10 thoughts on “BRRRRRRRRRRRRT, With A Twist

  1. Ahh! your point is the apparent twisting of the barrel group while rotating. an affect of the camera frame rate and speed of the barrels rotation. things are not as they appear. the barrels are not twisting as they rotate. they do vibrate while being fired, though. the gunmount is absorbing quite a bit of thrust from the firing.

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    • Ive seen video of prop aircraft taking off with it looking like the props are not even spinning or turning very slowly depending on the amount of throttle given because of that same frame rate issue.

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        • It has to do with the amount of time it takes for a shutter to sweep from the top of the photo or the CPU to scan the detector that makes the barrels appear to spiral. I’ve seen it as straight prop blades appear scimitar-shaped at certain speeds.

          Here’s a good example of that:

          Notice how the prop appears to change direction as it comes up to speed. Also notice that later the left-hand engine appears to be turning one direction, and the right-hand engine appears to be turning the other. This is because as the engines are started, number Two (third from left) is started first and brought up to 100 percent, then number Three (second from left, but these are Ja-PO-knee and they do things a little different). This has to do with the fact that Number Two and Number There engines drive the main generators and the order they come in determines what they power. Number Four (the left-most engine) also has a generator but it normally runs in standby, and comes in if either of the two main generators fails. When Number Four and Number One engines are started on the line, they’re normally only brought up to 70 percent, as there is no advantage to having them sit there at 100 percent; it’s just a waste of fuel. They are only brought up to 100 percent before taking the active runway.
          When the camera pans back, you can see Engines 4, 3 and 1 appear to be turning clockwise, and Number Two appears to be turning counterclockwise. Then, just before they taxi, Number Three appears to change direction as it comes up to 100 percent.

          Sorry for the rave, but I spent a lot of years working on and riding in these beasts and it still fascinates me how we did things and why.

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          • …and twenty years after they’ve been decommissioned nobody knows such details anymore which is a shame.
            It’s like those Navy ships rotting at their piers because they‘d need a software overhaul but nobody knows that language with which they’ve been programmed anymore.

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        • human eye/brain/image recognition sees at basically what you could call a frame rate of about 16 per second. probably wrong on that. film is done at twenty some fps so there is no stop motion effect in movies.
          aircraft flying down in the dirt at high speed use TFR coupled autopilot because at like nine hundred knots, what you see now is a quarter second behind you; if you are seeing a large tree in front of you it means your dead.
          digital video is so much more fun, remember the ghostly “tubes” people found in their phone/camera video a few years ago? turned out it was a affect caused by small birds zooming thru the frame. same deal.

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  2. This twisty-barrel illusion is not exactly a frame-rate issue. The apparent twisting is the rolling shutter effect, where the entire frame (each still image) is not captured all at once, but rather scanned line-by-line, as crazyeighter noted.

    Rolling shutter results in strange stuff like the “twisty” barrels, or things moving left to right (for this example of a horizontal raster scan) looking “smeared”. Rolling shutter and frame rate interact, but the old motion picture thing where the stagecoach wheels appear to be rotating backwards is aliasing. That’s an interaction between sampling rate (frames per second) and rotation rate.

    Thought experiment on aliasing. Suppose you’re taking photos of an old clock with a second hand. If you take a photo every 60 seconds, and make a little video of the frames strung together, it looks like the second hand (SH) isn’t moving, because each photo the SH is in exactly the same place. Take a photo each five seconds, and your little movie will show the SH moving “forward”. Take a photo every 30s and the SH hops between, say, 12 and 6. Take a photo each 65s, and the SH seems to be moving backwards.

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    • Damn it! I MEANT to say, take a photo every 55 seconds and the SH looks like it’s moving backwards.

      A photo every 65 sec gives the same effect as a photo every 5 sec, because 60 seconds is the period of one “rotation” of the second hand.

      /hangs head in shame

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