July – Animating GIFs in GNU Octave

Often it is nice to plot your results as an animation. The following code demonstrates the most effective way to write GIFs in Octave that I could find. Example code outputs an animation of a complex base-band signal symbol in noise. Code uses the quiver plot, but any other plot should do equally well. Matlab compatibility is not guaranteed.

Figure 8: Animated GIF of a noisy QAM symbol

The point is to loop through all frames and save them to a file after each other with functions getframe and imwrite.

function writeGIF()
  clear all;
  close all;
  clear imread;
  clear imwrite;
  
  N = 100; %%Number of frames.

  %%We use quiver plot as an example, but any other plot works equally.
  signal = 5 + 5*i; %%Baseband symbol.
  signal = signal + randn(1) + randn(1)*i ;   %%Add some Gaussian noise.
  quiver(0,0,real(signal), imag(signal))
  axis([[-10, 10],[-10, 10]])

  %%Write the first frame.
  frame = getframe(); %%Get the current frame.
  imwrite(frame.cdata,'gifu.gif','gif','writemode','overwrite',...
          'LoopCount',inf,'DelayTime',0.1, 'Compression' , 'lzw');  %%Write the frame to gifu.gif and use delay time 0.1 seconds.
  
  for iii = 1 : N
    iii
    %%Write GIF.
    signal = 5 + 5*i; %%Baseband symbol.
    signal = signal  + rand(1) + randn(1)*i ;   %%Add some Gaussian noise.
    quiver(0,0,real(signal), imag(signal))
    axis([[-10, 10],[-10, 10]])
    
    frame = getframe(); %%Get the current frame.
    imwrite(frame.cdata, 'gifu.gif','gif','writemode','append','DelayTime',0.1, 'Compression' , 'lzw'); %%Write the frame to gifu.gif.
  end
end

The 'Compression' setting is important to avoid insane file sizes and memory usage.

References:

• Octave Image Processing