Try this:
Y = reshape(X,257,257,3)
For example, a matrix with R, G and B columns, rearranged into a 2*4*3 array:
>> X = [11,12,13;21,22,23;31,32,33;41,42,43;51,52,53;61,62,63;71,72,73;81,82,83]
X =
11 12 13
21 22 23
31 32 33
41 42 43
51 52 53
61 62 63
71 72 73
81 82 83
>> reshape(X,2,4,3)
ans(:,:,1) =
11 31 51 71
21 41 61 81
ans(:,:,2) =
12 32 52 72
22 42 62 82
ans(:,:,3) =
13 33 53 73
23 43 63 83
