How to vectorize the following for...loop?

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Baraka Maiseli
Baraka Maiseli 2016-4-9
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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回答（1 个）

Kuifeng
Kuifeng 2016-4-9
% 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
Kuifeng 2016-4-9
%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
Baraka Maiseli 2016-4-9
编辑：Baraka Maiseli 2016-4-9
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
The two implementations produce different results.

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