Octave: Creating images from plots for web page

This post was written by eli on August 12, 2021
Posted Under: Linux,Software

This should have been a trivial task, but it turned out quite difficult. So these are my notes for the next time. Octave 4.2.2 under Linux Mint 19, using qt5ct plugin with GNU plot (or else I get blank plots).

So this is the small function I wrote for creating a plot and a thumbnail:

function []=toimg(fname, alt)

grid on;

saveas(gcf, sprintf('%s.png', fname), 'png');
print(gcf, sprintf('%s_thumb.png', fname), '-dpng', '-color', '-S280,210');

disp(sprintf('<a href="/media/%s.png" target="_blank"><img alt="%s" src="/media/%s_thumb.png" style="width: 280px; height: 210px;"></a>', fname, alt, fname));

The @alt argument becomes the image’s alternative text when shown on the web page.

The call to saveas() creates a 1200x900 image, and the print() call creates a 280x210 one (as specified directly). I take it that print() will create a 1200x900 without any specific argument for the size, but I left both methods, since this is how I ended up after struggling, and it’s better to have both possibilities shown.

To add some extra annoyment, toimg() always plots the current figure, which is typically the last figure plotted. Which is not necessarily the figure that has focus. As a matter of fact, even if the current figure is closed by clicking the upper-right X, it remains the current figure. Calling toimg() will make it reappear and get plotted. Which is really weird behavior.

The apparently only way around this is to use figure() to select the desired current figure before calling ioimg(), e.g.

>> figure(4);

The good news is that the figure numbers match those appearing on the windows’ titles. This also explains why the numbering doesn’t reset when closing all figure windows manually. To really clear all figures, go

>> close all hidden

Other oddities

  • ginput() simply doesn’t work. The workaround is to double-click any point (with left button) and the coordinates of this point are copied into the clipboard. Paste it anywhere. Odd, but not all that bad.
  • Zooming in with right-click and then left-click doesn’t affect axis(). As a result, saving the plot as an image is not affected by this zoom feature. Wonky workaround: Use the double-click trick above to obtain the coordinates of relevant corners, and use axis() to set them properly. Bonus: One gets the chance to adjust the figures for a sleek plot. If anyone knows how to save a plot as it’s shown by zooming, please comment below.


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