Matlab

Question

在 Matlab 中，我有一个矩阵，其中一些元素设置为零。例如：

对于每个为零的矩阵元素，我想找到最近的非零元素的索引。当可能有多个索引时，应返回所有索引。这里有什么聪明的解决方案，我怎样才能避免很多 for 循环的东西？

Answer 1

有一个高效的bwdist function in IPT that computes the distance transform:

M = [
    0 1 0 0 0
    2 5 0 3 0
    0 0 0 0 0
    0 5 0 2 1
];
[D,IDX] = bwdist(M~=0)

结果：

D =
    1.0000         0    1.0000    1.0000    1.4142
         0         0    1.0000         0    1.0000
    1.0000    1.0000    1.4142    1.0000    1.0000
    1.0000         0    1.0000         0         0

IDX =
           2           5           5          14          14
           2           6           6          14          14
           2           6           6          14          20
           8           8           8          16          20

returned IDX 包含 线性索引 M 中最接近的非零值。每个元素只有 return 个索引。

Answer 2

这是一种使用 bsxfun and mat2cell 的矢量化方法，它为每个单元格中的每个零元素存储非零最近元素的索引（按欧氏距离） -

%// Assuming A as the input matrix. Store rows, columns of zero and non-zeros
[rz,cz] = find(A==0);
[rnz,cnz] = find(A~=0);

%// Store zero pt indices
zero_pts = [rz cz];

%// Get squared euclidean distances
dists = bsxfun(@minus,rnz,rz.').^2 + bsxfun(@minus,cnz,cz.').^2;

%// Get all nearest XY indices of nonzeros for each zero pt
[R_idx,C_idx] = find(bsxfun(@eq,min(dists,[],1),dists));
idx = [rnz(R_idx) cnz(R_idx)];

%// Cut at each shifting positions and thus create a cell array, with a
%// cell of indices of non-zero nearest elements for each zero element 
nearest_nnonzero_pts = mat2cell(idx,histc(C_idx,1:max(C_idx)))

样本输入、输出-

输入：

>> A
A =
     0     1     0     0     0
     2     5     0     3     0
     0     0     0     0     0
     0     5     0     2     1

输出（零分）：

>> disp(zero_pts)
     1     1
     3     1
     4     1
     3     2
     1     3
     2     3
     3     3
     4     3
     1     4
     3     4
     1     5
     2     5
     3     5

输出（对应最近的非零点）：

>> celldisp(nearest_nnonzero_pts)
nearest_nnonzero_pts{1} =
     2     1
     1     2
nearest_nnonzero_pts{2} =
     2     1
nearest_nnonzero_pts{3} =
     4     2
nearest_nnonzero_pts{4} =
     2     2
     4     2
nearest_nnonzero_pts{5} =
     1     2
nearest_nnonzero_pts{6} =
     2     2
     2     4
nearest_nnonzero_pts{7} =
     2     2
     4     2
     2     4
     4     4
nearest_nnonzero_pts{8} =
     4     2
     4     4
nearest_nnonzero_pts{9} =
     2     4
nearest_nnonzero_pts{10} =
     2     4
     4     4
nearest_nnonzero_pts{11} =
     2     4
nearest_nnonzero_pts{12} =
     2     4
nearest_nnonzero_pts{13} =
     4     5

Matlab - 为每个零矩阵元素查找最近的非零元素的索引

Matlab - Find indices of nearest non-zero element for every zero matrix element

matrix

nearest-neighbor