Could anyone please explain this matlab code...?

function g = NeumannBoundCond(f)
[nrow,ncol] = size(f);
g = f;
g([1 nrow],[1 ncol]) = g([3 nrow-2],[3 ncol-2]); 
g([1 nrow],2:end-1) = g([3 nrow-2],2:end-1); 
g(2:end-1,[1 ncol]) = g(2:end-1,[3 ncol-2]);

[dpb]

Format code so it's legible...

Answer 1

[Roger Stafford]

The best way to explain your code is to see it in action. Let f be the following:

 f =
     1     2     3     4     5     6     7
     8     9    10    11    12    13    14
    15    16    17    18    19    20    21
    22    23    24    25    26    27    28
    29    30    31    32    33    34    35
    36    37    38    39    40    41    42
    43    44    45    46    47    48    49
    50    51    52    53    54    55    56

Then g will be this:

 g =
    17    16    17    18    19    20    19
    10     9    10    11    12    13    12
    17    16    17    18    19    20    19
    24    23    24    25    26    27    26
    31    30    31    32    33    34    33
    38    37    38    39    40    41    40
    45    44    45    46    47    48    47
    38    37    38    39    40    41    40

As you see, only the outer boundary of the f matrix has been changed. Its values are copies of the rectangle of values two places in from the boundary. The effect is, I suppose, some kind of Neumann-like condition on the outer normal derivative.

Note that this code has one more line than it really needed. It doesn't have to make a special case of the corners. It could have been written equivalently as this:

 g([1 nrow],:) = g([3 nrow-2],:); 
 g(:,[1 ncol]) = g(:,[3 ncol-2]);

[XIA SUN]

It's for some image segmentation related works i suppose.

Answer 2

[Walter Roberson]

In general, the form

array([A B], C)

refers to selecting the elements of the array in which the row is A or B, and the column is any of the values given by C. So (A,C(1)), (A,C(2)), (A,C(3))... (B,C(1)), (B,C(2)), ...

With A, B, C, D all scalar, array([A B], [C D]) would designate four locations, (A,C), (A,D), (B,C), (B,D)