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Thread: suggest how to implement autofocus for video projector

  1. #1
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    Default suggest how to implement autofocus for video projector

    I have a modified video projector that I need to add an autofocus to. The focusing ring already has a fast stepper motor added to it. If the focus had to be done on the middle of the projection frame I suppose I could just put a laser distance sensor in the optical path of the projection by having a 45 degree IR reflect, visible spectrum pass dichro attached after and maybe before the final focusing and zooming lens. But I want the ability to focus to any point in the frame. This requires the sensor to be able to be rotated, or if it goes through lenses, tilted in two axis. I could do that with a small gantry system with stepper motors and the distance sensor attached to it, or galvonometer mirrors that would reflect the distance sensor beam to a given point/angle, but there may be a better way or a better sensor for this I am not aware of which wouldn't require so many parts. Having a camera and software which focuses until software finds the sharpest result would work if it was fast enough (200 ms). As I understand with this method the lens is focused until the sharpest image is detected which is slow. This is a pretty unique use case and normally you don't need such a complex feature in video projectors, I know. Requirements are ability to measure distance up to 3 meters, accuracy about at least +-5cm up to 1 meter, +-10cm after that, which I believe is not much. Measuring on shiny or reflective surfaces don't need to be supported. Any ideas are welcome. Been brainstorming with friends but we couldn't come up with a better option to what I already have, yet.
    Last edited by shoujin; 04-18-2018 at 11:44.

  2. #2
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    Use a xbox kinect ?

  3. #3
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    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Quote Originally Posted by -bart- View Post
    Use a xbox kinect ?
    True autofocus is done with a camera and you run a Fourier Transform on the video to maximize high frequencies.

    In your case Google search for "Laserping".

    Steve
    Qui habet Christos, habet Vitam!
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    When I still could have...

  4. #4
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    I'm down to use the camera based autofocus too but I don't know if it can be made fast enough (say 200ms-ish). As far as I understand the camera doesn't know which side to move the zoom lens and tries both until it finds the sharpest result, right? If it went one way maybe I could get it fast enough.

  5. #5
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    The kinnect creates an entire 3d depth map pretty instantaneous. Ample time to move a stepper.

    Look on youtube for ideas on kinnect projector mapping, so much awesome stuff has been done.
    Moving one focus stepper is baby steps compared to some crazy shit out there.

  6. #6
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    Thanks for the ideas but I'm pretty familiar with kinect and similar devices such as Structure Sensor. While they give quick realtime results which are good eye candy for most of stuff people try to use them they are well, kind of shit. This has to do with limited number of points they shoot and the gaps between the points where software has to make guesses. It gets worse as distance increases, as point gap increases but point count remains the same. This is why body or hand tracking isn't that great. Size of the Kinect also makes it unsuitable. That said for the other 95% of similar projects Kinect would maybe be good enough.
    Last edited by shoujin; 04-19-2018 at 18:00.

  7. #7
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    Have you looked at the XV-11 Lidar, or the Benewake TFMINI Micro Lidar Module ?

    I havent tried them personally, but people seem to get good results from them.

  8. #8
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    Yup, results not that different than Kinect.

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