[HN Gopher] Wide angle lens distortion correction from lines
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Wide angle lens distortion correction from lines
Author : hugohadfield
Score : 124 points
Date : 2024-07-17 14:20 UTC (5 days ago)
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| lionkor wrote:
| This site could really use a mobile version[0]
|
| [0]: https://developer.mozilla.org/en-
| US/docs/Web/CSS/CSS_media_q...
| dwighttk wrote:
| Reader mode works alright
| Anotheroneagain wrote:
| You can get a rectilinear lens instead, you know.
| xeonmc wrote:
| and a spherical cow while you're at it.
| Anotheroneagain wrote:
| They're commonly available, not something exotic.
| TeMPOraL wrote:
| Are they? How do you store spherical cows? Do you need to
| roll them out to pasture and then back into the barn, or do
| they roll on their own?
| Anotheroneagain wrote:
| I mean you have to pick between fisheye and rectilinear
| lenses when you buy a wide angle lens. This is completely
| unnecessary, you only need to pick the lens that you
| actually want.
|
| Why does everybody doing as if I propose something
| outrageous or impossible?
| srean wrote:
| I think the down-voting was harsh, it usually gets
| corrected in no time.
|
| That said people here are interested in the different
| ways of solving a problem, if not for anything else but
| to tickle one intellectually. So yes rectilinear lenses
| exist, but that does not mean that computational methods
| are uninteresting or useless. For one thing, one need not
| purchase different kinds of lenses.
| krisoft wrote:
| > This is completely unnecessary, you only need to pick
| the lens that you actually want.
|
| It sounds like you are thinking only in the context of
| photography. In robotics and machine vision applications
| you often choose the fisheye lens because they are
| cheaper than the rectilinear lenses with the same FOV.
| (if a rectilinear lens is even available in the form
| factor and FOV you need.)
|
| So what people do in those situations is that they get a
| crappier lens and they calibrate it so the algorithms
| know how much to correct for its crappyness. That is
| where this kind of calibration really shines.
| epgui wrote:
| These aren't even fisheye examples.
| epgui wrote:
| Even the best and most expensive professional lenses have
| some barrel and or pincushion distortion.
|
| It's unreasonable to expect never to need any correction,
| and it's actually a really interesting, non-trivial
| problem, to tinker with.
| hug wrote:
| Sometimes you can, yes, if you are picking the lens with which
| a subject will be photographed -- you can get down as low as
| 9mm on 135-film area, and still buy a relatively rectilinear
| lens.
|
| Sometimes you can't get a rectilinear lens, though: If I want
| to shoot wide angle on my phone, curvilinear will have to do.
|
| Sometimes you don't even have a lens, you've just got a photo,
| and that photo is curvilinear.
|
| Novel ways to adjust for distortion are always nice to have in
| the toolkit.
| TeMPOraL wrote:
| Sometimes you don't even have a photo, but rather a
| synthesized image that looks _as if_ a lens was in use. Or,
| you 're looking through a translucent material, which acts as
| an ad-hoc lens with unknown parameters.
|
| Sometimes, you want to have a system that's able to self-
| correct after you slap a random lens on it as it's working.
|
| Point being, methods of on-line calibration without use of
| special calibration setup (like ArUco boards mentioned
| elsewhere) have a wide range of use cases and are always
| welcome.
| skhr0680 wrote:
| I know very few 35mm format lenses with NO distortion.
|
| The two I know of with the least distortion are actually primes
| from the 1980s. Nikon began allowing a small amount of
| distortion in their new prime designs circa 2010, choosing to
| correct it with an in-camera profile.
|
| It's not as bad as it sounds. Getting rid of that last bit of
| distortion may require relatively major tradeoffs in other
| areas like brightness.
| formerly_proven wrote:
| There's a number of lenses which prioritize distortion
| correction because they don't get to have lens profiles.
| Though even low distortion wide angle lenses generally retain
| low levels of high order distortion (i.e. straight lines
| become slightly wavy across the image, instead of having a
| large amount of low-order distortion, i.e. being simply bent
| strongly one way or another), see e.g. Laowa Zero-D lenses.
|
| I do actually think the OEM design approach is better
| overall. It's a lot easier to near-perfectly correct high
| amounts of low order distortion than it is to make lines with
| a slight amount of 6th? 8th? order distortion actually
| straight. Even if the resulting raw image of the OEM lens
| looks more like a fisheye than a rectilinear lens.
| nimbleal wrote:
| Zeiss cinema lenses (in particular master primes) have the
| least distortion I've come across
| tveyben wrote:
| Would one og these be the rectilinear 15mm?
|
| Great lens - and a huge front element...
|
| I used that in the 90'ies to make QTVR with a (really
| expensive) Kodak DCS 460 - 6MP on a DX sized CCD no display
| and the large 340MB PCMCIA disk ;-)
| jdietrich wrote:
| Optimising for low distortion means trading off against
| something else - sharpness, brightness, size, weight etc.
| Smartphone cameras have become so good because they're very
| intelligently optimised using a hybrid of hardware and
| software.
|
| DSLR/mirrorless users still use lens correction (either in-
| camera or as part of the post-processing pipeline) because even
| a big, heavy, expensive pro-quality lens is still imperfect in
| ways that are relatively easy to compensate for in software.
|
| https://www.canon-europe.com/pro/infobank/in-camera-lens-cor...
| Anotheroneagain wrote:
| This isn't about correcting for minor imperfections, but
| converting the image from the wrong kind of lens.
|
| See https://m.youtube.com/watch?v=6toiNmZ_e4I for the
| difference.
| eloisius wrote:
| Sometimes computer vision applications require rectilinear
| images, but you don't have a chance to choose the hardware, or
| it was chosen with other constraints in mind. No reason to dump
| on someone doing research to rectify an image in a novel way.
| srean wrote:
| A question for those who know optics: If the angle of incidence
| is past the critical angle of red do all of the visible spectrum
| get reflected without any chromatic effects ?
|
| Are there cameras that have a sensors laid out on a curve
| matching the expected surface on which the image is in focus ?
|
| I wonder why there are no cameras (apart from astronomical
| telescopes) that use reflection only for imaging. Such a camera
| would be too bulky to be practical ?
| zokier wrote:
| Mirror objectives do exist for dslrs:
| https://advancedphototech.wordpress.com/lenses/mirror-mirror...
| srean wrote:
| Thanks for the link. Learned something new.
| wdfx wrote:
| I have an old Nikon 500/8 ; gotta be honest, it's not very
| good.
| buildbot wrote:
| Some are not very good, but others are. I currently have 4
| different mirror lenses, two are not great but the other
| two are very good. One of which is an AF mirror lens!
|
| One thing is that they are extremely sensitive to shocks
| causing mirror misalignment - so if your lens has ever been
| dropped, it's probably lower performance than before due to
| the mirrors being out of wack.
| wdfx wrote:
| Mine's from 1974 according to a serial number lookup, so
| who knows what it's been through in the last 50 years.
| RobotToaster wrote:
| > Are there cameras that have a sensors laid out on a curve
| matching the expected surface on which the image is in focus ?
|
| Not a sensor, but some disposable film cameras have a curved
| film holder to compensate for low quality optics. Some
| panoramic film cameras do the same.
| Teever wrote:
| I had similar thoughts recently because I am working on a
| catadioptric system for a project at work.
|
| https://www.reddit.com/r/Optics/comments/oimvt0/curved_camer...
|
| https://www.digitalcameraworld.com/news/sonys-new-curved-ima...
|
| It appears that curved sensors maybe exist somewhere in a lab,
| and have been slightly commericialized, but I didn't see any
| 'buy now' buttons when I looked.
|
| I didn't dive too deep into it because It's not like I'm going
| to be changing the sensor in my design at this stage of the
| game, but it was an idea that a friend suggested when I talked
| about the limitations of the mirror based system that we're
| using.
|
| https://techxplore.com/news/2024-07-insect-autonomous-strate...
|
| This link popped up on hackernews a few days ago and I noticed
| that they were using a mirror in their optical system as well.
| I haven't had a chance to read beyond that promotional article
| above so I don't know how they're overcoming the depth of field
| limitations with this kind of optical set up.
| srean wrote:
| Very interesting. All the best for your project.
| PaulHoule wrote:
| In the early 2000s I was thinking about a machine vision camera
| that would use a mirror and a small lens to image a whole room,
| as seen from a corner. I figured it would take about 50
| megapixels to get the performance I wanted and at that time 5
| megapixels seemed like a lot.
|
| Today now that is no problem. A few years ago I saw this
|
| https://owllabs.com/products/meeting-owl-3
|
| at work, the fisheye lens on it is more compact than what I had
| in mind and it has enough pixels to pick out individuals
| speaking in a conference room.
| srean wrote:
| His dissertation looks very interesting
|
| https://hh409.user.srcf.net/index.html#PhDThesis
| PaulHoule wrote:
| I have been wanting to do this for my
|
| https://www.kandaovr.com/qoocam-ego
|
| after reading Lenny Lipton's books about stereo cinematography
| I've been debugging my stereograms and one thing I know is that
| the lenses on that thing have a little bit of pincushion
| distortion which means stereo pairs that are supposed to be
| perfectly aligned vertically aren't quite.
|
| I know DxO makes distortion correct filters for lens/camera pairs
| and I was sure I could make one by taking pictures of a grid but
| this gives a definite path to doing it.
| michaelt wrote:
| The ideas listed in the document are about correcting
| distortion when the image has already been taken and you can't
| control the scene.
|
| As you've got the camera in hand, you've got an even simpler
| option available: You can print a special pattern called a
| 'ChArUco board' [1] take pictures of it from a few different
| angles, then you can calculate the camera "intrinsics" (field
| of view, lens distortion parameters) and "extrinsics" (relative
| positions of your two cameras) based on those images.
|
| [1]
| https://docs.opencv.org/3.4/da/d13/tutorial_aruco_calibratio...
| qingcharles wrote:
| You can also use the patterns to generate DNG/RAW lens
| profiles which allow automatic lens correction in popular
| apps like Lightroom etc:
|
| Adobe Lens Profile Creator
|
| https://helpx.adobe.com/camera-raw/digital-negative.html
| syntaxing wrote:
| I swear I read something similar along the lines (pun intended)
| of this a couple years back but energy was not the Radon
| transformation, I forget what exactly it was. The hardest part of
| using this in production is that there is a lot of hand-tuned
| values, particularly during the edge detect portion which makes
| it difficult to scale. It's usually cheaper and easier to
| calibrate the camera at a mass scale in the factory using "old
| school methods.
| philsnow wrote:
| The last picture reminds me of what photos from my iPhone look
| like around the edges
| danilor wrote:
| I have difficulty understanding what the transformed image is
| equivalent to. This makes it feel like the picture was taken at a
| difference distance and focal length, but[1] it would look
| different if that were the case because the perspective would be
| different. Does this have any "physical" interpretation that
| would make it easier for me to understand? Like, cropping an
| image is equivalent to changing the focal length; what would this
| be equivalent to? A type of rectilinear lens?
|
| [1] With the exception maybe for a single plane in focus?
| oasisbob wrote:
| It sounds like you know this already, but as any portrait
| photographer would note, changing the focal length is not
| equivalent to cropping. It's roughly equivalent, at best.
|
| ie, Telephoto lenses bring a different perspective which
| includes distance compression. It's very apparent when
| photographing human faces.
| radiowave wrote:
| Changing the focal length doesn't inherently change the
| perspective, and (resolution and lens aberations aside) is
| exactly equivalent to cropping.
|
| What changing the focal length _does_ do is (e.g.) make you
| stand further back, and _that_ changes the perspective,
| causing distance compression, etc.
| TacticalCoder wrote:
| > I have difficulty understanding what the transformed image is
| equivalent to.
|
| As a non-photographer with zero knowledge about photography,
| the fixed image, with straight lines, feels much more natural
| to me.
|
| I'd say it reminds me of 3D games like, say, 3D game
| simulators?
|
| Are 3D games not reproducing lens deformation more or less
| correct from a "physics" point of view? I happen to be on
| vacation atm in an apartment on the beach on the ninth floor
| with a clear view: what I see is much closer to the "corrected"
| (not my word but TFA's author's one) version than to the other
| one.
| markerz wrote:
| As an artist, this the transformed image is what I would draw
| using 1-point perspective. Basically making everything straight
| lines. It intuitively feels a lot more natural and fits into
| our mental model of how the human world is shaped (i.e.
| everything is a rectangle)
|
| https://m.youtube.com/watch?v=qOojGBEsWQw
| brcmthrowaway wrote:
| What is the dofference between this and camera calibration?
| emtel wrote:
| This is cool, but couldn't you generate the correction
| transformation simply from knowing the lens geometry? I assume
| this is what my phone is doing when I take wide-angle pictures
| (which don't have any visible distortion)
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