How to create a mp4 video out of Octave/via plot

This is a follow up to question 54847941. I make videos from sequences of images created by Octave using the print command, for example:

clear;
for kt=1:100
  t(kt)=0.1*kt;
  clf,plot(t,sin(t),'linewidth',1);axis([0,10,-1,1]);
  fname=strcat('img',num2str(kt,"%0.2i"),'.png');
  eval(['print -dpng ' fname ' -r100']);
endfor

##use ffmpeg to make mp4 with:
##ffmpeg -framerate 20 -i img%02d.png -vf scale=1080:-1 Example1.mp4

This works when the imgaes are small, but if they are large, the print operations takes a very long time. What is an alternative to my method, using functions like getframe and imwrite?

Your initial code took 15.5s on my machine. The first optimization step is to keep the plot and update xdata and y data:

t = linspace (0, 10, 100);
for k = 1:numel(t) 

  y = sin (t(1:k));

  if (k == 1)
    p = plot (t(1:k), y, 'linewidth', 1);
    axis ([0, 10, -1, 1]);
  else
    set (p, "xdata", t(1:k));
    set (p, "ydata", y);
  endif
  fname = sprintf ("img%03i.png", k);
  print ("-dpng", "-r100", fname);
endfor

This takes 13.1s on my machine. Next we can use getframe:

out_dir = "temp_img";
mkdir (out_dir);
t = linspace (0, 10, 100);
for k = 1:numel(t) 
  y = sin (t(1:k));
  if (k == 1)
    p = plot (t(1:k), y, 'linewidth', 1);
    axis ([0, 10, -1, 1]);
  else
    set (p, "xdata", t(1:k));
    set (p, "ydata", y);
  endif
  fname = fullfile (out_dir, sprintf ("img%03i.png", k));
  imwrite (getframe (gcf).cdata, fname);
endfor
cmd = sprintf ("ffmpeg -framerate 20 -i ./%s/img%%03i.png -vf scale=1080:-1 Example1.mp4", out_dir)
system (cmd)

This takes 3.7s even with running ffmpeg .

Keep in mind that if you're creating images (I guess like your vortex videos) you haven't go though plot