Figure 1
Flow fields in the two possible frames of reference: “source” means all pixels at time t1 are mapped to a new location at time t2, while “target” means all pixels at time t2 are matched with a different previous location at the time t1.
Figure 2
Schematic of the flow field composition operation. To obtain the correct coordinate mapping xt1 → xt3 from Flows 1 and 2, it is necessary to select the flow vector from which starts at the end position of the tracked feature at time t2, or xt2. Therefore, it is not sufficient to simply add the vectors of the two flow fields: this would be equivalent to selecting the flow vector from which starts at xt1 (assuming the “source” frame of reference is used).