It's easier than ever to de-censor videos

> nothing does better than a simple black box

You still need to be aware of the context that you're censoring in. Just adding black boxes over text in a PDF will hide the text on the screen, but might still allow the text to be extracted from the file.

Ah yes, Mr. Swirl Face.

This was pretty different though. The decensoring algorithm I'm describing is just a linear search. But pixelation is not an invertible transformation.

Mr. Swirl Face just applied a swirl to his face, which is invertible (-ish, with some data lost), and could naively be reversed. (I am pretty sure someone on 4chan did it before the authorities did, but this might just be an Internet Legend).

A completely opaque shape or emoji does it. A simple black box overlay is not recommended unless you that’s the look you’re going for. Also very slightly transparent overlays come in all shapes and colors and are hard to recognize whether it’s a black box or another shape, so either way you need to be careful it’s 100% opaque.

Yeah most of the information is lost, but if you know the font used for the text (as is the case with a screencast of a macOS window), then you can try every possible combination of characters, render it, and then apply the averaging, and see which ones produce the same averaged color that you see. In addition, in practice not every set of characters is equally likely --- it's much more likely for a folder to be called "documents" than "MljDQRBO4Gg". So that further narrows down the amount of trying you have to do. You are right, of course, that the bigger the group of pixels, the harder it gets: exponentially so.

How many pixels are in `I` vs how many are in `W`? different averages as a result. With subpixel rendering, kerning, etc, there is minute differences between the averages of `IW` and `WI` as well, order can be observed. It is almost completely based on having so much extra knowledge: the background of the text, the color of the text, the text renderer, the font, etc, there are a massive amount of unknowns if it is a random picture, but if we have all this extra knowledge, it massively cuts down on the amount of things we need to try: if its 4 characters, we can make a list of all possibilities, do the same mosaic, and find the closest match, in nearly no time.

Depends how much data you have. If a 25x25 square is blurred to a single pixel, that will take more discrete information to de-blur than if it were a 2x2 square. So a longer video with more going on, but you can still get there.

It's not that you're utterly wrong; some transformations are irreversible, or close to. Multiplying each pixel's value by 0, assuming the result is exactly 0, is a particularly clear example.

But others are reversible because the information is not lost.

The details vary per transformation, and sometimes it depends on the transformation having been an imperfectly implemented one. Other times it's just that data is moved around and reduced by some reversible multiplicative factor. And so on.

TLDR: Most of the data is indeed "lost". If the group of pixels are big enough, this method alone becomes infeasible.

More details:

The larger the group of pixels, the more characters you'd have to guess, and so the longer this would take. Each character makes it combinatorially more difficult

To make matters worse, by the pigeonhole principle, you are guaranteed to have collisions (i.e. two different sets of characters which pixelate to the same value). E.g. A space with just 6 possible characters, even if limited to a-zA-Z0-9, that's 62*6 = 56800235584, while you can expect at most 2048 color values for it to map to.

(Side note: That's 2048 colors, not 256, between #000000 and #FFFFFF. This is because your pixelation / mosaic algorithm can have eight steps inclusive between, say, #000000 and #010101. That's #000000, #000001, #000100, #010000, #010001, #010100, #000101, and #010101.

Realistically, in scenarios where you wouldn't have pixel-perfect reproduction, you'd need to generate all the combos and sort by closest to the target color, possibly also weighted by a prior on the content of the text. This is even worse, since you might have too many combinations to store.)

So, at 25 pixel blocks, encompassing many characters, you're going to have to get more creative with this. (Remember, just 6 alphanumeric characters = 56 billion combinations.)

Thinking about this as "finding the preimage of a hash", you might take a page from the password cracking toolset and assume priors on the data. (I.e. Start with blocks of text that are more likely, rather than random strings or starting from 'aaaaaa' and counting up.)

> Walk past a closed bathroom stall fast enough and you can essentially do that with your own eyes

Only in the US. The rest of the world has doors without a gap at the sides.

The dither effect. Same as seeing through splayed fingers on a franticly oscillating hand.

"Sir, why do you keep running back and forth in the bathroom?"

I love line scan cameras. Wikipedia has an awesome photo of a tram taken with a line scan camera on the relevant wiki page[1].

I've just moved to a house with a train line track out front. I want to see if I can use a normal camera to emulate a line scan camera. I have tried with a few random YouTube videos I found [2].

I think the biggest issue I face is that there simply isn't the frame rate in most camera's to get a nicely detailed line scan effect.

---

[1]: https://en.wikipedia.org/wiki/Line-scan_camera

[2]: https://writing.leafs.quest/programming-fun/line-scanner

Similar to

https://hackaday.com/2024/08/17/olympic-sprint-decided-by-40...

I was not aware of those.

Reminds me of slit-scan as well. And of course rolling shutters.

Surprised I've never come across this idea before

Even today most cameras have some amount of rolling shutter—the readout on a high-megapixel sensor is too slow/can't hold the entire sensor in memory instantaneously, so you get a vertical shift to the lines as they're read from top to bottom.

Global shutter sensors of similar resolution are usually a bit more expensive.

With my old film cameras, at higher shutter speeds, instead of opening the entire frame, it would pass a slit of the front/rear shutter curtain over the film to just expose in a thousandth of a second or less time.

I am aware, and of course this works for any gap you might be walking past. Plenty of newer bathroom stalls here do not have a gap.

It can be much more than 2cm. The US really hates people for some reason.

Maybe you're referring to "aCropalypse". Also there was an issue once where sections with overpainted solid black color still retained the information in the alpha channel.

https://www.wired.com/story/acropalyse-google-markup-windows...

https://www.lifewire.com/acropalypse-vulnerability-shows-why...

You are thinking of John Meacham’s winning entry in the 2008 underhanded C contest https://www.underhanded-c.org/_page_id_17.html

https://www.underhanded-c.org/_page_id_17.html

There's tricks like this with embedded thumbnails.

I would guess that would be due to compression algorithms.

Step 5.5) Take a new screenshot of the image.

what about intentional adding data into image?

screenshot - im not convinced apple does not use invisible watermark to add info into image data. but for fact every photo you take with iphone, contains invisible watermark with your "phone serial number". to remove such watermarks, facebook is converting every picture you post for last 10 years... just weird extra con to using modern technology.

try to copy banknote on your printer, it will not print anything, just says error. + every page of text printed contains barely visible yellow marks containing again serial number of printer.

....

Maxis was simply ahead of its time.

Not forgetting subpixel rendering.

Luckily the current Microsoft screen snip utility is so buggy I often have to screen shot my edited screen shots anyway to get it to my clipboard.

News publications are also encouraged to do the same or even re-type the raw document. There was a story about how they shared raw scans of the leaked documents such that the yellow printer id dots were visible. That might have been for C. Manning?

This is similar to how I censor images on a cellphone. I use an editor to cover what I want to censor with a black spot, then take a screenshot of that edited image and delete the original.

That sort of makes me wonder if the best form of censoring would be solid black shape, THEN passing it through some diffusion image generation step to infill the black square. It will be obvious that it's fake, but it'll make determining the "edges" of the censored area a lot harder. (Might also be a bit less distracting than a big black shape, for your actual non-advisarial viewers!)

The part that might take some work is matching the motion correctly, with a pixelated area or blacked out rectangle it doesn't matter if it's exactly sized or moving pixel perfectly with the window. I haven't done any video editing in 20 years, so maybe that's not very difficult today?

That moving pixelation look is definitely cooler though. If you wanted to keep it without leaking data you could do the motion tracked screenshot step first (not pixelated, but text all replaced by lorem ipsum or similar) and then run the pixelation over top of that.

If any of you nerds reading this are into video editing, please steal this idea and automate it.

Someone's already emailed me the depixelated version of the paper I'm holding in the video attached to this blog post.

For pixelation you can use another technique invented for astronomy: drizzling [1].

[1] https://en.wikipedia.org/wiki/Drizzle_(image_processing)

Why not? Would you be willing to stake hypothetical customer data on your assumptions?

black box is black box, or other more pleasing color.

It would use information from the pixels around it though.

> Not to mention deeply and disturbingly unethical

Is it really deeply disturbingly unethical? Just FYI, it isn't their identities that are censored, but their genitals are pixelated due to Japanese laws.

So let me get this straight: Porn can be ethical - selling your nude features online can be ethical - doing the activities in porn consensually can be ethical - pleasuring yourself on other people doing so can be ethical - but using AI to infer nude features is "disturbingly unethical"?