A sample processing or assay instrument includes a moveable support. The moveable support defines a first pocket configured to receive a first object having a first machine-readable mark. The moveable support defines a second pocket configured to receive a second object having a second machine-readable mark. The moveable support also includes a first fiducial machine-readable mark and a second fiducial machine-readable mark. The instrument also includes an image capture device that captures a first image including the first fiducial machine-readable mark and the first machine-readable mark of the first object. The image capture device captures a second image that includes the second fiducial machine-readable mark and the second machine-readable mark of the second object. The instrument also includes a processor configured to associate information decoded from the first and second machine-readable marks with first and second locations on the moveable support.