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#
8.1 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 anim01.html in the browser will display the animation.

    #!/bin/sh
    #               GMT ANIMATION 01
    #
    # Purpose:      Make web page with simple animated GIF of sine function
    # GMT progs:    gmtset, gmtmath, psbasemap, pstext, psxy, ps2raster
    # Unix progs:   awk, mkdir, rm, mv, echo, convert, cat
    #
    # 1. Initialization
    # 1a) Assign movie parameters
    . gmt_shell_functions.sh
    width=4i
    height=2i
    dpi=125
    n_frames=18
    name=`basename $0 '.sh'`
    # 1b) Do frame-independent calculations and setup
    angle_step=`gmtmath -Q 360 $n_frames DIV =`
    angle_inc=`gmtmath -Q $angle_step 10 DIV =`
    gmtset DOTS_PR_INCH $dpi
    psbasemap -R0/360/-1.2/1.6 -JX3.5i/1.65i -P -K -X0.35i -Y0.25i \
            -Ba90g90f30:,-\\312:/a0.5f0.1g1WSne -Glightgreen \
            --PAPER_MEDIA=Custom_${width}x${height} --ANNOT_FONT_SIZE=+9p > $$.map.ps
    # 2. Main frame loop
    mkdir -p $$
    frame=0
    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=`gmtmath -Q $frame $angle_step MUL =`
            if [ $frame -gt 0 ]; then       # First plot has no curves
    #               Plot smooth blue curve and dark red dots at all angle steps so far
                    gmtmath -T0/$angle/$angle_inc T SIND = $$.sin.d
                    psxy -R -J -O -K -W1p,blue $$.sin.d >> $$.ps
                    gmtmath -T0/$angle/$angle_step T SIND = $$.sin.d
                    psxy -R -J -O -K -Sc0.1i -Gdarkred $$.sin.d >> $$.ps
            fi
            #       Plot red dot at current angle and annotate
            sin=`gmtmath -Q $angle SIND =`
            echo $angle $sin | psxy -R -J -O -K -Sc0.1i -Gred >> $$.ps
            echo $angle | awk '{printf "0 1.6 14 0 1 LT a = %3.3d\n", $1}' \
                    | pstext -R -J -O -K -N -Dj0.1i/0.05i >> $$.ps
            psxy -R -J -O -T >> $$.ps
            if [ $# -eq 0 ]; then
                    mv $$.ps $name.ps
                    gmt_cleanup .gmt
                    gmt_abort "$0: First frame plotted to $name.ps"
            fi
    #       RIP to TIFF at specified dpi
            ps2raster -E$dpi -Tt $$.ps
            mv -f $$.tif $$/$file.tif
            echo "Frame $file completed"
            frame=`gmt_set_framenext $frame`
    done
    # 3. Create animated GIF file and HTML for web page
    convert -delay 20 -loop 0 $$/*.tif $name.gif
    cat << END > $name.html
    <HTML>
    <TITLE>GMT Trigonometry: The sine movie</TITLE>
    <BODY bgcolor="#ffffff">
    <CENTER>
    <H1>GMT Trigonometry: The sine movie</H1>
    <IMG src="$name.gif">
    </CENTER>
    <HR>
    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 Imagemagick's convert tool to make an animated GIF file
    with a 0.2 second pause between frames, set to repeat forever.
    <HR>
    <I>$name.sh: Created by $USER on `date`</I>
    </BODY>
    </HTML>
    END
    # 4. Clean up temporary files
    gmtset DOTS_PR_INCH 300
    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.

Figure 8.1: Animation of a simple sine function.

Next: 8.2 Examining DEMs using Up: 8. Creating GMT Animations Previous: 8. Creating GMT Animations Contents Index