# (11) Wrapping the Marbles on a Sphere

Using grdmix, we are blending the NASA day and night views from the Blue and Black Marble mosaic images using a day-night mask set for the summer solstice midnight in Hawaii on June 20, 2020. In addition, we adjust the colors using the intensities derived from the slopes of the earth relief grid. We spin the view point around at 24 frames per second where each frame advances the viewpoint 0.25 degrees of longitude.

    #!/usr/bin/env bash
    #
    # Blending the NASA day and night views from the Blue and Black Marble mosaic
    # images using a day-night mask set for the summer solstice midnight in Hawaii
    # on June 20, 2020.  In addition, we adjust the colors using the intensities derived
    # from the slopes of the earth relief grid.  We spin around at 24 frames per second
    # where each frame advances the viewpoint 0.25 degrees of longitude.
    #
    # DEM:    @earth_relief_02m
    # Images: @earth_day_02m @earth_night_02m from the GMT data server
    #
    # The finished movie is available in our YouTube channel as well:
    # https://youtu.be/nmxy9yb2cR8
    # The movie took ~2.75 hour to render on a 24-core MacPro 2013.

    # 1. Create background plot and data files needed in the loop
    cat << 'EOF' > pre.sh
    gmt begin
      # Set view longitudes 0-360 with steps of 0.25 degree
      gmt math -T-180/180/0.25 -o0 T = view.txt
      # Make global grid with a smooth 2-degree day/night transition for the 2020 solstice.
      gmt grdmath -Rd -I2m -rp $(gmt solar -C -o0:1 -I+d2020-06-20+z-10) 2 DAYNIGHT = daynight.grd
      # We will create an intensity grid based on a DEM so that we can see structures in the oceans
      gmt grdgradient @earth_relief_02m -Nt0.75 -A45 -Gintens.grd
      # Make sure our remote files have been downloaded
      gmt which -Ga @earth_day_02m @earth_night_02m
    gmt end
    EOF
    # 2. Set up main script
    cat << 'EOF' > main.sh
    gmt begin
      # Let HSV minimum value go to zero and faint map border
      gmt set COLOR_HSV_MIN_V 0 MAP_FRAME_PEN=faint
      # Blend the day and night Earth images using the weights, so that when w is 1
      # we get the daytime view, and then adjust colors based on the intensity.
      gmt grdmix @earth_day_02m @earth_night_02m -Wdaynight.grd -Iintens.grd -Gview.tif
      # Plot this image on an Earth with view from current longitude
      gmt grdimage view.tif -JG${MOVIE_COL0}/30N/21.6c -Bafg -X0 -Y0
    gmt end
    EOF
    # 3. Run the movie, requesting a fade in/out via white
    gmt movie main.sh -Sbpre.sh -C21.6cx21.6cx50 -Tview.txt -Nanim11 -Lf -Ls"Midnight in Hawaii"+jBR -H8 -Pb+w1c -Fmp4 -V -W -Zs