As I discuss in my Close Listening introduction ...
Charles Lock, "Petroglyphs in and of perspective," Semiotica 100-2/4, 405-420 (1994)
The burden of his essay is to discuss the difference between seeing something as a visual marking and something as a semiotic marking, which is a crucial point in terms of visual poetry, and relates to a basic issue of poetic function (where "symbolic" sounds are, in some approaches to poetry, perceived nonsymbolically). Lock characterizes the difference as that the "pictorial" requires perspective on what is seen, the semiotic not: "the absence or presence of perspective marks the crucial difference between pictorial and symbolic signs, both of which are visual." For a text is the only visual sign system that, as Lock puts it, is entirely free of perspective.