that removes your favorite halftone dots.

Images are black and white halftone and linear. The originals have 600 dpi, but what we send with the application will be half as much, so maybe 2500 pixels or less.

So what are my options? If I write a custom downsampling algorithm, will it be fast enough for real time on these devices? Are there any other tricks I can do? Will it work to simply avoid the aspect ratio that have the most visual effects of moire?

When you zoom in, there are certain peaks where moire effects are worse. Is there a way to calculate what these points are and just zoom in, which is not so bad?

Any suggestions are welcome. I have very little experience in processing images and signals, but I get the opportunity to learn. I don't know anything about bursts and sharpness and other jargon, so please be verbose.

Edit:

At the moment, at least we click on dynamic scaling. We will support scaling to full magnification, but not arbitrary scaling. I hope to reconsider this in the future.

Answer 1

The difficult problem, in general, is interesting in particular. I doubt that there is a simple solution - perhaps if we could imagine an almost perfect grayscale (monochrome images with halftones placed in a perfect grid), but that would hardly work for a scanned image.

If you are interested in mathematics and / or want to get some bibliography for research, this thesis may be useful (I have not read it)

You can also find descreening algorithms, plugins, etc. to get ideas.

Answer 2

Moire effects are due to smoothing. The bias is due to a too low sampling frequency compared to the frequency content of the signal / image.

I see no way to avoid this without applying blur. If you choose the blur filter well enough, you can get results that don't look “blurry” at all.

Since blurring is easy to implement, I would do downsampling with blurring and show it to the client. If they are satisfied with the results, then everything should be fine.

The only other options that I see are:

• Custom downsampling method. If someone else hasn’t come up with it, I don’t think this is an option, since you yourself claim that I have little experience in processing images / signals.

• Transform comics into a vector format that will give endless zoom.