emgucv

Use EMGU to get "real world" coordinates of pixel values

There are a number of calibration tutorials to calibrate camera images of chessboards in EMGU (OpenCV). They all end up calibrating and then undistorting an image for display. That's cool and all but I need to do machine vision where I am taking an image, identifying the location of a corner or blob or feature in the image and then translating the location of that feature in pixels into real world X, Y coordinates.

Pixel -> mm.

Is this possible with EMGU? If so, how? I'd hate to spend a bunch of time learning EMGU and then not be able to do this crucial function.

Solution

  • Yes, it's certainly possible as the "bread and butter" of OpenCV.

    The calibration you are describing, in terms of removing distortions, is a prerequisite to this process. After which, the following applies:

    PointProjection

    The Intrinsic calibration, or "camera matrix" is the first of two required matrices. The second is the Extrinsic calibration of the camera which is essentially the 6 DoF transform that describes the physical location of the sensor center relative to a coordinate reference frame.

    All of the Distortion Coefficients, Intrinsic, and Extrinsic Calibrations are available from a single function in Emgu.CV: CvInvoke.CalibrateCamera This process is best explained, I'm sure, by one of the many tutorials available that you have described.

    After that it's as simple as CvInvoke.ProjectPoints to apply the transforms above and produce 3D coordinates from 2D pixel locations.

    The key to doing this successfully this providing comprehensive IInputArray objectPoints and IInputArray imagePoints to CvInvoke.CalibrateCamera. Be sure to cause "excitation" by using many images, from many different perspectives.