To associate knot heads correctly, GradeScan locates the pith of the tree even if the pith is not actually in the board.
This is a color image of a twenty foot long, Green Douglas Fir 2 x 8.
This is an optimizer image showing of the same board showing defects on the top face and the location of the pith above the board. The optimizer screen displays the pith in three colors, red when above the board, white when it is interior to the board (or actually on the surface) and blue when the pith is below the board.
This is an optimizer image of a different board with pith mostly below but that touches the bottom face for a short distance. The pith location is indicated even when the pith is located opposite a narrow face. A Quality Control Technician can easily tell at a glance if the lumber scanner is working properly.
Another way to look at this is to lay the calculated pith location over actual images of the board. Below are color images of the four faces of a board that has internal pith.
Note that the board images have been adjusted so that the Top face is normal, the Bottom face is seen as if you are looking through the board from the Top and out the Bottom face. Similarly, the Near face is normal and the Far face is seen from looking through the board. The yellow circles identify where the pith approaches the top or bottom face of the board.