Calibrating with a mirror???

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    I am nearly finished building an AR sandbox! I am at the point that I need to calibrate my kinect and projector. The projector is not a short throw so I need 70″ to get a 40″x30″ image. I mounted a mirror 40″ above the sand and will mount the projector 30″ below the mirror.
    My issue is that the projector does NOT have an option to flip the image! I can buy another projector but I would like to test things out with this one first.
    My question: can I calibrate properly using a mirrored image that is not flipped? Is there something in the software that can flip the data at the camera?


    As far as I’m aware, you do not need an image flip mode on your projector. You can just set up the sandbox as usual, and the last step in the calibration process takes care of the orientation.

    70″ above the surface seems like a lot, even for a non-short throw projector. Ours is not short throw and it rests at 40″ like in the image here: Have you tested the setup yet?


    I have not tested it yet. Both of the projectors I have need 60″-70″ throw to make a 40″x30″ image. I have looked at several other models that are not short throw and they come in about the same range. Some have a larger zoom range but still need 60″ throw.
    I am not against buying a short throw projector but I’d like to get everything setup and make sure it will get enough use to make the purchase worth it.
    I would just like to avoid going through calibration and find that it adjusts the image on the opposite side of the sand movement, lol.

    The projector I have with the flip orientation option is my main home theater projector so I would prefer not to use it for this. The one I want to use is just a cheap projector that will be replaced if this projects works out.

    Is it easy to start over on calibration if it fails?
    What brand/model of projector are you using?


    You might want to have a setup like this


    It worked for me…


    I had considered a tall setup like yours but I need it to be able to go through doorways without being taken down.
    I purchased a new projector this weekend and it nearly fills the box with my current setup and it has the flip orientation option. At least with the new projector I only need to move my overhead mount/mirror up 2.5 inches. Hopefully I will get that done tomorrow.


    Just to put this info out there…
    If using a mirror, the flip orientation option in the projector is a must AND the Kinect sensor will need to be mounted the opposite direction than seen in setups without a mirror.
    I mounted my projector with the top facing the sandbox so I could mount the bottom to my side supports. The Kinect is mounted in the same direction as the projector.

    Oliver Kreylos

    That is not correct. Your projector does not need a mirroring option, and the Kinect does not have to be in any special orientation.

    In principle, the entire sandbox calibration procedure (base plane equation, box corners, projection matrix) can be run with a mirrored projector. If the projector is your only display, you’ll have to be able to read the menu entries backwards, and have to collect box corners in lower-right, lower-left, upper-right, upper-left order, i.e., as a mirror image of the noted order, but other than that everything works.

    I overlooked mirrored setups in the SARndbox application itself, meaning that nothing is displayed with a mirrored calibration (it’s a one-line insertion of glFrontFace(GL_CW) if anyone’s curious). I’ll fix that in the future, but for now the solutions are thus:

    When using an Nvidia graphics card, the driver has the option to flip the image horizontally or vertically. Open nvidia-settings, go to “X Server Display Configuration,” find the screen associated with your projector, select it (if it isn’t already), and choose either “Reflect along X” or “Reflect along Y” from the “Orientation” drop-down menu. Then apply and confirm.

    Alternatively, run
    $ xrandr
    in a terminal to get the list of video outputs on your computer. For example, I have:

    DVI-I-2 connected 1600x1200+0+0
    DVI-I-3 connected 2560x1600+1600+0

    the first being my secondary monitor, and the second being my primary. Find your projector, for example by its resolution, note its output name (first column of xrandr’s output), and then flip it by
    $ xrandr --output <output name> --reflect x

    For example, to flip my secondary, I would
    $ xrandr --output DVI-I-2 --reflect x

    To undo the reflection, run
    $ xrandr --output <output name> --reflect normal


    Thanks! That is some good information. When I attempted to calibrate I went off my “intended” bottom left, bottom right, etc. When I was done and I moved sand in the intended bottom left the projected effect occurred in the upper right, lol. That’s what made me think I needed to also flip the Kinect.
    After making adjustments I was able to get it calibrated properly but this information would have been easier and faster for me!


    I’ve come across a similar problem with the Kinect raw image being mirror (left to right) with respect to the projected image, presumable due to the relative orientation of the Kinect camera vs the projector.

    Was there a final answer as to the correct calibration order to fix the mirroring?

    The recommended corner order is
    1. Bottom left
    2. Bottom right
    3. Top left
    4. Top Right

    But is this with respect to the camera view or the projected view (in my case these are mirrored left to right)

    I’m guessing my problem will go with changing the order of the corner inputs? Am I correct in this assumption



    You can check out beautify similar photos here:

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