POV Staff (video)
- Thread starter yanekm
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mortonkopf
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@yanekm its great to see your build process. Thanks for sharing. Interesting to see that you went with three aspect LEDs rather than two aspect. Was this for brightness or clarity of image?
The brightness is more than sufficient, actually I don't usually even use the full brightness capability of the LEDs. Experimenting with 2, 3 and 4-way prisms in the pursuit of image quality. I do not want the individual pixels to be visible, or the light field to be made of discernable lines with darkness in between; but neither do I want to sacrifice (too much) resolution. Light-shaping diffusion film from http://luminitco.com/ does interesting things.
@mortonkopf BTW, one of these located at the center of gravity is an effective way to sync up the image to the rotation speed in near-real-time: http://www.frys.com/product/1908343
mortonkopf
Well-known member
that is a nice small component, I guess that you are using it to trigger the image at a give point, rather than do sampling of on off to create an average?
The switch triggers an interrupt, which keeps track when it was last triggered, and if the time since the previous state change is within a designated range (a "reasonable rotation rate") and within some percentage of previous two measurements ("steady rotation"), it changes a global variable, which is used elsewhere in the main loop to control the rate at which pixels are painted out. So, conceptually, it's a boolean phase-locked loop with a tolerance for missing or spurious inputs.
mortonkopf
Well-known member
That is very nice, thanks for the clarification. Its certainly a step above my feel for 120bmp. I like the feedback process to store and update the global variable, continually syncing to the rotation speed. When it comes to group work, though, I think pre programmed non-changing output rate is the option, as all the visuals remain synced. But for a single performer, your process sounds great.
You could have the detected speed of motion control the timing of the *current* image/animation, so its aspect ratio, etc., will always look good; while keeping a completely separate independent timer for when to switch images, which would remain in sync with the group.
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happyinmotion
Well-known member
Hey, that's looking pretty good. I have a heap of questions:
What camera did you use to get those recordings? I've tried and failed to get good video of my staff.
How effective is the mercury tilt switch if the staff is spinning around a point away from the centre? I went with a gyro coz my hands wander up and down the staff all the time.
What diameter tubing are you using? Is it stiff enough?
IS the mic able to pick up sound over the wind noise from spinning? How is it isolated from mechanical noise (e.g. hands moving on the staff)?
What camera did you use to get those recordings? I've tried and failed to get good video of my staff.
How effective is the mercury tilt switch if the staff is spinning around a point away from the centre? I went with a gyro coz my hands wander up and down the staff all the time.
What diameter tubing are you using? Is it stiff enough?
IS the mic able to pick up sound over the wind noise from spinning? How is it isolated from mechanical noise (e.g. hands moving on the staff)?
mortonkopf
Well-known member
I am interested in this too, trying to get decent video is a real pain. The colours just wash out. I found recently that the inbuilt MacBook camera gives a reasonable impression of what you see: https://youtu.be/_YUVUlHGxPU
The two sizes of tubing I have experimented with both have an inside diameter of .75 inches, with thin-walled and outside diameter of .875, and thick-walled with an OD of 1 inch. Neither is sufficiently rigid without an inner core, the thinner one is more pleasant to hold, the thicker one is more protective. With a hardwood or aluminum core, both are more than rigid enough.
The solid-state gyro+accelerometer is the better sensor, but the mercury tilt switch is much less expensive, easier to interface to, can also operate a sleep-wake circuit independent of the processor. Its effectiveness during staff rotation is surprisingly good; it must be located with the sensitive point of the drop of liquid metal very precisely at the center of gravity. It does not much matter where your hands are, when free to rotate, the object will rotate about its CG. The code discards errant readings by only looking for consistent sequences of on-off timings as in a boolean phase-locked-loop. Thus, having a few missed transitions does not cause a problem, as long as the speed remains approximately consistent, and as soon as you establish a new speed, it will get picked up by the loop.
I have not yet done enough with microphone input to conclusively speak to that.
Of the camera, I did not do the photo/videography, others more capable in those areas than I did, and what I know is that lower shutter speeds are better; orientation of the plane of motion to the focal plane of the camera matters; and if possible, at least approximately sync the rpm to the shutter speed, for the clearest image.
The solid-state gyro+accelerometer is the better sensor, but the mercury tilt switch is much less expensive, easier to interface to, can also operate a sleep-wake circuit independent of the processor. Its effectiveness during staff rotation is surprisingly good; it must be located with the sensitive point of the drop of liquid metal very precisely at the center of gravity. It does not much matter where your hands are, when free to rotate, the object will rotate about its CG. The code discards errant readings by only looking for consistent sequences of on-off timings as in a boolean phase-locked-loop. Thus, having a few missed transitions does not cause a problem, as long as the speed remains approximately consistent, and as soon as you establish a new speed, it will get picked up by the loop.
I have not yet done enough with microphone input to conclusively speak to that.
Of the camera, I did not do the photo/videography, others more capable in those areas than I did, and what I know is that lower shutter speeds are better; orientation of the plane of motion to the focal plane of the camera matters; and if possible, at least approximately sync the rpm to the shutter speed, for the clearest image.
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