[HN Gopher] Sir, there's a cat in your mirror dimension
___________________________________________________________________
Sir, there's a cat in your mirror dimension
Author : zdw
Score : 259 points
Date : 2024-05-14 16:42 UTC (6 hours ago)
(HTM) web link (lcamtuf.substack.com)
(TXT) w3m dump (lcamtuf.substack.com)
| kiernanmcgowan wrote:
| A great example of the time-frequency (or space-frequency in this
| case) duality of fourier transforms. The math of the FT doesn't
| care about the "direction" that your going for the transform, so
| function that look similar in time/frequency will have similar FT
| in the frequency/time space.
|
| In this case, embedding the frequency plot of the cat in the
| space plot of the women means that the FT of the women will cause
| the cat to appear, and vis-versa.
| jessetemp wrote:
| That post just gets better all the way through.
|
| I can't believe I never realized the frequency domain can be used
| for image compression. It's so obvious after seeing it. Is that
| how most image compression algorithms work? Just wipe out the
| quieter parts of the frequency domain?
| abetusk wrote:
| Yep, this is how both MP3 (and Ogg-Vorbis) and JPEG all work.
| Picking the weights for which frequencies to keep is,
| presumably, chosen based on some psychoacoustic model but the
| coarse description is literally throwing away high order
| frequency information.
| dylan604 wrote:
| > chosen based on some psychoacoustic model
|
| Does audio encoding use a similar method of using matrices to
| pick which frequencies get thrown away? Some video encoders
| allow you to change the matrices so you can tweak them based
| on content.
| astrange wrote:
| Audio is one dimensional, so it doesn't use matrices but
| just arrays (called subbands).
|
| And you can't get too hard into psychoacoustic coding,
| because people will play compressed audio through all kinds
| of speakers or EQs that will unhide everything you tried to
| hide with the psychoacoustics. But yes, it's similar.
|
| (IIRC, the #1 mp3 encoder LAME was mostly tuned by
| listening to it on laptop speakers.)
| dylan604 wrote:
| I know one mix studio that has a large selection of
| monitors to listen to a mix through ranging from the
| highest of high end studio monitors, mid-level monitors,
| home bookshelf speakers, and even a collection of
| headphones and earbuds. So when you say "check it on
| whatever you have available", you have to be a bit more
| specific with this guy's setup
| alexlarsson wrote:
| You don't generally completely wipe the high frequencies, just
| encode it with less bits.
| pushedx wrote:
| DCT is also often used as a substep in more complex image (or
| video) compression algorithms. That is, first identify some
| sub-area of the image with a lot of detail, then apply DCT to
| that sub-area and keep more of the spectrum, then do the same
| for other areas and keep more or less of the spectrum. This is
| where the quantization parameters that you have seen for video
| compression algorithms affect the behavior.
| astrange wrote:
| Images are not truly bandlimited, which means they can't be
| perfectly represented in the frequency domain, so instead
| there's a compromise where smaller blocks of them are encoded
| with a mix of frequency domain and spatial domain predictors.
| But that's the biggest part of it, yes.
|
| Most of the problem is sharp edges. These take an infinite
| number of frequencies to represent (= Nyquist theorem), so
| leaving some out gets you blurriness or ringing artifacts.
|
| The other reason is that bandlimited signals infinitely repeat,
| but realistic images don't - whatever's on the left side of a
| photo doesn't necessarily predict anything about whatever's on
| the right side.
| rocqua wrote:
| How are images not bandlimited? They don't get brighter than
| 255, 255, 255 or darker than 0,0,0
| astrange wrote:
| Bandlimited means limited in the frequency domain, not the
| spatial domain.
|
| (Also, video is actually worse - only 16-235. Good thing
| there's HDR now.)
| rocqua wrote:
| There is more to it. Often the idea isn't just that you throw
| away frequencies, but also that data with less variance is
| possible to encode more efficiently. And it's not just that
| high frequency info is noise, it also tends to be smaller
| magnitude.
| toast0 wrote:
| I'm a little bit slow with all this stuff, can somebody confirm
| this is the process:
|
| a) take photo of woman and photo of cat
|
| b) DCT cat into the frequency domain
|
| c) composite the frequency domain cat into the visual image of
| the woman
|
| d) if you DCT the composite image, you get the cat back? (or more
| specifically, you get the visual cat and the frequency domain
| woman composited; but the visual cat dominates)
| woopsn wrote:
| Yep, that's it.
| readthenotes1 wrote:
| This article makes the case that "steganography" should be
| renamed to "catography"
| tzot wrote:
| "Stegatography" can be an even more appropriate choice if you
| speak one of the languages born around the European side of the
| Mediterranean sea.
| selimthegrim wrote:
| https://stegato.com/
| hi-v-rocknroll wrote:
| It's clearly a startup in Los Gatos. Herding is the premium
| add-on.
| ryandrake wrote:
| It's a very cool and interesting steganographic application! Want
| to hide an illicit image inside an innocent image? Just convert
| it to frequency domain and composite it onto the other image. As
| long as the viewer knows how to transform it back, you have a
| covert way to send images that is potentially hard to detect.
| bee_rider wrote:
| It would be hard to detect if the other party didn't know what
| to look for, but easy if they did.
|
| If you combined your hidden image with a one-time-pad it should
| be indistinguishable from noise, right? And noise would be
| expected in a lossily compressed image. I wonder if anyone has
| done that. It seems like we'd probably never know unless they
| told us!
| woopsn wrote:
| In most photos with a recognizable subject, spectral energy will
| be concentrated around the origin (the upper left corner) as it
| is here
|
| https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_pr...
|
| The same is true for the DCT of the woman. Meanwhile, the subject
| of a photo is typically located towards the frame's center. This
| helps minimize interference between the space and frequency
| domain data in the composite, thus preserving kitty's expression
| when the transform is inverted
|
| https://substackcdn.com/image/fetch/w_1456,c_limit,f_webp,q_...
|
| (and vice versa for the woman)
| jameshart wrote:
| And this is only true of the DCT - 2D Fourier transforms of
| images usually concentrate the data near the center of the
| image.
| whimsicalism wrote:
| its the same fundamental effect
| nico wrote:
| It makes me think how the lens of the camera, is focusing the
| light/image at the center of the sensor, so it would make
| sense that data is also denser at the center, where the lens
| concentrated more light
| jameshart wrote:
| So... I think you're a bit confused about how lenses work
| and what they do (they don't focus all the light into the
| middle, they focus light from one plane onto another one.
| They only focus light from the center of the frame onto the
| center of the image - that's why it's an image)
|
| _But_ ... there is something interesting about what
| 'focusing' looks like in the frequency domain, and the
| difference between the frequency-space-transform of a
| sharply focused image and a blurred image - or of the same
| image focused at different focal planes - shows up as a
| prosecutable transformation in the frequency space; which
| means you can apply transformations in frequency space that
| cause focus changes in the image domain like a lens does.
| meindnoch wrote:
| What are you talking about?
| emeraldd wrote:
| Considering the title of the article, this comment had me
| thinking of some supernatural. Took me way to long to realize
| what it was talking about...
| woopsn wrote:
| Clarifying, the specter of a hidden animal will usually take
| the form of a diffuse sparkle or blur, typically hovering off
| to the person's side and somewhat above them, and as a result
| when carried through to the "other side" cannot possess what
| remains of the person in that domain (because they are
| returned to the origin in turn).
| whimsicalism wrote:
| If the cat were more focused in the upper left, I don't think
| this demo would work as well. DCT will have lots of high
| magnitude low frequency components which will drown out the cat
| if it is near the top left.
| lucianbr wrote:
| Also the fact that JPEG throws away a lot of data without us
| noticing is hardly a discovery, rather the stated purpose of
| the compression algorithm.
| rmnclmnt wrote:
| From what I remember from some student project many years ago,
| this technique is the basis for robust digital watermarking for
| any kind of signals, be it images or audio.
|
| Of course the main application is to detect copyrighted material
| even after signals being heavily processed (e.g. ripped or cam'd
| movies, provided by JPEG-2000).
|
| If anyone in the movie industry can provide some more technical
| details, I'm all ears!
| dookahku wrote:
| this reminds me of Hough transformations.
| ziofill wrote:
| One interesting thing is that in the quantum description of
| position and frequency (i.e. position and momentum if you account
| for hbar), it is not possible to cram two different functions
| into one in this way because functions that differ by a position-
| dependent phase are different quantum states.
| baobabKoodaa wrote:
| Can someone please ELI5 for me?
|
| I don't understand how the cat is encoded in the image that has
| both woman and cat. I assume the visible pixels are in some way
| slightly altered to encode the cat?
| pwg wrote:
| @toast0 answered your question here:
| https://news.ycombinator.com/item?id=40357927
| xuhu wrote:
| How is the DCT of the two images done here exactly ? Clearly 8x8
| tiles like in JPEG are not used, otherwise the similar blurry
| background tiles would still look similar in the DCT composite.
| Are the 2D DCT basis functions not a thing in this case ?
| TZubiri wrote:
| Touch grass
| sim7c00 wrote:
| i know nothing of this stuff, but it reminds me of aphex twin and
| venetian snares encoding images into their sounds. is that a
| similar thing somehow? i thinknfor venetian snares the track was
| something like song for my cat. if you'd use certain tools, the
| frequencies would show a picture of a cat.
|
| edit: venetian snares was an album, songs about my cats. you can
| find it on youtube, unsure if i can link it.
| jontutcher wrote:
| A similar fun trick was used by Aphex Twin (and others) to make a
| weird face appear in the audio spectrogram of one of his tracks:
| https://news.ycombinator.com/item?id=8509105
| sim7c00 wrote:
| love this, venetian snares too. thanks for confirming haha, i
| wasnt sure how they did it! cool memories =) thx! didnt know
| which one it was from aphex twin. these guys are magicians :D
___________________________________________________________________
(page generated 2024-05-14 23:00 UTC)