Earth is being invaded by space aliens. You are Earth's last defense. Based on the Moon, you must shoot the aliens before they get down to you and destroy your ship. Otherwise, Earth is lost.
Controls:
LEFT ARROW - move left
RIGHT ARROW - move right
SPACEBAR - fire missile
ESCAPE - quit game
Unlike the other testbed programs, "Invaders" only uses scrolling during the opening and closing scenes. Actual gameplay relies just on sprites to render the play field. There are a possible 8 active sprites: 1 ship, 1 missile, and 6 aliens. A stock IBM PC cannot update all 8 sprites 60 times a second without CGA snow or image anomalies so alien movement is interleaved, one alien per frame. It is hard to recognize that the aliens aren't moving in sync.
The LORES library won't clip sprites to the map boundaries, but it will clip them to screen boundaries (an implementation performance decision). In order to get clipped sprites, "Invaders" creates a tile boundary surrounding the playfield. The view is set inside this boundary so the sprites will be nicely clipped to the screen edges.
The background was created from a NASA "Earthrise" picture. The slicer.c program in the MAPEDIT directory is run on a modern computer and will convert the Portable Net RGB image into either a dithered or a closest match 4 BPP IRGB version. It will then slice the image in 16x16 pixel tiles, reducing duplicate tiles, and saving it in a format to be loaded by the LORES library (or further edited with MAPEDIT).
These are more of the tricks needed to effectively use this library and graphics mode on a 4.77 MHz IBM PC.