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Great work Gary!
and thanks nex.

Hmm..I may be way off on this but I was thinking more of a strobe light effect. There is another word that describes this effect but it escapes me and I guess its similar to the strobe light effect. As for the curving I think its due to the different angular(?) speed of the blade. If you do an analysis on a prop blade, the point furthest fr the center moves fastest, that makes it look curved and possibly the part that looks like its detached is because your camera shutter is moving at a speed similar to the other 'points'(the parts you captured) but not the points thats 'invisible'. It doesn't pick it up hence making it look like its not there. Hope this helps!..or perhaps it confused you more, in that case, sorry!! :)

The light reflecting off of the moving propeller is deflected and passes through the glass at an oblique angle. I'm guessing that you would be able to see it without electronic capture devices (if you could process the image fast enough).

I think I know. It must be that the phone takes pictures by scanning from top to bottom or visa versa. It must be a very fast scan, so that by the time the scan line has has moved up, the propeller has moved up and gets rescanned.
I can't explain exactly why it makes that pattern.
A long time ago I used to use an old computer camera that had a very slow scan and was able to get some wierd effects.

The aircraft has a stobe light either on top or underneath th fuselage. It gives the impression of stop action to a relatively poor camera.

Thanks, nex!

Also, of course! The flickering of the fluorescent tube makes some nice sense to me. I can't recall if I was still on the old CRT for that picture. I'm on a flat panel these days, but the upgrade was somewhere about the time I took the picture. Thanks for the help.

Gary, you got that animation to fit in pretty much perfectly, I'm very impressed!

As to your question about the multiple monitor images: No, the camera certainly doesn't scan multiple times. The picture would look very different in that case! It depends on which kind of monitor you have. If it's an LCD with a backlight, then that backlight is a flourescent tube, which naturally flickers, albeit _very_ fast in this case. CRTs are only ever lit at a single pixel, and while the electron ray is moved back up to the top, the screen really is unlit for a while, and this interval is indeed called the blanking interval. However, the afterglow of a TV's phosphor layer is quite long, so the tube doesn't really go completely dark. It can, however, be so dark that it looks completely blank with some exposure settings ... it's just unlikely that you'll find the entire screen completely dark, as the blanking interval isn't that long compared to the afterglow duration. (I'm guessing a bit here.)

from the fencecheck.com article linked above, there are some other
cool distortion examples:

http://www.rit.edu/~andpph/text-slit-scan.html


Warren

Edited by author 02-23-2007 11:05 AM
It's the scan line effect as Gary Fixler demonstrated. I've used this effect in some art by moving hard-copy portrait photos of people around on a flatbed scanner.

Here's a link to a simple study in lateral transfer using a Bell phonecard.

http://img528.imageshack.us/img528/6763/bellcallingcardnz7.jpg

Edited by author 02-23-2007 08:33 AM
How about a nice visual? I just mocked this up in Maya. I didn't get everything exactly matched up, but that's to be expected with the ambiguous angle, and position of the prop housing, the probable differences between the original camera, and my camera's focal lengths, the contours, and lengths of the blades, and even the number of them. This was the best fit I could get fudging some things around in about 15 minutes, and I ended up needing 5 props to get both the camera scan-line, and the prop rotation to work for ideal linear movements. This doesn't necessarily mean the plane's prop had 5 blades. It's just what I could get to work for me.

That said, the yellow line represents the camera scanning in the image row-by-row, just like an old TV scanline, at what would in real life be a very fast rate, but up against the speed of the prop spinning is relatively slow. You can see that each row that's recorded takes a single-pixel tall slice of the current state of the blades as they spin. Note in particular that the yellow tip of the propeller blades actually makes its appearance on both ends of the detached blade. That's because your seeing the same thing get scanned 2x, as the blade dips into the retreating scan-line, and pulls back out again. This should be more clear in my animation, each version being about 1.4MB

http://www.garyfixler.com/prop/prop.avi
http://www.garyfixler.com/prop/prop.mpg

Also, feel free to poke around with the 200 source images:

http://www.garyfixler.com/prop/

Another interesting version of this effect can be seen with camera tosses. In the following, I set my camera to take a picture after a short delay, and heaved it spinning into the air in my room. You can see my TV and computer monitor, and if you notice, they're heavily bowed in each instance. This is because both displays create their images with scan lines, so while the camera's moving, the rows are being drawn, meaning the camera keeps pointing in a slightly different direction for each row, creating warped images.

http://www.flickr.com/photos/garyfixler/42565941/

As to why there are many separate images of each screen, it's either that the scan lines take a break between each refresh, meaning the screens are actually dark for a moment before the next refresh, or the camera takes its long exposure by scanning multiple times, pausing in between each. I lean toward the former, as the red LED from my mouse creates an unbroken trail.

RaGe, you are correct as far as the imager technology is concerned. Pretty much all tiny, extra-cheap digital cameras measure the image line by line, which results in a distinctive distorsion when your subject is in motion. This is what makes the blades curved (AFAIK the actual propeller blades of that plane are really straight). It is the exact same effect that causes one blade to seem detached.

Here's how you could make yourself understand what's going on: Stick a piece of graph paper on a wall, put a fan in front of the wall, and some sort of light behind the fan. Use a small light, or put the light source in the other corner of the room, so you have a sharp shadow of the fan on the graph paper. Now trace the outline of the shadow, but only one line at a time. Manually rotate the fan (don't plug it in) by, say, 5 degrees between lines. If you experiment a little, you'll see that contiguous areas can easily become detached.

So from all the posts I've read here and from all the external pictures linked, I gather that :

* The sensor was more likely a CMOS than a CCD
* CMOS uses and Electronic Rolling Shutter, which produces effects similar to the old vertical shutters

This gives me a fairly satisfactory explanation for why the blades are curved. This curved effect is more pronounced in the pictures of Helicopter blades that some people linked to.

However I am still in the dark about why one of the blade pieces seems detached.. Help!

"has anyone ever created a 1-bit sensor?"

Combining multiple frames gives you more bits per pixel (higher quality, less noise), not more pixels.

(1-bit sensors aren't that uncommon, though. But you'll need to move either the sensor or put a moving or rotating mask or mirror in front of it to get something useful. Satellite instruments often use moving sensors or mirrors, missile navigation systems sometimes use a rotating mask in front of a single sensor.)

The strobe effect has been explained here sufficiently. Why do they propeller blades appear to be bent? Because they are, due to a differential in air pressure from the front to the rear of the blades. In the pre-jet years, airlines would never mention this because they didn't want to scare people
into thinking the blades might break off.

Edited by author 02-23-2007 12:23 AM
I have several pics showing this same effect:
- http://www.flickr.com/photos/edans/47661377/
- http://www.flickr.com/photos/edans/73256529/
- http://www.flickr.com/photos/edans/107268768/

One of my commentators, Herme Garcia, provided an explanation here (in Spanish):
http://itsp.typepad.com/voip/2005/12/ingeniero_de_gu.html

Translating from his text, the effect is a result of the Nyquist Theorem, there's a mathematical explanation here:
http://cnx.org/content/m10791/latest/

The theorem says that in order to recompose an analog signal from its samples, these samples have to be taken with a minimum sampling rate exceeding the double of its maximum frequency component (Nyquist frequency). If the sampling is done at a lower frequency, as is the case in the CCD of many digital cameras, you get "aliasing", the same effect that produces the wheels of a car to look like they are spinning backwards or the bands in a CRT monitor when you see it through a camera.

Hope it helps...

Enrique Dans
http://www.enriquedans.com

the end of the prop is moving faster than the parts closer to the rotor linkage. The ccd does not record all lines of information simultaneously therefore some information at the top of the frame actually occurs before the information recorded at the bottom. Solved.

Does this effect only happen with single-blade "clapboard" type shutters, or with telescoping multi-blade shutters, or both?

I think that the propeller is falling apart and the plane is about to crash.

I would agree with jima

"Basically, each line of the CMOS sensor is exposed at a different time. So, for example, the first line is exposed from time 0 to time 30, line 2 is exposed from time 2 to time 32, line 3 is time 4 to time 34, etc."

The prop is moving during the time the image is being captured line by line. You can make weird effects with a slow scanner by moving the piece while you are scanning.

Good shot too.
-Chris
eagleapex.com

Edited by author 02-22-2007 09:47 PM
Wait, I know why this happens! You see, as the...

Oh, never mind, you guys know it too. So much for the big mystery.

I'm an aviation photographer, so I've seen this effect before and I think I understand it quite well. If you want to read a forum where some aviation photographers figure it out (with more examples of pictures where this happens);

http://www.fencecheck.com/forums/index.php/topic,6032.0.html

http://www.fencecheck.com/forums/index.php...10276.html#msg10276

http://www.fencecheck.com/forums/index.php...77532.html#msg77532

Edited by author 02-22-2007 09:31 PM
Andy is partly right. It's just a strobe effect. The "floating" part of the blade is traveling at a different speed than the rest of the blade, because the blade is curved. The rest of the blades are really there because the turbo prop has multiple blades.

Mystery solved.

By the way, I'm a professional photographer that specializes in industrial applications and high speed subjects.

Andy Bee is right.

i shoot video for a living and love the effect of shooting rapidly moving landscape (eg out a car window) at a high shutter speed (1/1000 sec) Still frames of this type of footage look exactly like the photo -- i will ofen see sharp, repeated images scanned across a single frame.

so i would conclude that the camera in use has probably stopped down to 1/500 or 1/1000 of a second -- and probably doesn't scan quickly enough to accurately "freeze" all the blades in a single position. successive scans are painting the blades (and pieces of blades) in different positions

I reckon the propeller blades are forming a pressure wave which is refracting the light in a spooky way. The pressure next to a propeller blade would be enormous, and large variations in air density are certainly enough to change the refractive index of the air.

Let's not overlook the possibility of an updraft that might have nudged the plane in a weird way at just the right time to make the picture come out in a way that is not immediately accountable to the intuition.

Doesn't what we're seeing here just mean there's a hole in the toilet?

saw this linked from boingboing,
I have a few more examples. taken in st. louis last summer.
http://dontclickitdude.com/space/1.JPG
http://dontclickitdude.com/space/2.JPG
http://dontclickitdude.com/space/3.JPG
http://dontclickitdude.com/space/4.JPG

I think the answer is a lot simpler. Here is a link to a blog about the "leaning effect" found in old photos of fast moving objects, such as racing car wheels appearing to be ovals.

http://www.largeformatphotography.info/for...dex.php/t-2255.html

And a similar blog concerning the same effect with new cameras.

http://photo.net/bboard/q-and-a-fetch-msg?msg_id=004sR9

Analog photographers seem to be familiar with this effect, which comes from the camera and the timing of its shutter and/or the design of its electronic imaging.

Thanks to all you people for very interesting reading here. Obviously the people who come to this place are a cut above normal bloggers. (And I really loved the 'graviton' theory too).

Edited by author 02-22-2007 05:54 PM
This was discussed a few years ago on a digital camera forum:
http://forums.dpreview.com/forums/read.asp...010&message=7163509

This post has a diagram to help explain what is going on:
http://forums.dpreview.com/forums/read.asp...010&message=7153995

Note that the blade is moving in the same direction as the image is being captured on the camera. this causes the prop to appear to be moving much slower than it actually is. Apparently this is most prevalent in cheaper digital cameras.

Edited by author 02-22-2007 05:31 PM
I'll point out that the propeller blades on some modern turboprops are curved to start with, hence the "curved" appearance of the propeller is most likely not a artifact of the CCD "scanning"- see wikipedia: http://commons.wikimedia.org/wiki/Image:He...s.propeller.arp.jpg

An article describing this is at wikipedia under "scimitar propeller"

http://www.answers.com/topic/scimitar-propeller

uh...your propeller has fallen off! get the f*** off that plane now!

I would agree with the last post /m26 on the first point. Considering this is from a cellphone, it's quite likely a CMOS sensor, not a CCD. I disagree, that it has much to do with whether or not a flash was used.

This is quite likely caused by the "rolling shutter" used on many CMOS sensors. There's a quick write up here:
http://wiki.elphel.com/index.php?title=Electronic_Rolling_Shutter

Basically, each line of the CMOS sensor is exposed at a different time. So, for example, the first line is exposed from time 0 to time 30, line 2 is exposed from time 2 to time 32, line 3 is time 4 to time 34, etc.

An interesting side effect to this (besides making straight lines curved) is that it's theoretically possible for a thin line moving vertically through the field to either be completely invisible, or, conversely, to cover the entire frame.

Interesting answers. There is no flash for one...it would flare in the thick glass! The shutter speed is much slower than the prop speed so it looks like a faze thing. The same illusion that we see by eye in high speed spoked devices. The blades are dark so they block the light in faze.

Interiot,

Your example has absolutely nothing to do with how fast the CCD is read out.

1) The sensor is CMOS not CCD

2) The effect is entirely a result of using a flash and leaving the shutter open longer.

As has been already established, but just to add a little more to the mix. This is nothing more than a CCD with a very slow progressive scan readout.

This is another example of the same thing:
http://www.flickr.com/photo_zoom.gne?id=8798593&size=m

Some additional discussion on it and other interesting optical happenings with older cameras:

http://maisonbisson.com/blog/post/10531/

Lastly, here's a camera phone shot from a moving car (around 70mph)

http://www.flickr.com/photo_zoom.gne?id=323264975&size=m

A less extreme example of the same phenom.

Edited by author 02-22-2007 04:02 PM
The curved blades are definitely a slow-scan artifact. And turboprops traditionally have several blades, so that may account for all the blades in the image. I believe the free-floating blob is a blade on the far side of the rotation, moving in the same direction of the scan, i.e. the sensor was scanned from top to bottom, and the blades were spinning clockwise.

If the blades are spinning *much* faster than the scan, they could appear in multiple places in the picture. I can't tell for sure from the shrunken picture, but there should be thin horizontal gaps in the original picture. In a smoothly reduced version, I'd expect the blades to appear translucent.

I doubt Nathan's theory .. I'd expect the blades to be translucent if multiple scans were merged. But I'm intrigued by the theory now .. has anyone ever created a 1-bit sensor?

[Edit] Wups, I didn't realize the thumbnail linked to a large version. There are no gaps in the big version. Did you see the plane stopped? How many blades does it have?

If the originator of this article could identify the camera/ cameraphone that took the original image, I might be able to research and identify the sensor chip used and determine what the scan rate/ algorithm might be.

Looks like your shutter is open for maybe 1/64th of a second, the video chip captures the data every 1/500th of a second and stores these bits as a composite. The space between the blade images might then be the video chip refreshing.

No idea if this is true, but its a thought.

Similar effects can occur, even on faster CCDs:

http://paperlined.org/old/photoblog/minor/...tion_funniness.html

Note, the link/ flickr image by karl frankowski several entries below IS a scanline artifact- notice the distinct "breaks" in the propeller.

It's called "slow-scan photography". I have a weird old Kodak digicam that does it. The problem is (as stated by others) that the camera takes a non-zero amount of time to read out the sensor line-by-line. It's clearly a very fast scan, but it's not instantaneous, and the prop moves fast enough to cause trouble.

It's more of a problem on cheap CMOS sensors than on CCD sensors.

You can repeat this by taking pictures of other fast-moving objects (like a fan) or by spinning the camera itself.

I'll disagree and say "camera-based multiple images, not scanline artifact"- the camera sensor is using multiple images internally to the camera to create the final image. Not a mechanical shutter (most point-and-shoot camera's don't use them)and not photoshopped- just the camera's solution in hardware/firmware to increase sensitivity.

My problem with other answers based on a "scanline" issue is because I think if it was related to line-by-line scanning lag, there would be distinct breaks in the propeller position- such as distinguishable jumps in the position of the edge of the propeller related to the difference in scanning times. As proof I offer that you see 2 or more images of the entire blade in the same row and column- if it was truly scan-line artifacting, you wouldn't see the same blade in multiple postions along the same "scanline"- either horizontal or vertical (since I don't know what orientation this camera uses to read the CCD or CMOS chip)

However, if the sensor/imaging chip is read "~almost instantaneously" but multiple times and then those multiple images are combined in the camera's firmware to make 1 final picture- (say, each chip scan requires 10 milliseconds, if read exactly 5 times (since their are portions of the same propeller blade 5 times= 50ms )then you can increase the sensitivity of the CCD chip by 5x and in most cases not notice (unless you have a fast-moving object that's moving between full-chip reads. CCD chips do this to improve sensitivity because, unlike film, they can't just remain "active" for longer times to improve response- doing so would risk over-saturating the individual pixel sites on the silicon chip and rendering the image useless.

I believe this is what's happening here... the issue of using/combining multiple frames to improve sensor response is fairly well known. (astronomy, motion studies, Hubble Telescope, low-light imaging) Astronomers routinely may combine 100,200, 500 or more images to get a high-sensitivity final image

I saw the same thing just last week and blogged it. The effect was most pronounced when the engines were at full speed, but coming in to land with the engines slowing down, the propeller looked normal in the camera.

http://www.peteconnolly.co.uk/blog/wordpre...e-propeller-effect/

First of all, the propeller blades are all within the disc of the rotating propeller. Draw an ellipse where the propeller should be and you'll see that it lines up.

The fragmentation appears to be a scanline effect - not all lines of the CCD are read into the framebuffer at the same time. (My guess would be that the purpose of this is to be able to put the weakest CPU and smallest framebuffer possible in the phone for better costs and battery life.)

Any way, as a result, the propeller is in different places when different lines are read. A quick google says turboprops are spinning at between 1000 and 1700 RPM at speed, so the 5 blades we see might be a 1200 RPM propeller scanned in 1/2500 of a second total, with each scanline being exposed for a much smaller fraction of that time, thus eliminating motion blur on individual lines.

In other words, the effect is like the distortion you see when photocoying your hand while moving it (only sped up). As a mater of fact, you could probably duplicate the effect by spinning a pencil at the right speed on a copier or scanner's glass.
Another way to think about it is that it's the wagon wheel effect (wheels in westerns appear to spin backwards at certain speeds due to the movie camera shutter), only in space instead of time.

It would be interesting to test this hypothesis by photographing other fast-moving objects with camera phones.

If the camera was a digital SLR (or a film SLR for that matter), it may be an effect of the shutter curtain. At shutter speeds faster than 1/60 second or so, the focal-plane shutter curtain is adjusted to form a slit which travels across the image sensor (or film), exposing the area under the slit for the desired length of time, but still traveling across at about 1/60 second.

I took some similar ones with my camera phone whilst moving fast in cars and on trains. The first one was a mistake, and I was instantly intrigued. It's a bit like being drunk in a photo...

http://img443.imageshack.us/img443/8818/spookybl0.jpg

Scan rate, but perhaps also something about the inside of the prop moving slower then the outside?

hey autumn,

"Gravity is caused by substructural elementary particles called "gravitons". The propellers in a plane are monopolar grids of magnets..."

Those gravitons only work on freeways, where the metal in the front bumpers of cars and trucks collect them as you speed along. Which is why when the freeway is slowing down and you change lanes, the lane you are now in slows down and the lane you just left speeds up*.

Gravitons are the main cause of traffic jams, stuck sliding glass doors and big jars with recalcitrant lids. =)

*Your mileage may vary.

Yep, the floating "propellor blade" is really made just by the tip of one of the real blades - but it creates the impression of being a part of blade attached to an axis that's not on the wing of the plane! But if you look at the outer part of the "blade" as being part of the circle that the spinning props make, you'll see it's centered on the prop's axis. By the way, where is this? Somewhere on the St. Lawrence?

Yes, this is definitely related to the scan rate of the CCD..

I've wondered the same thing.
And taken the same photo.

Don't forget that the plane is traveling forward at great speed,
and the camera is getting a piece of the prop in the airspace
where it had been before (yeah, I know you are traveling at the
same speed). It is an optical illusion, similar to old photos of
race cars on the speedway, where the tires actually look like
foreward-facing ovals, instead of perfectly round.

That's my story, and I'm sticking to it.

Aeroplanes are actually powered by the gravitational forces of the earth. When a mechanical flying machine takes off, a magnetic vortex formed by the engines activates. Gravity is caused by substructural elementary particles called "gravitons". The propellers in a plane are monopolar grids of magnets that, when spun, create a positive ion charge in the air. This charge takes gravitons from the aeroplane and the area inside and around it and expends them through the vortex, creating what is basically a hole in gravity. The aeroplane uses this loss of gravity to fly.

The propellers are covered in a special diamond-shaped carbon paneling. This paneling has the ability to reflect and move gravitons. The shape and structure of the propellers help the plane move itself forward by taking the spare gravitons and projecting them directly in front of the aeroplane, losing some gravitons, heat, and infrared light in the process. Thus, instead of the flying machine falling into the sea, it “falls” forward.

I assume you used a digital electronic camera obscura. Certain digital camera obscuras have the ability to capture infrared light in addition to regular light. What you’re viewing is a rather interesting phenomenon known as “inverse electromagnetic refraction”. While you can see the propellers spinning yourself, you won’t be able to capture the image on electrofilm. This is because the propellers generate a strong, controlled electromagnetic field (to propel the gravitons ahead of the plane, as explained above). Infrared light generated by the reflection of gravitons on the carbon matrix is reflected off of the propeller. Because of the electromagnetic field, the infrared light bends at a slight angle around the individual fins of the propeller. Thus, your camera is able to “see” around the propeller, giving the illusion of the fins hanging midair.

Yep. Scan rate of the CCD. It's not taking the picture in one moment in time, but rather slices of time for each row of the ccd. It's basically the same concept as linear-strip photography.

here's some stuff i've done related to this: http://yorgle.cis.rit.edu/Image/2003/05/

also check out this page: http://www.rit.edu/~andpph/articles.html

not quite the same, but more like related to the opposite: http://seriss.com/people/erco/2001/

Thes scan rate thing makes sense. There could also be some "stabilization" algorithm in there that is going haywire over such a fast moving object.

I bet you could even figure out how fast the propeller is going based on the scan rate of the CCD that you used. But maybe that is a conversation for Slashdot, not Boing Boing. ;-)

exactly as blitzcat says.

You can shake your cameraphone left and right while it's exposing and you'll get a wavy distorted image. (works in bright light; low light may just blur the entire image.)

Digital camera. It's analogous to the strobe effect you'd see when you hold a fan in front of a TV.

The 'camera' sensor is only exposing part of the frame at a time. It looks like its scanning in horizontal sections from top to bottom. Because the propeller is moving faster than the camera is scanning, the propeller is exposed in different places several times.