An object tracking system that includes a sensor that is configured to capture frames of at least a portion of a global plane for a space. The system is configured to receive a first frame from the sensor. The first frame includes a region-of-interest (ROI) marker within the space. The system is further configured to identify pixel locations within the first frame corresponding with the ROI marker and to define a zone for subsequent frames from the sensor corresponding with the pixel locations. The system is further configured to receive a second frame from the sensor, to detect an object within the zone, and to identify the object. The system is further configured to determine a person is within the second frame and to modify a digital cart that is associated with the person based on the identified object.