When grass is grown from seed on a vertical surface, it can record complex images much as photographic film does: Each germinating blade produces chlorophyll in proportion to the light that reaches it. Stronger light produces greener grass, and blades deprived of light grow, but produce no chlorophyll, leaving them yellow. “In a sense we have adapted the photographic art of producing pictures on a sensitive film to the light sensitivity of emergent blades of young grass.”
They shine negatives of a picture through a projector to produce a light onto a canvas that has been coated with a growing medium and real grass seeds. If you are interested in more of the process, here is a LINK