How to vectorize the following for...loop?

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Baraka Maiseli 2016-4-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/278057-how-to-vectorize-the-following-for-loop

评论： Kuifeng 2016-4-10

Dear colleagues,

I have written the following MATLAB code, with two for...loops, which compute the distance map of an image. However, the code is relatively slower, and I want to speed it up using vectorization techniques. I am still a newbie in code optimization; still learning. Any idea on how to vectorize the code, please share:

function [ res ] = computeDT( v, d1, d2 )
%%COMPUTEDT -- Compute the distance field of an input image, img
%  v   -- Input image
%  res -- Output image
%%%%%%%Initialize the input image
v = v([1,1:end,end],[1,1:end,end]); 
v(v~=0) = inf;
% Weights [0 +d1 +d2]
%%Forward pass
M1 = size(v,2); % Number of columns
M2 = size(v,1); % Number of lines (rows)
for k1=2:M1-1
    for k2=2:M2-1
        v(k1,k2) = min([v(k1 - 1, k2 - 1) + d2, v(k1 - 1, k2) + d1,... 
                        v(k1 - 1, k2 + 1) + d2, v(k1, k2 - 1) + d1, v(k1, k2)]);
    end
end
%%Backward pass
for k1=M1-1:-1:2
    for k2=M2-1:-1:2
        v(k1,k2) = min([v(k1, k2), v(k1, k2 + 1) + d1, v(k1 + 1, k2 - 1) + d2,...
                       v(k1 + 1, k2) + d1, v(k1 + 1, k2 + 1) + d2]);
    end
end
res = v(2:end-1,2:end-1);
% save res.mat
% subimage(mat2gray(res))
end

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Kuifeng 2016-4-10

can you give an example value/matrix of v, d1, d2?

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Answer 1

Kuifeng 2016-4-9

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/278057-how-to-vectorize-the-following-for-loop#answer_217131

在 MATLAB Online 中打开

% maybe make some changes to the following code could help,

 [rows cols] = size(ans);
v1 = v(1:rows-1, 1:cols-1);
v2 = v(2:rows, 1:cols-1);
v3 = v(1:rows-1, 2:cols);
v4 = v(2:rows, 2:cols);
result = min[v1+d1, v2+d2, v3, v4, v5]; %for example only, revise accordingly

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Kuifeng 2016-4-9

在 MATLAB Online 中打开

 %for example in the 'forward pass', 
  [rows cols] = size(v);
v1 = v(1:rows-2, 1:cols-2);
v2 = v(1:rows-2, 2:cols-1);
v3 = v(1:rows-2, 3:cols);
v4 = v(2:rows-1, 1:cols-2);
v5 = v(2:rows-1, 2:cols-1);
result = min(min(min(min(v1 + d2, ...
    v2 + d1), ...
    v3 + d2), ...
    v4 + d1), ...
    v5);

Baraka Maiseli 2016-4-9

编辑：Baraka Maiseli 2016-4-9

在 MATLAB Online 中打开

Testing the code snippet (for the forward pass), as it is, I get:

     Inf   Inf   Inf   Inf   Inf   Inf   Inf
     Inf   Inf   Inf   Inf   Inf   Inf   Inf
     Inf   Inf   Inf   Inf   Inf   Inf   Inf
     Inf   Inf   Inf     0     1   Inf   Inf
     Inf   Inf   Inf     1   Inf   Inf   Inf
     Inf   Inf   Inf   Inf   Inf   Inf   Inf
     Inf   Inf   Inf   Inf   Inf   Inf   Inf

But the old code, which I use as reference, gives:

     Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf
     Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf
     Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf
     Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf
     Inf   Inf   Inf   Inf     0     1     2     3   Inf
     Inf   Inf   Inf   Inf     1     2     3     4   Inf
     Inf   Inf   Inf   Inf     2     3     4     5   Inf
     Inf   Inf   Inf   Inf     3     4     5     6   Inf
     Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf   Inf