Intrinsic Calibration Mean

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This topic contains 3 replies, has 3 voices, and was last updated by  sotoglzz 3 years, 10 months ago.

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  • #101685

    sotoglzz
    Participant

    Hi Oliver, we have finished our AR Sandbox and is amazing, thank you, Oliver.

    I have a doubt about intrinsic calibration, I really don’t understand what do mean intrinsic camera parameters and Depth conversion formula and how does RawKinectViewer calculate these numbers?

    I really appreciate your answers

    Thanks in advance Alberto GT

    #101686

    Oliver Kreylos
    Keymaster

    Intrinsic calibration defines how a depth camera such as the Kinect converts from two-dimensional depth images to three-dimensional geometry. Without intrinsic calibration, the surfaces reconstructed by depth cameras would not be a 1:1 match to their real counterparts. While each Kinect is individually calibrated at the factory, and contains the resulting calibration data in its firmware, the calibration is not particularly good. Most importantly, a factory-calibrated Kinect will reconstruct a flat plane as a gently curved bowl. In the AR Sandbox, this would lead to elevation contour lines that are not completely flat when viewed from the side, or water that might appear to flow uphill.

    The custom intrinsic calibration procedure in the Kinect package is rather complex, but it can correct for almost all of these problems.

    In technical terms, the result of intrinsic calibration is a 4×4 homogeneous projection matrix that transforms depth-valued depth image pixels (px, py, d, 1) into 3D positions (wx, wy, wz, w). (These 4-vectors are homogeneous points; to convert them to regular affine points, the first three components are divided by the fourth, which is then dropped. So the affine counterparts of the two given vectors would be (px, py, d) and (x, y, z), respectively.)

    The depth conversion formula is a part of the intrinsic 4×4 matrix that describes how a depth value d as reported by the Kinect is converted into a metric distance z in centimeters. Concretely, the conversion is z = A / (B – d), where A and B are device-dependent constants determined during calibration. A is usually around 32000, and B is around 1090.

    #101687

    maphew
    Participant

    Thank you Oliver. That explains what I was seeing one evening when I attempted recalibration 4 times in a row, in order to correct a “fishbowl” effect I was seeing.

    I thought the camera was broken, but it turned out I’d forgotten to add the fix camera perspective ‘-fpv’ parameter.

    I don’t pretend to understand the matrix projection math overview but I appreciate you taking the time to introduce it. Someday I may be sufficiently inspired to dig in. :=)

    #101690

    sotoglzz
    Participant

    Thank Oliver, is very useful this information. I appreciate very much your answers.
    Thank you.

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