Explain code just starting with matlab ?

Can any one explain this part of code in details please !

if true
 for i=1:length(b)
      inds=round(1:factor:size(b{i},1));
      b{i}=b{i}(inds,:);
  end
end

and below the hole part of code :

    %construct voronoi
if ~boundary
    b=bwboundaries(BW);
else
    b={BW};
end
if factor>1
    for i=1:length(b)
        inds=round(1:factor:size(b{i},1));
        b{i}=b{i}(inds,:);
    end
end
i=1;
inds=[];
while i<=length(b)
    if size(b{i},1)<4
        b(i)=[];
        continue;
    end
    inds(i)=length(b{i}); %#ok<AGROW>
    i=i+1;
end
inds=[0 cumsum(inds)];
p=cell2mat(b);
[v e]=costumVoronoi(p);

[Julia]

I don't know anything about voronoi-diagrammes, but I suppose your programme is some tool for generating them? It would certainly be helpful if you posted the exact nature of b (I suppose it is a cellarray?), factor, BW and the functions bwboundaries and costumVoronoi.

My guess for the first part: Factor is some number, not necessarily an integer. You generate an indexvector inds with the entries 1 round(1+factor) etc. Then you replace the matrix in b{i} with a matrix which has only the rows 1 round(1+factor) etc (the numbers from inds).

[mika]

Thank you julia

Below the hole code :

   function [skel v e]=squelette(BW,varargin)

 iptcheckinput(BW,{'numeric' 'logical'},{'real' 'nonsparse' '2d'}, ...

              mfilename, 'BW', 1);

 trim=0;

 factor=-1;

 boundary=false;

 for i=1:length(varargin)

    switch lower(varargin{i})

        case 'trim'

            trim=varargin{i+1};

        case 'fast'

            factor=varargin{i+1};

        case 'boundary'

            boundary=true;

            if nargout~=2

                error(['If boundary is specified, the function cannot 
generate'...

                        char(10) 'the BW inage of the skeleton.' 

 char(10)...

                        'Use [v e]=voronoiSkel(...) instead'])

            end

            if size(BW,2)~=2 

                if size(BW,1)~=2

                    error('If you use the ''boundary'' option, the imput 
must be a 2 x n or n x 2 matrix')

                else

                    BW=BW';

                end

            end

    end

 end

 if trim<1;  trim=pi;  end

 if factor<0; factor=1; end;

 if ~islogical(BW) && ~boundary

    BW = (BW ~= 0);

end

 %construct voronoi

 if ~boundary

    b=bwboundaries(BW);

 else

    b={BW};

end

 if factor>1

    for i=1:length(b)

        inds=round(1:factor:size(b{i},1));

        b{i}=b{i}(inds,:);

    end

end

 i=1;

 inds=[];

 while i<=length(b)

    if size(b{i},1)<4

        b(i)=[];

        continue;

    end

    inds(i)=length(b{i}); %#ok<AGROW>

    i=i+1;

end

 inds=[0 cumsum(inds)];

 p=cell2mat(b);

 [v e]=costumVoronoi(p);

 %clear bad vertices (bv) which are outside of the object.

 if ~boundary

    rv=round(v);

    M=max(p);

    m=min(p);

    bv=v(:,1)<m(1)|v(:,1)>M(1)|v(:,2)>M(2)|v(:,2)<m(2);

    bv2=find(~bv);

    tmp=sub2ind(size(BW),rv(bv2,1),rv(bv2,2));

    bv2(BW(tmp))=[];

    bv=[find(bv); bv2];

 else

    bv=find(~inpolygon(v(:,1),v(:,2),p(:,1),p(:,2)));

end

 be=ismember(e(:,3),bv)|ismember(e(:,4),bv);

 e=e(~be,:);

 clear bv2 m M rv tmp;

 % build distance table

 D=cell(size(b));

 for i=1:length(D);

    tmp=diff(b{i});

    tmp=sqrt(sum(tmp'.^2)');

    D{i}=[0 ;cumsum(tmp)];

end

 % trim

 be=false(size(e,1),1);

 for i=1:size(e,1)

    i1=find(inds>=e(i,1),1,'first')-1;

    i2=find(inds>=e(i,2),1,'first')-1;

    if i1~=i2; continue; end;

    offset=inds(i1);

    contourDistance=abs(D{i1}(e(i,1)-offset)-D{i1}(e(i,2)-offset));

    contourDistance=min(contourDistance,D{i1}(end)-contourDistance);

    realDistance=norm(p(e(i,1),:)-p(e(i,2),:));

    if (contourDistance<realDistance*trim)

        be(i)=1;

    end

 end

 % keep only good edges

 e=e(~be,3:4);

 outputVertices=(nargout>1);

 outputSkel=(nargout~=2);

 if (outputSkel) 

    skel=false(size(BW));

    for i=1:size(e);

        v1=v(e(i,1),:);

        v2=v(e(i,2),:);

        t=linspace(0,1,max(ceil(1.3*norm(v2-v1)),4));

        x=v1(:,1).*t+(1-t).*v2(:,1);

        y=v1(:,2).*t+(1-t).*v2(:,2);

        inds=unique(round([x' y']),'rows');

        skel(sub2ind(size(skel),inds(:,1),inds(:,2)))=1;

    end

end

 if outputVertices

    tmp=1:length(v);

    tmp=~ismember(tmp,e(:,1:2));

    inds=cumsum(tmp);

    e(:,1)=e(:,1)-inds(e(:,1))';

    e(:,2)=e(:,2)-inds(e(:,2))';

    v=v(~tmp,:);

end

 if nargout==2

    skel=v;

    v=e;

end

end

 function [v e]=costumVoronoi(V)

 % Calculates the voronoi diagram of the vertices in V.

 % v should be a n x 2 real matrix.

 % qhull (www.qhull.org) MUST be executable from the current directory.

 %

 % Output: v is the matrix of the Voronoi vertices.

 %         e is the matrix of the Voronoi edges. 

 %            each row of e represents an edge in the following way: 

 %            e(k,[1 2]) are the row indices (in V) of the points which

 %            generated the k-th edge. 

 %            e(k,[3 4]) are the row indices (in v) of the endpoints of the

 %            k-th edge.

 %         

% write to temp file

 namein=tempname;

 fid=fopen(namein,'w');

 fprintf(fid,'%d\n%d\n',2,size(V,1));

 fprintf(fid,'%d %d\n',V');

 fclose(fid);

 [a s]=dos(['qhull v p Fv TI ' namein]);

 if (a~=0)

    delete(namein);

    error(['qhull returned an error:' char(10) s]);

end

 [junk s]=strtok(s);

 [Nv s]=strtok(s);

 Nv=str2double(Nv);

 [v pos]=textscan(s,'%f %f',Nv);

 v=cell2mat(v);

 s=s(pos:end);

 [Ne s]=strtok(s);

 e=double(cell2mat(textscan(s,'%d %d %d %d %d',Ne)));

 e=e(:,2:5);

 e(:,1:2)=e(:,1:2)+1;

 e(e(:,3)==0,:)=[];

 e(e(:,4)==0,:)=[];

 %clean up

 delete(namein);

end

source Skeletonization using voronoi

 by Yohai

[mika]

any ideas please !

thx