Video Feedback Cancellation technique - Uncles & Angels

  • Hi all

    Uncles & Angels is a video/dance collaboration where the choreographer/dancer Nelisiwe Xaba performs right up against a screen, interacting with multiple duplicates of herself delayed and looped from live inputs. The camera faces the screen head-on... and of course usually there would be feedback. I found a neat trick to cancel this feedback, which I chatted about to some members of this community some time back (years!)... but my university was in the process of trying to make a workable commercial patent out of the trick so I held off discussing it in detail. In the end the patent idea proved to be more trouble than it was worth, and was from the start contrary to my instinctual impulse to share useful info, so I would like to post this method.
    The audience sees this (Audience View image), while the camera sees this (Clean Signal image). 
    The idea came to me while working with stereoscopic projections and the quite wonderful 3D Virtual Black back projection material from DaLite. And it is really simple: the screen material is completely black (without projected light), and is non-depolarising. I fit a polarising projection filter on the projector (which has to be DLP), and a perpendicular polarising filter on the camera, which cancels the projected light. The camera then sees Neli against a pure black backdrop, perfect for live duplicating, delaying and comping with additive blends - and does not register any of the projected image.
    The lighting is of course tricky as any non-polarised light falling on the screen will show up on camera and be amplified in the duplications.
    In any case, that's it. Let me know if anyone finds this useful, will be interesting to see if there are other applications!
    Take care

    d23e00-audience-view.jpg d9dbba-clean-signal-01.jpg

  • nice solution!

  • genius! thanks mockej!

    let's try  :)
  • Izzy Guru

    Can't beat a bit of live feed video delay ;)

  • Damn, it's a bit addictive...

  • excellent

  • Tech Staff

    Thats clever.. Love it.