How to find critical points - maxima,minima and saddle points in an image?

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Lalith kumar
Lalith kumar 2014-10-9
编辑： Matt J 2014-10-9
Hi Matlab community, I am new to Matlab and sorry if my question is lame. I would like to find the critical points in an image (maxima,minima,saddle points). I am able to find out the maxima and minima using the 'imregionalmax,imregionalmin'function. But i dunno how to find out the saddle points in an image. I really need help, please help me out.

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回答（1 个）

Matt J
Matt J 2014-10-9
编辑：Matt J 2014-10-9
How about,
Cx=diff(Image,2,1); Cx(end+2,:)=nan;
Cy=diff(Image,2,2); Cy(:,end+2)=nan;
[i,j]=find( Cx.*Cy<=0);
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Lalith kumar
Lalith kumar 2014-10-9
编辑：Lalith kumar 2014-10-9
Hi Matt,
Thank you so much for your fast reply. When i tried the code which you suggested, i faced a few hiccups.
[i,j]=find( Cx.*Cy<=0); --> Matrix dimensions must agree.
I think this is due to the cx(end+1,:)=nan and cy(:,end+1)=nan part.
I tried to implement the second order derivative test(hessian matrix). But i am not sure on how to implement that on an image, because for a function, the test is fairly simple and straight forward.
I tried calculating the second order derivative by using this snippet of code :
im=double(dicomread'MR_017.dcm'); %calculating the partial derivatives of the image. [imx,imy]=gradient(im); [imxx,imxy]=gradient(imx); [imyx,imyy]=gradient(imy);
I am not sure on how to proceed after this, because i have never done a test of this sort on an image.
Matt J
Matt J 2014-10-9
编辑：Matt J 2014-10-9
I think this is due to the cx(end+1,:)=nan and cy(:,end+1)=nan part.
Whoops. Should be end+2 everywhere...

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