[Sam]

There are a lot of assumptions, and it is quite difficult for me to assess your needs.
Looking at your other posts, it seems that you are scanning holes on a wing and I already provided a partial answer here:
//www.sinclairbody.com/forum/Thread-Camera-r...ossibility

Finding a blob and its relative distance from the camera axis is fairly easy to do in pixel.
However, can you guarantee that the camera is perpendicular to the hole's surface and that the relative distance from the camera to the holes is constant?

If so, you can retrieve the pixel/mm ratio in the XY plane experimentally. For instance, place a ruler over the hole and the camera at your target, take the snapshot, divide the ruler length in pixels by the ruler length in mm. It should be good enough to iterate and converge to the hole centre. The examples in our documentations have every bit and pieces to achieve this.

If not, you need to consider a lot of variables.

• How do you align the Z axis of the camera to be perpendicular with the holes? Consider using the moments of the blobs.
  
• How do you dynamically find the distance from the camera to the hole? Consider having a reference measurement/marker.
  
• How do you compensate for defects?