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What does “frequency” mean in an image?
- ChrisSD 7y agoWhen stackexchange or wikipedia articles appear on HN I always wonder what the context is. Presumably something triggered interest in this topic recently? In this case image frequency can be useful in many things from image/video compression to hashing images (to efficiently compare if images are similar, even in the presence of cropping etc). So I'm curious as to what in particular motivated posting this now?
- OldManAndTheCpp 7y agoI think it's just interesting to a lot of the HN audience. A modal HN reader stumbled across the post, they found it interesting, they thought it would be good to post on HN, and it was. I think it's interesting to the modal HN viewer because although FFTs are pretty neat intrinsically, but most people are exposed to FFTs through sound processing (if they are exposed to Fourier Transforms at all, I don't think they were a required part of the CS curriculum at my university, they were only required for the EE courses), so the application of FFTs to images seems like total magic. The explanation that jpegs use FFTs is ho-hum if you spend a bunch of time doing signal processing at your day job, but there are many intellectually curious developers who spend their day job gluing together CRUD apps.
- arpa 7y agoIf I recall correctly, frequency anomalies also show image doctoring, ala photoshop?
- ChrisSD 7y agoOh undoubtedly it's interesting. But interesting things often go by with few votes. Often there's some reason it quickly gains the upvotes needed to appear of the front page. For example, if the topic is mentioned in another thread.
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- thaumasiotes 7y ago> When stackexchange or wikipedia articles appear on HN I always wonder what the context is. Presumably something triggered interest in this topic recently? A lot of the time it's because the link was relevant to a discussion in another thread, and someone pulled it out of that discussion to submit at the top level.
- zbobet2012 7y agoThe best answer to this question is actually provided by Ouss which is only at 1 up vote. Showing the 1d case for frequency of an image is far more illuminating than showing the 2d case. Most people with a mathematics, or music background of any level understand frequency in terms "hertz". Aka speed of vibration. Translating it to speed of change of pixel value per pixels is difficult without the 1d case.
- microcolonel 7y ago> The best answer to this question is actually provided by Ouss Could you share a direct link? I don't see any such comment here.
- tempguy9999 7y agoYeah, got me too. Ouss is actually a respondent in the linked StackOverflow article.
- takeda 7y agohttps://photo.stackexchange.com/a/112702 https://photo.stackexchange.com/a/112702
- sidpatil 7y agohttps://photo.stackexchange.com/a/112702 https://photo.stackexchange.com/a/112702
- OldManAndTheCpp 7y agoIt was the second post by the time I got there, but the rankings are fluid so here's a permalink: https://photo.stackexchange.com/a/112702 https://photo.stackexchange.com/a/112702
- YeGoblynQueenne 7y agoYeah, I don't know. Ouss' answer doesn't quite spell out what the repeating event is in the context of the image. He shows a formula that calculates pixel intensities with an "f" cosntant, but doesn't explain what that "f" is and what it does to the pixel intensities. I mean, it's so easy to say "frequency in an image is the rate of change of pixel intensity". Sometimes when someone asks a question about something simple you have to just spell it out as simple as possible. (Another thing that works for me given a 2d image like Ouss' example is "think of looking at a sine wave from above" much as this makes no real sense).
- mattkrause 7y agoThe one thing missing from these answers is that “spatial frequency” (as this is called in vision/neuroscience) is a biologically-meaningful concept too. Neurons in early visual areas extract spatial frequency information, and it’s maintained in (largely) separate channels as that information flows through the brain. There’s a neat Dali painting which sort of abuses these properties to combine two portraits: http://archive.thedali.org/mwebcgi/mweb.exe?request=record;id=152;type=101 http://archive.thedali.org/mwebcgi/mweb.exe?request=record;i...
- YeGoblynQueenne 7y agoI honestly can't see Abraham Lincoln there, except the small portrait at the bottom left. Is that the point? I just don't get modern art- anything after the impressionists. I normally make an exception for the surrealists, like Dali, but this one is not in the "exception" group, I think.
- close04 7y agoYou have to be far away (20m away for the original) or squint a bit/cross your eyes and the big picture starts looking like the tiny portrait of Lincoln that you mention. You can also save that picture on your device and look at the generated thumbnail. Resizing it makes it a lot more obvious.
- YeGoblynQueenne 7y agoCheers, I managed to see it eventually.
- Jarwain 7y agoFor me to see Lincoln, I tried holding my phone at arm's length; it was better when my friend held the phone and I stood back
- sp332 7y agoThe small portrait is reproduced in the larger one. The pixels of the beard are around the bottom of the window, the ear is the lighter blocks to the left, and his right eye is her head in the center.
- knolan 7y agoWe also use this approach in physics to describe things like fluid flow in a computational domain. https://en.m.wikipedia.org/wiki/Spectral_method https://en.m.wikipedia.org/wiki/Spectral_method
- lostlogin 7y agoThis is very useful - I’m an MR tech and image formation is the job. It is entirely dependent on how we obtain the frequency information, what order we obtain it in and which corners we cut to obtain it. Image artefacts can be interpreted and the cause corrected. Blurring, movement, phase wrap, slice wrap, ringing and a host of others are directly related to the way the raw data is obtained.
- mattkrause 7y agoK-as-in-"Kill Me" space!
- unlinked_dll 7y agoI like to think of frequency as the quantum of a pattern.
- vermarish 7y agoThat sounds interesting, could you elaborate more?
- unlinked_dll 7y agoA pattern is something that repeats. Frequency is a mathematical description of repetition or cycles [1] . That's why spectral techniques and their relatives are so useful when dealing with patterns and information. So if you want to think about how patterns are constructed, frequency components are useful as the building blocks of those patterns. And from the other angle, if you stop thinking about frequency as "cycles per unit" and more in terms of just cycles or repetitions, then it's more intuitive when you talk about spatial frequency. [1] or to get really into it, it's one part alongside magnitude/phase components of linear/nonlinear combinations of time varying exponentials which abstract any pattern/signal/system.
- hazeii 7y agoAn interesting - and somewhat intuitive - observation is that the sharper the focus the higher the spatial frequencies go (this is pretty obvious when viewing a composite video signal on an oscilloscope while adjusting the camera focus - the peaks are 'pointiest' at best focus). In the 1D case, consider a dashed line. With perfect focus this is equivalent to a square wave (which is a sum of all the odd harmonics of the fundamental frequency). Loosely, the blurrier the image of the dashes the lower the frequency content - eventually blurring to 50% grey which is essentially DC.
- knzhou 7y agoIn physics, we call the spatial analogue of temporal frequency the "wavenumber", or if you're thinking quantum mechanically the "momentum". Neither seems to have caught on elsewhere though.
- blattimwind 7y agoConversely, moiré patterns can be understood as a result of high spatial frequencies being folded to lower frequencies due to insufficient band limiting (provided by an optical low pass filter in front of the image sensor) and thus violating Shannon's principle. This isn't an accident, we simply prefer sharper images that possibly contain spatial aliasing over less sharp (i.e. less high frequency content) images that never contain spatial aliasing. The reason why moiré patterns are generally colourful is because each pixel is not sampled at a point, but with the colors distributed over some area. Hence each color component of a pixel sees a slightly phase shifted component of the signal, i.e. it gets rainbowy. You would expect the moiré exhibited by a planar sensor like the Foveon X3 to show this effect to a lesser degree.
- dr_dshiv 7y agoI find spiritual comfort in the fact that all functions can be reduced to sine waves in a Fourier transform. "It's all vibrations, man". AMIRITE?