[MATLAB图像处理] 如何找出图片RGB三层都小於某数值的座标

img_R = img(:,:,1);
img_G = img(:,:,2);
img_B = img(:,:,3);
k=0;
for i=1:length(img_R(:,1))
for j=1:length(img_R(1,)
if img_R(i,j)<200&&img_G(i,j)<200&&img_B(i,j)<200
k=k+1;
r_R(k)=i;c_R(k)=j;
end
end
end
idx_R = [r_R',c_R'];
哎，没找到注册账号的地方，就这样吧