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Comment by axiolite | Hacker News Reader

You can take TWO screenshots, moments apart, open in GIMP, paste one over the other and choose any one of these laying modes:

Lighten, Screen, Addition, Darken, Multiply, Linear burn, Hard Mix, Difference, Exclusion, Subtract, Grain Extract, Grain Merge, or Luminance.

https://ibb.co/DDQBJDKR

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> You can take TWO screenshots, moments apart, open in GIMP, paste one over the other and choose any one of these laying modes:

You actually don't need any image editing skill. Here is a browser-only solution:

1. Take two screenshots.

2. Open these screenshots in two separate tabs on your browser.

3. Switch between tabs very, very quickly (use CTRL-Tab)

Source: tested on Firefox

reminds me of this: https://www.reddit.com/r/LifeProTips/comments/5jdzsx/lpt_use...

I went cross-eyed on my screenshot, and I couldnt read the word, but I did notice some artifacts

This was used in some early-20th-century astronomical setting, I think to detect supernovae. I can't find any documentation now, but my memory is that it was called "blink testing" or something similar, where one switched rapidly between two images of a star field so that changes due to a supernova would stand out.

https://en.m.wikipedia.org/wiki/Blink_comparator

That's it exactly! Thanks.

You could also just record a video.

Hah, indeed, that was my first thought. This is clearly for fun though, it’s a cool project idea

What does that accomplish? You can just read the web page as-is...

Are you going to share your two screenshots, and provide those instructions, with others? That seems impractical.

Video recording is a bit less impractical, but there you really need a short looping animation to avoid ballooning the file size. An actual readable screenshot has its advantages...

> use CTRL-Tab

Thank you forever for this, I ever used Ctrl-Page up/down for that.

Or use Blink Comparison from F-Droid.

Is it possible to modify the webpage to make the pattern of the text go down and the pattern of the background do up?

Yes: https://jsfiddle.net/kx6stbcL/

Neat idea.

A friend of mine made a similar animated GIF type captcha a few years ago but based on multiple scrolling horizontal bars that would each reveal their portion of the underlying image including letters, and made a (friendly) bet that it should be pretty hard to solve.

Grabbing the entire set of frames and greyscaling them, doing an average over all of them and then applying a few minor fixups like thresholding and contrast adjustment worked easily enough as the letters were reveleaed in more frames than not (I don't think that would affect the difficulty much though if it were any diffierent). After that the rest of the image was pretty amenable to character recognition.

That's reminiscent of a (possibly apocryphal?) method I once read about to get "clean" images of normally crowded public places - take multiple photos over time, then median each pixel. Never had the opportunity to try it myself, but I thought it sounded plausible as a way to get rid of transient "noise" from an otherwise static image.

That's a real method:

https://digital-photography-school.com/taking-photos-in-busy...

https://petapixel.com/2019/09/18/how-to-shoot-people-free-ph...

But it only works well if the crowds move out of the way reasonably quickly. If we're taking about areas packed with people all blocking a certain area, and you need hours of shots, the change in ambient lighting over time will have negative effects on the end photo.

Ah, that's the method indeed! Thanks!

There's an analogue method too - just do a very long exposure of a busy street. There is so little reflected light from people (relatively speaking) that it barely registers on the film/sensor, hence they are 'invisible' in the final shot. That's why old street photos are sometimes empty of people, because both lenses and film were slow, leading to long exposures.

Out of sheer curiosity, I put three screenshots of the noise into Claude Opus 4.1, Gemini 2.5 Pro, and GPT 5, all with thinking enabled with the prompt “what does the screen say?”.

Opus 4.1 flagged the message due to prompt injection risk, Gemini made a bad guess, and GPT 5 got it by using the code interpreter.

I thought it was amusing. Claude’s (non) response got me thinking - first, it was very on brand, second, that the content filter was right - pasting images of seemingly random noise into a sensitive environment is a terrible idea.

> pasting images of seemingly random noise into a sensitive environment is a terrible idea.

Only if your rendering libraries are crap.

I think they mean prompt injection rather than some malformed image to trigger a security bug in the processing library

The LLM is the image processing library in this case so you are both right :)

> pasting images of seemingly random noise into a sensitive environment is a terrible idea

BLIT protection. https://www.infinityplus.co.uk/stories/blit.htm

I've found taking two screenshots and adding them as separate layers works well, and then setting one as Difference, and then tweaking the opacity.

Here it is in Pixelmator Pro: https://i.moveything.com/299930fb6174.mp4

Bottom layer normal, second layer grain extract, top layer vivid light. This completely blacks out the whole area outside of the text.

Or just copy the text from the url. Not very secure, really. :D

Or just ... record a video of the screen.

What tool do you use to make such a video ?

On macOS it's built in. The screenshot tool can do it. For more professional work I use ScreenFlow.

Computer vision mode: and each screenshot together.

But then that would be a video, not a screenshot

Layered images do not a video make. Sequential images distributed over time do.

Yeah if this became popular, we'd have another Show HN for a tool that automated that.

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