Matlab在这张图片中找不到圆圈

Question

MATLAB代码，不返回任何内容：

A = imread('2.jpg');
imshow(A)
Rmin = 20;
Rmax = 70;
[centersBright, radiiBright] = imfindcircles(A,[Rmin Rmax],'Sensitivity',0.8);
viscircles(centersBright, radiiBright,'LineStyle','--');