I’m a great admirer of this technique for creating images in grass by controlling local photosynthesis.
Today’s invention is related but perhaps more prosaic.
It’s a robot lawnmover the depth of cut of which can be varied in the course of its transit across a lawn.
It would therefore leave behind patterns corresponding to the greyscales in an image.
Pictures could be programmed into one’s grass including those which are distorted so as to appear standing up from the ground (when viewed, eg, from one’s house).
People with gardens alongside motorways, trainlines or runways might be paid to have their robot mowers advertise certain brands.