zero crossing

I convolved my image with laplavian and now i want to use zero crossing . What is a concept of zero crossing in image processing .How can i apply it in MATLAB on laplacian convoled images ? Please help me, thanks.

Answer 1

[Matt Fig]

x = 0:.01:2*pi;
f = @(x) sin(x).*cos(x);  % A randomly chosen function.
% Next find the zero crossings of f(x)...
rt = [fzero(f,0),fzero(f,1.5),fzero(f,3),fzero(f,5),fzero(f,6.5)];
plot(x,f(x),'b',rt,zeros(size(rt)),'or','markers',8,'markerfacec','r')
title('The zero crossings of sin(x)*cos(x)','fonts',14,'fontw','bold')

[Tina]

but i have to apply zero crossing on image ..

[Matt Fig]

Ah, it would have been good to mention that in your original post.

Answer 2

[David Young]

In the context of images, the classic reference is the book "Vision" by David Marr, in which the idea of zero-crossings is heavily exploited in his theory (now, I think, largely superseded) of early visual processing in animals. Any reasonable academic library should have a copy of this seminal text.

Images do not have zero-crossings; however if you convolve an image with a differencing operator such as a Laplacian of Gaussian (or Difference of Gaussians) the result will usually have zero-crossings. These are simply the zero-level contours between positive and negative regions of the array; their positions may be estimated in a discrete array by interpolation, or crudely approximated by finding pixels which have an opposite-sign neighbour.

If you have the Image Processing Toolbox, then the 'log' option of the edge function incorporates an implementation of zero-crossing detection.

Answer 3

[Walter Roberson]

As you mention images: possibly what is intended is that you find the phase of the image at each location by using fft2(). I am, though, a bit fuzzy as to how you would use that to determine zero crossings, as in most cases images would have many many zero crossings.

Answer 4

[farshad bayat]

see this link: http://homepages.inf.ed.ac.uk/rbf/HIPR2/zerocdemo.htm