Images were extracted from the videotape record of polyp behavior using a PCVISION frame grabber and the OPTIMAS image analysis software. A series of programs, written in the OPTIMAS macro language, were used to analyze each binary image to determine polyp length and diameter at multiple points along the longitudinal axis of the polyp.
The algorithm first extracts the polyp outline and draws an axis from the polyp base to its maximal extent normal to the base. This vertical axis is then marked with 11 equidistant points and lines are drawn through these points normal to the vertical axis (Bio. Bull. 196: 1-17, Fig. 1A). The points at which these lines intersect the polyp outline on either side are then marked (Fig. 1B). Next, the midpoint of each of these polyp cross-sections is determined and a line is drawn from midpoint-to-midpoint (Fig. 1C). Finally, a new line is drawn normal to each midpoint-to-midpoint segment at the distal terminus (i.e., towards the polyp tip) of each midpoint segment (Fig. 1D). The length of these normal segments constitute our measures of polyp diameter.
Polyps are often bent. A bend is detected whenever adjacent midpoint lines subtend an angle >53 degrees (Fig. 1E). When this occurs, the algorithm continues as described above, but with horizontal rather than vertical sections until the endpoint of the polyp is reached.
Locating the polyp tip requires special treatment. To do so, the algorithm treats the final section differently than all others. The protocol is that the final segment is traversed pixel by pixel, with the algorithm described above repeated for each one ppixel increment (i.e., with each one pixel increment, a segment is drawn normal to the base and its midpoint marked, line segments are drawn connecting such midpoints for adjacent one pixel sections, and lines normal to them are established). Finally, with the endpoint detected, the length of the polyp is calculated from the best fit 4th-order polynomial to the midpoint-to-midpoint line segments running from the base to the tip (Fig. 1F). Assuming radial symmetry, polyp volume was estimated from these measurements of polyp length and polyp diameters, using the extended Simpson's rule (Press et al., 1992).