Alpha blending

Alpha overlaying two images¶ ↑

Image credit: NASA

Overlaying mars on top of the earth using a bit of transparency yields:

require "imgrb"

image_bg = Imgrb::Image.new("nasa_earth_small.png")
image_fg = Imgrb::Image.new("nasa_mars_small.png")

alpha = 180 #Out of 255
alpha_channel = Imgrb::Image.new(image_fg.width, image_fg.height, alpha)
image_fg.set_channel(3, alpha_channel)
image_alpha_overlay = image_fg.alpha_over(image_bg)

image_alpha_overlay.save("earth_mars_overlay.png")