(1) Animation of the sine function

Our first animation is not very ambitious: We wish to plot the sine function from 0-360 and take snap shots every 20. To get a smooth curve we must sample the function much more frequently; we settle on 10 times more frequently than the frame spacing. We place a bright red circle at the leading edge of the curve, and as we move forward in time (here, angles) we dim the older circles to a dark red color. We add a label that indicates the current angle value. Once the 18 frames are completed we convert them to a single animated GIF file and write a plain HTML wrapper with a simple legend. Opening the HTML page in anim01.html the browser will display the animation.

#               GMT ANIMATION 01
# Purpose:      Make web page with simple animated GIF of sine function
# GMT modules:  gmtmath, psbasemap, pstext, psxy, psconvert
# Unix progs:   printf, mkdir, rm, mv, echo, convert, cat
# Note:         Run with any argument to build movie; otherwise 1st frame is plotted only.
# 1. Initialization
# 1a) Assign movie parameters
. gmt_shell_functions.sh
# 1b) Do frame-independent calculations and setup
angle_step=`gmt math -Q 360 ${n_frames} DIV =`
angle_inc=`gmt math -Q ${angle_step} 10 DIV =`
gmt psbasemap -R0/360/-1.2/1.6 -JX3.5i/1.65i -P -K -X0.35i -Y0.25i \
	-BWSne+glightgreen -Bxa90g90f30+u\\312 -Bya0.5f0.1g1 \
	--PS_MEDIA=${width}x${height} --FONT_ANNOT_PRIMARY=9p > $$.map.ps
# 2. Main frame loop
mkdir -p $$
while [ ${frame} -le ${n_frames} ]; do
	# Create file name using a name_##.tif format
	file=`gmt_set_framename ${name} ${frame}`
	cp -f $$.map.ps $$.ps
	angle=`gmt math -Q ${frame} ${angle_step} MUL =`
	if [ ${frame} -gt 0 ]; then	# First plot has no curves
#		Plot smooth blue curve and dark red dots at all angle steps so far
		gmt math -T0/${angle}/${angle_inc} T SIND = $$.sin.d
		gmt psxy -R -J -O -K -W1p,blue $$.sin.d >> $$.ps
		gmt math -T0/${angle}/${angle_step} T SIND = $$.sin.d
		gmt psxy -R -J -O -K -Sc0.1i -Gdarkred $$.sin.d >> $$.ps
	#	Plot red dot at current angle and annotate
	sin=`gmt math -Q ${angle} SIND =`
	gmt psxy -R -J -O -K -Sc0.1i -Gred >> $$.ps <<< "${angle} ${sin}"
	printf "0 1.6 a = %03d" ${angle} | gmt pstext -R -J -F+f14p,Helvetica-Bold+jTL -O -K \
		-N -Dj0.1i/0.05i >> $$.ps
	gmt psxy -R -J -O -T >> $$.ps
	[[ ${frame} -eq 0 ]] && cp $$.ps ${ps}
	if [ $# -eq 0 ]; then
		gmt_cleanup .gmt
		gmt_abort "${0}: First frame plotted to ${name}.ps"
#	RIP to TIFF at specified dpi
	gmt psconvert -E${dpi} -Tt $$.ps
	mv -f $$.tif $$/${file}.tif
	echo "Frame ${file} completed"
	frame=`gmt_set_framenext ${frame}`
# 3. Create animated GIF file and HTML for web page
${GRAPHICSMAGICK-gm} convert -delay 20 -loop 0 $$/${name}_*.tif ${name}.gif
cat << END > ${name}.html
<TITLE>GMT Trigonometry: The sine movie</TITLE>
<BODY bgcolor="#ffffff">
<H1>GMT Trigonometry: The sine movie</H1>
<IMG src="${name}.gif">
We demonstrate how the sine function <I>y = sin(a)</I> varies with <I>a</I> over
the full 360-degree interval.  We plot a bright red circle at each
new angle, letting previous circles turn dark red.  The underlying
sine curve is sampled at 10 times the frame sampling rate in order to reproduce
a smooth curve.  Our animation uses GraphicsMagick's convert tool to make an animated GIF file
with a 0.2 second pause between frames, set to repeat forever.
<I>${name}.sh: Created by ${USER} on `date`</I>
# 4. Clean up temporary files
gmt_cleanup .gmt

Make sure you understand the purpose of all the steps in our script. In this case we did some trial-and-error to determine the exact values to use for the map projection, the region, the spacing around the frame, etc. so that the final result gave a reasonable layout. Do this planning on a single PostScript plot before running a lengthy animation script.


Animation of a simple sine function.