# (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.

    #!/usr/bin/env bash
    #               GMT ANIMATION 01
    #
    # Purpose:      Make web page with simple animated GIF of sine function
    # GMT modules:  math, basemap, text, plot, movie
    # Unix progs:   echo, convert, cat
    # Note:         Run with any argument to build movie; otherwise 1st frame is plotted only.

    if [ $# -eq 0 ]; then	# Just make master PostScript frame 0
    	opt="-Mps -Fnone"
    else	# Make animated GIF
    	opt="-A+l"
    fi
    # 1. Create files needed in the loop
    cat << EOF > pre.sh
    gmt math -T0/360/20 T SIND = sin_point.txt
    gmt math -T0/360/2 T SIND = sin_curve.txt
    gmt begin
    	gmt basemap -R0/360/-1.2/1.6 -JX3.5i/1.65i -X0.35i -Y0.25i \
    	-BWSne+glightgreen -Bxa90g90f30+u@. -Bya0.5f0.1g1 --FONT_ANNOT_PRIMARY=9p
    gmt end
    EOF
    # 2. Set up the main frame script
    cat << EOF > main.sh
    gmt begin
    #	Plot smooth blue curve and dark red dots at all angle steps so far
    	last=\$(gmt math -Q \${MOVIE_FRAME} 10 MUL =)
    	gmt convert sin_curve.txt -Z0:\${last} | gmt plot -W1p,blue -R0/360/-1.2/1.6 -JX3.5i/1.65i -X0.35i -Y0.25i
    	gmt convert sin_point.txt -Z0:\${MOVIE_FRAME} | gmt plot -Sc0.1i -Gdarkred
    #	Plot bright red dot at current angle and annotate
    	gmt plot -Sc0.1i -Gred <<< "\${MOVIE_COL0} \${MOVIE_COL1}"
    	printf "0 1.6 a = %3.3d" \${MOVIE_COL0} | gmt text -F+f14p,Helvetica-Bold+jTL -N -Dj0.1i/0.05i
    gmt end
    EOF
    # 3. Run the movie
    gmt movie main.sh -Sbpre.sh -C4ix2ix125 -Tsin_point.txt -Zs -Nanim01 -D5 $opt

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.