An initial member (having non-zero coordinates) of such a set is chosen from 

 the operand data vector, the coordinates of this point are set equal to zero in 

 the operand data and a mask (width = 2'nl, and height = 2"n2) is centered upon 

 tliis point. The set is augmented if an operand point (having non-zero 

 coordinates) falls within the mask. The mask is then centered upon the new 

 point and the search continues. The search terminates when the set is deemed 

 "acceptable" if it possesses at least n3 members. Ideally, this process yields one 

 centroid corresponding to each outline— unless the outlines of two organisms 

 are merged. The user can PLOT the centroids over the original data to confirm 

 this correspondence and recompute the centroids using new parameters if the 

 correspondence is not adequate. The structure of "centroid data" is quite 

 similar to that of video data except that each coordinate of a centroid is 

 represented v^th 1 5-bit (rather than 8-bit) precision in attempt to exploit the 

 greater accuracy resulting from the averaging procedure. Centroid data is 

 displayed and edited in the same fashion as video data. 



The PICK operator provides an alternative method to abstract points of 

 interest from video data. The use of this operator for direct analysis of a 

 videotape, thereby, bypassing the acquisition and processing of video files, is 

 discussed above (see "COUPLING TO RESEARCH ENVIRONMENTS"). The 

 user may also PICK points associated with each outline by means of the 

 terminal cursors. The PICK operator is indispensable for investigations of 

 rotational and flectional movement. For example, the user might elect to study 

 the orientations of the longitudinal body axes of a group of organisms (i.e., 

 simple rotation in a single plane). The longitudinal axis may be defined as a 

 vector extending from the tail to the head of an organism. Thus, the user 

 selects an ordered pair of points corresponding to each outline by entering 



*PICK/VI BUGS.VI BUGS.TH 



and using the cursors to specify first the "tail" and then the "head" associated 

 with each outline. The resultant file "BUGS.TH" possesses the structure of 

 centroid data with each "tail point" immediately followed by the correlated 

 "head point". These points are then segregated using the PLUCK operator. For 

 example, the two commands 



*PLUC BUGS.TH BUGS.T 1 , 2 

 *PLUC BUGS.TH BUTS.H 2, 2 



respectively produce the files "BUGS.T'-containing all first elements of the 

 ordered pairs (viz., the "tail points)-and "BUGS.H"-containing all second 

 elements of the ordered pairs (viz., the "head points). This type of analysis 

 may be extended to encompass larger collections of points (or n-tuples), 

 thereby providing for the study of flectional movement (e.g., the angles of 

 propulsive appendages vnth respect to the longitudinal axis). 



261 



