Find blocks iof non-zero values in Matrix

Hi,

I have a matrix m=(120x240) of values and randomly distributed zeros.

I would like to find all contiguous blocks of size 20x6 without any zeros. And if possible the min. value inside the block should be above predifined threshold (thr=13).

Is this possible without a loop, because I need to "search" many matrices.

Thank you

Answer 1

[Matt J]

Yes, the locations of the blocks are given by,

f=@(z,k,dim) movmin(z,k,dim,Endpoints='fill');

locations = f( f( m ,6,2) ,20,1) > threshold ;

Answer 2

[John D'Errico]

Sure. It should even be pretty easy.

A = randi(200,[120,240]); % make up a matrix

First, I'll convert it to binary, where a 1 corresponds to a non-zero value greater than 13.

thresh = 13;
Abin = A > thresh;

That works, since zero is less than 13 anyway. Now just use conv2, with a 20x6 array for the convolution kernel, and the valid flag. find will do the job then.

Aconv = conv2(Abin,ones(20,6),'valid');
[r,c] = find(Aconv == 20*6)

r = 7×1

2 3 2 3 54 55 56

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c = 7×1

96 96 97 97 214 214 214

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r and c will be the locations of the upper left corner of each located block in the original array. In this case, there were two blocks found, ech slightly larger than the 20x6 requirement.

[CSCh]

Thank you so much, however my question was not very precise, my goal is to have at the end a cell containing the blocks, assuming there are 2 blocks in the matrix I would like to have blocks={20x6 double}, {20x6 double}.

[Matt J]

my goal is to have at the end a cell containing the blocks, assuming there are 2 blocks in the matrix

That will be very inefficient. Note that with a 20x6 sliding window, the existence of one block with a minimum at the threshold means there are potentially 120 nearby blocks containing the same minimum. So you will end up duplicating a lot of data. Also, there is no way in Matlab to iterate through a cell array with anything faster than a for-loop, so if speed is the goal, cell arrays are a bad choice for the container.

[CSCh]

What would be a better choice for a container? A Table? My version contains a while loop, that was very slow. Both method would be very nice to have, blocks how are totally seperated and a version where a sliding window would be allowed.