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Thread: volumetric fog screens (existing and ideas for improvements)

  1. #11
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    Interesting, although isn't this just another take on Peppers Ghost? In which case there are better ways to achieve this:

    https://www.youtube.com/watch?v=1pjZ98p9k3c

  2. #12
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    I saw your post when you first put it up and read it as ghost peppers. I had absolutely no idea what you meant and moved on. On a whim I clicked the link today expecting something ridiculous. Opps. I sure read that wrong. Never heard of that method before and it is amazing. Thanks for sharing.

  3. #13
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    It's hard to tell if it's volumetric or just peppers on the surface of the smoke. Unless the projector was mounted below and projecting up through the smoke in differing layers, something impossible to do, I'd suggest that this is just peppers ghost even though the image appears to be to the eye to be volumetric in the smoke.

    As I see it, there's a simple question to ask if it's volumetric - how do you get the light to pass through the outer layers of a dense cloud of smoke without reflecting off and illuminating the outer particles only to illuminate and reflect off the particles in the centre of the smoke mass? I'd suggest that just as in a projection from below it's impossible to get light to pass through some layers of reflective particles without reflecting off them only to then reflect off others at a specific depth.

    In my opinion, whilst the smoke may be volumetric, what you're seeing is a projection onto the surface of the smoke just like with a water screen or with a peppers ghost effect onto glass / mylar film. The clever part is the projection is very 3D in appearance but I'd suggest that's down to the source rather than the smoke juts like Pepper's Ghost. I'd like to be proven wrong, but that's what I suspect.

    The only examples I've seen of true holographic projection are made with lasers where the laser is able to produce light by causing what I understand to be plasma gas to form at a specific point in space by heating the air causing the emission of light. They aren't very advanced in terms of detail:


    https://www.youtube.com/watch?v=WkkZcFWgr4Q

  4. #14
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    Jan 2019
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    I wouldn't call the existing one in my first video volumetric as you can get only around 6 individual views across the cylindrical air flow. You'd need something like 300 views to get a volumetric or autostereoscopic multiview display. But it isn't a simple Pepper's ghost screen either. There is a new view every 30 degrees, so it is a multiview display.

    Aside from lasers atomizing air particles in mid air there's also swept volume volumetric displays which I mentioned in my OP. The projection screen is translucent and rotates constantly or vibrates up and down or left to right at very rapid speeds. The DLP projector is modified to project in monochrome instead of RGB which allows to reach several kHz frames per second and refresh rate which is enough to project enough views for a mutliview display so each eye sees a different perspective and also enough to project many slices of a 3d volume per frame for a volumetric display. Perspecta was such a device, check the third video in my OP. It has been long discontinued but these days you can buy the Voxon VX1 which works very similarly but still costs several thousand dollars.
    Downside with swept volume volumetric displays and swept volume multiview displays is the noise the rotating screen makes and the fact that the "hologram" only appears in mid air but you can't reach out and touch it as the screen is in an enclosure and rotating dangerously fast.

    That is why laminar flow got me interested.
    For an ordinary Pepper's ghost you can just use water, PC fans and a cheap Ultrasonic Atomizing Piezoelectric Transducers. You don't need to use a fogger using fog juice.
    But the more I search on Mie scattering, the more it seems that there is no way to narrow the scatter angle to get more views rather than every 30 degrees or so. For a volumetric or multiview you want ideally new view every 1.5 degrees.
    Only solution then remains either rotating a fog or water particles screen suspended in air somehow which I do not believe is possible or blowing one 2d fog screen, blowing it away, then blowing another one at a slightly different angle. But for a minimum refresh rate volumetric display you'll need to blow a new screen and blow away previous one 1500 times each second. We are talking about 600 km/h blowing speeds. Even for a narrow stream of tiny atomized water particles, I don't know if it is safe for the skin and eyes. I really doubt it is, but if it is, I can help with everything else like modifying cheap projectors to project in kHz speeds, encoding, streaming via HDMI and decoding such framerate videos and syncing the rotating mirror and blower with the video projector.

  5. #15
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    Update: never mind, bought a cheap usb humidifer to test mie scattering. Indeed the projection on the back is visible only around 5% so it will work

    Guys I need some help from anyone knowledgable and experienced with how mie scattering behaves with lasers or video.
    For a 360 degree cylindrical screen as in the first video I'd need the video projection to only be visible from the front (viewer facing the projection beam origin) and not from the back, otherwise if I continuously rapidly rotate the projection beam to cover a cylindrical viewing area, the video projected on the back side of the cylinder will be visible as well which should only be visible when looking from the opposite side of the cylindrical laminar flow screen. This will cause two different views to be imposed over each other and ruin the results. Something like 10% being projected back is fine as our vision will ignore that small dim interference but anything more is a no-no.

    I think with fog fluid screen the laser images or video projector images are visible from both back and front but that may be due to the size of the atomized glycol particles being smaller and resulting in rayleigh scattering rather than mie.



    Even though this project is relatively cheap to build and test, it will still cost money and time spent so I wouldn't want to learn the hard way that what I built doesn't behave as I had expected due to my lack of understanding how mie scattering will influence video projection visibility based on the viewing angle.
    Last edited by caimgin; Today at 04:51.

  6. #16
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    Jan 2019
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    What general rules and equations can I use to determine how small I can and should go with the diameter of the straws and how long with their length before I achieve worse or same results than if I hadn't?
    I hope those of you who have built the bigger laminar flow screens did it based on more than only trial and error.

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