# (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 10. 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 36 frames are completed we convert them to a single mp4 file.

    #!/usr/bin/env bash
    #               GMT ANIMATION 01
    #
    # Purpose:      Make simple MP4 of sine function
    # GMT modules:  math, basemap, text, plot, movie
    # Unix progs:   echo, convert, cat
    #
    # The finished movie is available in our YouTube channel as well:
    # https://youtu.be/5m3gRhFFFLA

    # 1. Create files needed in the loop
    cat << 'EOF' > pre.sh
    gmt math -T0/360/10 T SIND = sin_point.txt
    gmt math -T0/360/1 T SIND = sin_curve.txt
    gmt begin
    	gmt basemap -R0/360/-1.2/1.6 -JX22c/11.5c -X1c -Y1c \
    	-BWSne+glightskyblue -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 ${MOVIE_FRAME} 10 MUL =)
    	gmt convert sin_curve.txt -qi0:${last} | gmt plot -W1p,blue -R0/360/-1.2/1.6 -JX22c/11.5c -X1c -Y1c
    	gmt convert sin_point.txt -qi0:${MOVIE_FRAME} | gmt plot -Sc0.1i -Gdarkred
    #	Plot bright red dot at current angle and annotate
    	gmt plot -Sc0.1i -Gred <<< "${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 -Chd -Tsin_point.txt -Vi -D5 -Zs -Nanim01 -Fmp4

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.