# (2) Examining DEMs using variable illumination

Our next animation uses a gridded topography for parts of Colorado (US). 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 to where the light source is located.

    #!/usr/bin/env bash
    #               GMT ANIMATION 02
    #
    # Purpose:      Make simple MP4 of an illuminated DEM grid of the US Rockies
    # GMT modules   math, makecpt, grdcut, grdimage, inset, plot, text, movie
    # Unix progs:   cat
    #
    # The finished movie is available in our YouTube channel as well:
    # https://youtu.be/WQ6JrtEu_Fk
    # The movie took 1 minute 45 seconds to render on a 24-core MacPro 2013.

    # 1. Create files needed in the loop
    cat << 'EOF' > pre.sh
    gmt begin
    	gmt math -T0/360/1 T -o0 = angles.txt
    	gmt makecpt -Coleron -T500/4500 -H > main.cpt
    	gmt grdcut @earth_relief_15s -R-114/-103/35/40 -Gtopo.nc
    gmt end
    EOF
    # 2. Set up the main frame script
    cat << 'EOF' > main.sh
    gmt begin
    	width=$(gmt math -Q ${MOVIE_WIDTH} 1.5c SUB =)
    	gmt grdimage topo.nc -I+a${MOVIE_COL0}+nt2 -JM${width} -Cmain.cpt \
    		-BWSne -B1 -X1c -Y0.53c --FONT_ANNOT_PRIMARY=9p
    	gmt inset begin -DjBR+w2.1c/2.4c+o0.2c -F+gwhite+s+p
    	echo 0.5 0.5 | gmt plot -R0/1/0/1 -JX? -Sc2c -Gwhite -Wthinner -Y-0.15
    		echo 0.5 0.5 ${MOVIE_COL0} | gmt plot -Sk@azimuth/1.0c -W1.75p -Gred
    		echo Light Source | gmt text -F+cTC+f9 -Y0.1
    gmt end
    EOF
    # 3. Run the movie
    gmt movie main.sh -Chd -Nanim02 -Tangles.txt -Vi -Sbpre.sh -Zs -Fmp4