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#
8.2 Examining DEMs using variable illumination

Our next animation uses a gridded topography for parts of Colorado (US); the file is distributed with the tutorial examples. Here, we want to use grdimage to generate a shaded-relief image sequence in which we sweep the illumination azimuth around the entire horizon. The resulting animation illustrates how changing the illumination azimuth can bring out subtle features (or artifacts) in the gridded data. The red arrow points in the direction of the illumination.

    #!/bin/sh
    #               GMT ANIMATION 02
    #
    # Purpose:      Make web page with simple animated GIF of a DEM grid
    # GMT progs:    gmtset, gmtmath, grdgradient, makecpt, grdimage psxy, ps2raster
    # Unix progs:   awk, mkdir, rm, mv, echo, convert, cat
    #
    # 1. Initialization
    # 1a) Assign movie parameters
    . gmt_shell_functions.sh
    width=3.5i
    height=4.15i
    dpi=72
    n_frames=36
    TDIR=../../tutorial
    name=`basename $0 '.sh'`
    # 1b) setup
    del_angle=`gmtmath -Q 360 $n_frames DIV =`
    makecpt -Crainbow -T500/4500/500 -Z > $$.cpt
    gmtset DOTS_PR_INCH $dpi
    R=`gmt_get_gridregion $TDIR/us.nc`
    # 2. Main loop
    mkdir -p $$
    frame=0
    while [ $frame -lt $n_frames ]; do
            # Create file name using a name_##.tif format
            file=`gmt_set_framename $name $frame`
            angle=`gmtmath -Q $frame $del_angle MUL =`
            dir=`gmtmath -Q $angle 180 ADD =`
            grdgradient $TDIR/us.nc -A$angle -Nt2 -M -G$$.us_int.nc
            grdimage $TDIR/us.nc -I$$.us_int.nc -JM3i -P -K -C$$.cpt -B1WSne -X0.35i -Y0.3i \
            --PAPER_MEDIA=Custom_${width}x${height} --ANNOT_FONT_SIZE=+9p > $$.ps
            echo 256.25 35.6 | psxy -R$R -J -O -K -Sc0.8i -Gwhite -Wthin >> $$.ps
            echo 256.25 35.6 $dir 0.37 | psxy -R$R -J -O -Sv0.02i/0.05i/0.05i -Gred -Wthin >> $$.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 10 -loop 0 $$/*.tif $name.gif
    cat << END > $name.html
    <HTML>
    <TITLE>GMT shading: A tool for feature detection</TITLE>
    <BODY bgcolor="#ffffff">
    <CENTER>
    <H1>GMT shading: A tool for feature detection</H1>
    <IMG src="$name.gif">
    </CENTER>
    <HR>
    We make illuminated images of topography from a section of Colorado and
    vary the azimuth of the illumination (see arrow).  As the light-source sweeps around
    the area over 360 degrees we notice that different features in the data
    become hightlighted.  This is because the illumination is based on data
    gradients and such derivatives will high-light short-wavelength signal.
    Again, our animation uses Imagemagick's convert tool to make an animated GIF file
    with a 0.1 second pause between the 36 frames.
    <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

As you can see, these sorts of animations are not terribly difficult to put together, especially since our vantage point is fixed. In the next example we will move the ``camera'' around and must therefore deal with how to frame perspective views.

Figure 8.2: Animation of a DEM using variable illumination.

Next: 8.3 Orbiting a static Up: 8. Creating GMT Animations Previous: 8.1 Animation of the Contents Index