Calibration error: Some tie points have negative projection weights

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    Hi Oliver.

    Thanks for putting this awesome project in to the public space. I have just finished building one, but I am stuck in the “CalibrateProjector” step.

    It gives me the following message: “Calibration error: Some tie points have negative projection weights. Please start from scratch.”

    My setup is based on a NEC np610s projector with its focal point placed 1 m above the plane along the edge of the bounding box. The box is 1 m x 1.3 m.

    I have mounted the kinect 1.3 m above the bottom plane.

    I am using a cd covered with white paper and the intersecting lines for calibrating.

    Is there something that is wrong with this setup, or am I doing it wrong. I would appreciate a hint now that I am so close to completion 🙂

    Kind regards Kasper


    Hi Kasper, it’s likely that the error is due to the measured point being below the existing surface plane. Try hitting F2 to recalculate the surface plane after each F1 point capture until it figures out the actual texture surface.

    Also if you’re just using the sand surface then make sure it’s reasonably flat and level. Preferable for the first capture is a flat board or card. You may have to alter height manually in (? Sorry can’t remember the the filename) to raise or lower the bed height.

    OF bigger concern…. It looks like you’re using a short throw projector so you will need to make sure the Kinect and projector have the same ‘viewpoint’ ie centered above the bed otherwise the image has a very different geometry to the Kinects camera and will never align well.(yes my hard learning to here).

    Good luck


    Hi Grant

    Thanks for taking the time to reply.

    I have been hitting F2 to recapture the plane, and I was calibrating with an empty sandbox, so the surface was completely level.

    Regarding the projector, it is true that the angle of projection is very different than that of the kinects view. I had gotten the impression that mounting along the edge of the box would work. Since i am using a projector with an even shorter throw than the MW632ST, then the angle is steeper than it would have been with the suggested projector.

    I have ordered the Benq projector, and really hope that will solve the problem. If not I have a serious funding problem for my sandbox 🙂

    Again thanks for your input!

    Oliver Kreylos

    The issue might be that you are capturing tie points that are all in the same plane. The resulting linear equation system will be either under-determined or very ill-conditioned. During the first 12 tie points you collect, aim to get some that are as low as possible, and some that are above where you anticipate your sand surface is going to be when it’s piled up.

    Having different projector and camera positions should not be a problem. But when capturing tie points, ensure that the calibration disk is facing the kinect, not the projector.

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