【求助】关于Demos里的一个程序！

[firefly]

Image Processing Toolbox Demos里面的Measuring Image Features里的例子Measuring the Radius of a Role of Tape的程序有一处看不太明白，程序如下：
RGB = imread('tape.png');
imshow(RGB);
text(15,15,'Estimate radius of the roll of tape',...
'FontWeight','bold','Color','y');
I = rgb2gray(RGB);
threshold = graythresh(I);
BW = im2bw(I,threshold);
imshow(BW)；
dim = size(BW);
col = round(dim(2)/2)-90;－－－？此处为什么要减90？什么意思啊？
row = min(find(BW(:,col)));
connectivity = 8;
num_points = 180;－－－－？此两处的赋值为什么是8和180？可以是别的吗？
contour = bwtraceboundary(BW, [row, col], 'N', connectivity, num_points);

imshow(RGB);
hold on;－－－－？hold on 什么意思？

plot(contour(:,2),contour(:,1),'g','LineWidth',2);
x = contour(:,2);
y = contour(:,1);

% solve for parameters a, b, and c in the least-squares sense by
% using the backslash operator
abc=[x y ones(length(x),1)]\[-(x.^2+y.^2)];
a = abc(1); b = abc(2); c = abc(3);

% calculate the location of the center and the radius
xc = -a/2;
yc = -b/2;
radius = sqrt((xc^2+yc^2)-c)

% display the calculated center
plot(xc,yc,'yx','LineWidth',2);

% plot the entire circle
theta = 0:0.01:2*pi;

% use parametric representation of the circle to obtain coordinates
% of points on the circle
Xfit = radius*cos(theta) + xc;
Yfit = radius*sin(theta) + yc;

plot(Xfit, Yfit);

message = sprintf('The estimated radius is %2.3f pixels', radius);
text(15,15,message,'Color','y','FontWeight','bold');


谢谢先！