# (52) Image and grid manipulation

This example highlights how GMT can be used to manipulate images in conjunction with grids, using grdmix (as well as grdmath). We start with two geotiff images for the day (@earth_day) and night (@earth_night) Earth views and compute a day/night smoothed grid to reflect the state on June 22, 2000 at midnight in Hawaii. The day/night grid is used to blend between the two images with grdmix, and we add more life to the plot by modifying the colors based on intensities computed from a co-registered DEM. The final image is then overlaid on an Earth viewed from a point near Florida.

```#!/usr/bin/env bash
# GMT EXAMPLE 52
#
# Purpose:     Mixing images and overlaying them on the Earth
# GMT modules: grdgradient, grdimage, grdmath, grdmix, grdsample, solar

res=20m # Change to 15m, 10m, 06m, 05m, 04m, 03m, 02m, 01m, 30s for higher resolutions

gmt begin ex52

# Use the location of the Sun at noon on Jun 22, 2000, Hawaii Standard Time
# Make a global grid with a smooth 2-degree transition across day/night boundary.
gmt grdmath -Rd -I\${res} -r \$(gmt solar -C -o0:1 -I+d2000-06-22T24:00+z-10) 2 DAYNIGHT = w.grd

# We will create an intensity grid based on a pixel-registered DEM so that we can see structures in the oceans
gmt grdgradient @earth_relief_\${res}_p -Nt0.5 -A45 -Gintens.grd

# Blend the earth_day and earth_night geotiffs using the weights, so that when w is 1
# we get the earth_day, and then adjust colors based on the intensity.

gmt grdmix @earth_day_\${res} @earth_night_\${res} -Ww.grd -Iintens.grd -Gview.tif

# Plot this image on an Earth with view from over Mexico
gmt grdimage view.tif -JG80W/30N/18c -Bafg
# Clean up after use
rm -f w.grd intens.grd view.tif
gmt end show
```