Someone was asking recently about designs/logos made of stripes where the design was picked out by changes in the stripe thickness. I have in mind a similar, but different effect:
I've seen designs where the image appears, at first glance (especially when you look at it quite close up) to be just lines, usually diagonal. But then, if you look at it from a little further away, or move your head and the page relative to each other, you see text or a graphic element, subtly hidden. It's not a change in the direction of the lines, nor an immediately noticeable change in the line thickness (unline the previous person's question).
(a) do you know what I'm talking about?
(b) if so, has this thing got a name?
(c) how's it done?