This SceneBox callback gets the 2-D mouse coordinates, and then uses Pick to determine the “named” object under the mouse. For example, hitlist is a 5x5 pixel pick box around x,y; up to 1000 items returned, but just the leaf names. The format of the return is determined by the last argument (1 returns a simple array, 0 returns a sorted (by depth) list of arrays).
Track2d=function({x, y, m, k},
hitlist = theSceneBox<<pick( x, y, 5, 5, 1000, 1 );
if ( nrow(hitlist) > 0, // something IS in the pick box
... hitlist[1..n] // are names that you put in the display list
) );
Contrast this with a call back Track3d function, where the pick rectangle is always 1x1 and picking only happens when the mouse moves. This is almost always what you want, but points are difficult to pick because the 1x1 pick area is the same small size as the point. This function lets you pick without a mouse move.
The Track3d function always provides a depth-sorted list of arrays; each array can describe multiple names in a hierarchy (pushname and popname construct a hierarchy of objects). The sorting can be very slow when thousands of objects are selected. The final argument (1, above) controls whether the pick function replaces the sorted list of arrays with a simple array. The simple array contains only the “leaf” names, not higher level names.