RGB = imread('tape.png');     %讀取圖片，數據存在RGB變量中；

I = rgb2gray(RGB);                    %將RGB轉換為灰度圖像數據；
threshold = graythresh(I);             %獲取灰度圖像轉換時的閾值；
BW = im2bw(I,threshold);               %將圖像轉換位2進制數據
dim = size(BW);                        

col = round(dim(2)/2)-90;
row = min(find(BW(:,col)));

connectivity = 8;
num_points   = 180;
contour = bwtraceboundary(BW, [row, col], 'N', connectivity, num_points);

imshow(RGB);
hold on;
plot(contour(:,2),contour(:,1),'g','LineWidth',2);

x = contour(:,2);
y = contour(:,1);


abc=[x y ones(length(x),1)]\[-(x.^2+y.^2)];
a = abc(1); b = abc(2); c = abc(3);
xc = -a/2;
yc = -b/2;
radius  =  sqrt((xc^2+yc^2)-c)

plot(xc,yc,'yx','LineWidth',2);


theta = 0:0.01:2*pi;
Xfit = radius*cos(theta) + xc;
Yfit = radius*sin(theta) + yc;
plot(Xfit, Yfit);
message = sprintf('通過計算得到的半徑是 %2.3f pixels', radius);
text(15,15,message,'Color','y','FontWeight','bold');