Creating animation with multiple plots in Octave - animation

Question

I'm using Octave to write a script that plots a function at different time periods. I was hoping to create an animation of the plots in order to see the changes through time.
Is there a way to save each plot for each time point, so that all plots can be combined to create this animation?

Answer 1

It's a bit of kludge, but you can do the following (works here with octave 4.0.0-rc2):
x = (-5:.1:5);
for p = 1:5
plot (x, x.^p)
print animation.pdf -append
endfor
im = imread ("animation.pdf", "Index", "all");
imwrite (im, "animation.gif", "DelayTime", .5)
Basically, print all your plots into a pdf, one per page. Then read the pdf's as images and print them back as gifs. This will not work on Matlab (its imread implementation can't handle pdf).

Answer 2

This creates an animated gif
data=rand(100,100,20); %100 by 100 and 20 frames
%data go from 0 to 1, so lets convert to 8 bit unsigned integers for saving
data=data*2^8;
data=uint8(data);
%Write the first frame to a file named animGif.gif
imwrite(data(:,:,1),'/tmp/animGif.gif','gif','writemode','overwrite',...
'LoopCount',inf,'DelayTime',0);
%Loop through and write the rest of the frames
for ii=2:size(data,3)
imwrite(data(:,:,ii),'/tmp/animGif.gif','gif','writemode','append','DelayTime',0)
end

Answer 3

Had to come chime in here because this was the top Google result for me when I was looking for help with this. I had issues with both answers, and some other issues, too. Notably:
For Rick T's answer, the code snippet doesn't write a plot figure, it just writes matrix data. Getting the plot window was a pain.
For carandraug's answer, writing to a PDF took a very long time and made a gigantic PDF.
On my own machine, I'm pretty sure I used apt-install to get Octave, but the getframe function I found referenced in other answers wasn't found. Turns out I had installed version 4.4, which was from 2018 (>3 years old).
I removed the old version of Octave sudo apt remove octave, then installed the new version with snap. If you try octave from a terminal without it installed it should prompt you to the snap install - be sure to include the # 6.4.0 or whatever is included in the command.
Once I had the current version installed, I got access to the getframe command, which is what lets you convert directly from a figure handle to image data - this bypasses the hackey (but previously necessary step) in #carandraug's answer where you had to write to PDF or some other image as a placeholder.
I used #RickT's answer to make my own MakeGif function, which I will share with you all here. Note that MakeGif stores the filename in a persistent variable, meaning it is retained across calls. If you change the filename it will make (or overwrite!!) the new file. If you need to overwrite the current file (i.e., running the same script multiple times and want new results) then you can use clear MakeGif between calls and that will reset the persistentFilename.
Here is the code for the MakeGif function; code to test it with is provided after this:
function MakeGif(figHandle, filename)
persistent persistentFilename = [];
if isempty(filename)
error('Can''t have an empty filename!');
endif
if ~ishandle(figHandle)
error('Call MakeGif(figHandle, filename); no valid figHandle was passed!');
endif
writeMode = 'Append';
if isempty(persistentFilename)|(filename!=persistentFilename)
persistentFilename = filename;
writeMode = 'Overwrite';
endif
imstruct = getframe(figHandle);
imwrite(imstruct.cdata, filename, 'gif', 'WriteMode',writeMode,'DelayTime',0);
endfunction
And here is the code to test the function. There's a commented-out call to clear MakeGif between the blue and green colors. If you leave it commented out it will append the green sine wave to the blue sine wave, resulting in alternating colors after every cycle - again the filename is persistent in the function. If you uncomment that call then the MakeGif function will treat the green's call as "new" and trigger the overwrite of the blue sine wave and all you'll see is green.
clear all;
time = 0:0.1:2*pi;
nSamples = numel(time);
figHandle = figure(1);
for i=1:nSamples
plot(time,sin(time + time(i)),'Color','blue');
drawnow;
MakeGif(figHandle, 'test.gif');
endfor
% Uncomment the 'clear' command below to clear the MakeGif persistent
% memory, which will trigger the green sine wave to overwrite the blue.
% Default behavior is to APPEND a green sine wave because the filename
% is the same.
%clear MakeGif;
for i=1:nSamples
plot(time,sin(time + time(i)),'Color','green');
drawnow;
MakeGif(figHandle, 'test.gif');
endfor
I spent several hours on this after being super dissatisfied with laggy screen captures so I really hope this helps someone in the future! Good luck and best wishes from the Age of Covid lol.

Answer 4

#Chuck thanks for that code; I've been using it to save 1500-frame GIFs of simulation output, and I find that after maybe ~500 frames the time to save the next frame to the output during the call to MakeGif starts to become ... unnerving. I guess imwrite reads and writes the entirety of the output file at each call that includes the 'WriteMode','Append' pair. At frame 1500 my output is 480Mb so that becomes untenable.
An apparent rescue for this is hinted at in the doc for Octave 7.1.0's imwrite, with the suggestion that you can pass it a 4-dimensional array and write the entire image sequence with one call. I haven't been able to make this work, though: Calling imwrite that way seems to simply write the very first image in the sequence into every frame in the output file.