Reshaping long vertical RGB image array into layered horizontal sections in Octave

Question

I'm trying to reshape a long vertical image array (multidimensional RGB array) into layered horizontal sections in Octave.

I have a very long vertical image array (32734x1x3). How can I reshape the image array so I turn the long vertical image into a layered horizontal image array of 52x640x3 while the rest of the array is filled in with 0's to make it "square-ish"

I was looking at reshape but couldn't figure out how to reshape an array in multiple dimensions while also creating the layered horizontal sections.

Simple Example: What space is left over in the array is filled in with zero's.

My logic/test was:

a=[1:32734]';

num_cols_wanted=640
num_of_rows_calc=ceil(size(a,1)/num_cols_wanted) %use ceil to get whole number rounded up
num_cells_to_add=mod(size(a,1),num_cols_wanted) %extra cells needed to even array out

b = zeros(num_of_rows_calc, num_cols_wanted); %preallocate 

b=reshape(a,[num_of_rows_calc,num_cols_wanted]); %place reshaped a array into preallocated b array

At the end of the test script I'm having issues reshaping the original array a into the larger preallocated array b. I get an error can't reshape 32734x1

PS: I'm using Octave 5.2

Answer 1

When using reshape, the number of input elements must be equal to the number of the output (reshaped) elements.

In your case: length(a) must be equal to num_of_rows_calc*num_cols_wanted.

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Executing b = zeros(...) and then b = reshape(...) just overwrites the value of b (filling b with zeros doesn't help).

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• You may fill b with zeros (create a "long" vector), and copy a elements to the beginning of a:

   b = zeros(num_of_rows_calc*num_cols_wanted, 1);
   b(1:length(a)) = a;
  

• After having b with correct number of element, we can reshape b:

    b = reshape(b, [num_cols_wanted, num_of_rows_calc])';
  

  Note: reshaping a vector in OCTAVE orders the elements by columns (top to bottom) first.
  For ordering by rows, we may reshape to cols x rows and transpose the result.

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Complete code sample:

%a=[1:32734]';
a = (1:10)';

num_cols_wanted=3; %640;
num_of_rows_calc=ceil(size(a,1)/num_cols_wanted); %use ceil to get whole number rounded up
num_cells_to_add=mod(size(a,1),num_cols_wanted); %extra cells needed to even array out

b = zeros(num_of_rows_calc*num_cols_wanted, 1); %Create a vector of zeros with desired number of elements.

b(1:length(a)) = a; %Copy a into the b - keeping the zeros at the end of b (we could also add zero padding at the end of a).

%Reshape to num_cols_wanted x num_of_rows_calc and transpose, because OCTAVE ordering is "column major".
b = reshape(b, [num_cols_wanted, num_of_rows_calc])'; %reshape b array into preallocated b array

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Result:

b =

    1    2    3
    4    5    6
    7    8    9
   10    0    0

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Example for 3D output:

a = cat(3, (1:10)', (21:30)', (31:40)');

a = squeeze(a); % Remove redunded dimentsion

num_cols_wanted=4;%640;
num_of_rows_calc=ceil(size(a,1)/num_cols_wanted); %use ceil to get whole number rounded up
num_cells_to_add=mod(size(a,1),num_cols_wanted); %extra cells needed to even array out

b = zeros(num_of_rows_calc*num_cols_wanted, 3); %Create 3 columns matrix of zeros with desired number of elements.

b(1:length(a), :) = a; %Copy a into the b - keeping the zeros at the end of b (we could also add zero padding at the end of a).

%Reshape to 3 x num_cols_wanted x num_of_rows_calc and permute, because OCTAVE ordering is "column major".
b = reshape(b, [num_cols_wanted, num_of_rows_calc, 3]); %reshape b array into preallocated b array
b = permute(b, [2, 1, 3]);