convert image into 3XN matrix

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bes
bes 2012-9-14
I have a dem data(an image of 6000X5200). I need to convert the data and create a 3XN matrix with row number, coloum number and Z value (pixel value of the image represent the Z value).
orgim = imread('dem'); % read DEM data
nentry = 0;
for ii = 1:size(orgim,2)
for jj = 1:size(orgim,1)
nentry = nentry +1;
mydata(nentry, :) = [ii, jj, orgim(jj,ii)];
clear X Y Z;
end
end
This works but takes time. Is there any optimum way to convert the multi-dimentional 6000X 5200 matrix as 3XN matrix, without loop
  2 个评论
Andrei Bobrov
Andrei Bobrov 2012-9-14
[x,y] = ndgrid(1:size(orgim,1),1:size(orgim,2));
mydata = [y(:),x(:),orgim(:)];
Jan
Jan 2012-9-14
"clear X Y Z"?! This line can obviously omitted.

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采纳的回答

Andrei Bobrov
Andrei Bobrov 2012-9-14
编辑：Andrei Bobrov 2012-9-14
[x,y] = ndgrid(1:size(orgim,1),1:size(orgim,2));
mydata = [y(:),x(:),orgim(:)];
or
mydata = [fliplr(fullfact(size(orgim))), orgim(:)];

更多回答（2 个）

Jan
Jan 2012-9-14
Although Andrei's solution is smarter, I suggest the pre-allocation as general programming schema:
orgim = imread('dem'); % read DEM data
mydata = zeros(numel(orgim), 3)); % Pre-allocate!!!
nentry = 0;
for ii = 1:size(orgim,2)
for jj = 1:size(orgim,1)
nentry = nentry +1;
mydata(nentry, :) = [ii, jj, orgim(jj,ii)];
end
end
This can be noticably faster already.
bes
bes 2012-10-3
Thanks all. Your comments helped me a lot.

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