plot - Octave: Function to display greyscale matrix shows and all black image instead

Question

I'm doing assignment 3 from Andrew Ng's Machine Learning course on Coursera. The assignment comes with a .mat containing a 5000x400 matrix (denoted X) with values between 0 and 1 corresponding to a gray scale. Each row represents and image of size 20x20.

The assignment comes with a pre-made function that is used to display the images in a sort of grid, called DisplayData(X, width).

The problem is, this function only displays an all black figure. Since this code is course standard and I didn't mess with it at all, my guess is something must be wrong with my Octave. I'm using popOS and installed Octave via terminal with 'apt'. I'm working with Octave console in an open shell. Anyhow, I'll leave the function's code here:

function [h, display_array] = displayData(X, example_width)
%DISPLAYDATA Display 2D data in a nice grid
%   [h, display_array] = DISPLAYDATA(X, example_width) displays 2D data
%   stored in X in a nice grid. It returns the figure handle h and the 
%   displayed array if requested.

% Set example_width automatically if not passed in
if ~exist('example_width', 'var') || isempty(example_width) 
    example_width = round(sqrt(size(X, 2)));
end

% Gray Image
colormap(gray);

% Compute rows, cols
[m n] = size(X);
example_height = (n / example_width);

% Compute number of items to display
display_rows = floor(sqrt(m));
display_cols = ceil(m / display_rows);

% Between images padding
pad = 1;

% Setup blank display
display_array = - ones(pad + display_rows * (example_height + pad), ...
                       pad + display_cols * (example_width + pad));

% Copy each example into a patch on the display array
curr_ex = 1;
for j = 1:display_rows
    for i = 1:display_cols
        if curr_ex > m, 
            break; 
        end
        % Copy the patch
        
        % Get the max value of the patch
        max_val = max(abs(X(curr_ex, :)));
        display_array(pad + (j - 1) * (example_height + pad) + (1:example_height), ...
                      pad + (i - 1) * (example_width + pad) + (1:example_width)) = ...
                        reshape(X(curr_ex, :), example_height, example_width) / max_val;
        curr_ex = curr_ex + 1;
    end
    if curr_ex > m, 
        break; 
    end
end

% Display Image
h = imagesc(display_array, [-1 1]);

% Do not show axis
axis image off

drawnow;

end

question from:https://stackoverflow.com/questions/66054138/octave-function-to-display-greyscale-matrix-shows-and-all-black-image-instead

Answer 1

Using the command graphics_toolkit('gnuplot'); solved the problem!