We create the png files in analogy to the gif example, hence we will discuss only the generation of the movie here. In order to compose a avi file from the png files we are using Mencoder. Gnuplot is able to directly start Mencoder by its system command.

# Create movie with mencoder
ENCODER = system('which mencoder');
if (strlen(ENCODER)==0) print '=== mencoder not found ==='; exit
CMD = 'mencoder mf://animation/*.png -mf fps=25:type=png -ovc lavc -lavcopts vcodec=mpeg4:mbd=2:trell -oac copy -o huygen.avi'
system(CMD)

The first two lines check if Mencoder is available and quit gnuplot if not. The Mencoder command itselfs gets the directory containing the png files mf://animation/*.png, frames per second and input type-mf fps=25:type=png, video -ovc and audio -oac options, and finally of course the output file -o huygen.avi.

In order to generate a webm video file for a web site, ffmpeg can be used to convert the video.

$ ffmpeg -i huygen.avi huygen.webm

Fig. 1 Animation of Bessel function (code to produce this figure, loop)

Fig. 1 shows a gif animation of a Bessel function of first kind and zeroth order. Gnuplot has a gif output terminal, but the result will be smoother if we first create png files with Gnuplot and then the animated gif file with another program. In order to create a set of png files we have to loop through the time value t and different output files.

# initializing values for the loop and start the loop
t = 0
end_time = 1
system('mkdir -p animation')
load 'bessel.plt'

The above code sets a start value for the running time variable t and the end_time variable at which the looping should be stopped. Then a folder for the output png files is created and the loop file bessel.plt is loaded. The content of this loop file is shown below.

# bessel loop
t = t + 0.02
outfile = sprintf('animation/bessel%03.0f.png',50*t)
set output outfile
splot u*sin(v),u*cos(v),bessel(u,t) w pm3d ls 1
if(t<end_time) reread;

Here we update the time t, create a file name with a different number and plot out Bessel function. Finally the last line checks if our time t is smaller than the end_time variable, and executes the loop again if this is the case.

After we have created the png files in the animation folder, we start GIMP and load all the files as layers (File > Open as Layers). After this we save all layers together as an animated gif file.