This is Persistent Illusion. And I’ve been known to knit. 😉
And the end result of those two facts is that many people get here by searching for “illusion knitting.”
I thought I’d create something for them to find, so here is a sample. It’s not blocked, so it’s not straight. But at least you can get the idea.
From a straight-on view, it appears to be just a simple striped knit. There’s a subtle red heart pattern that’s hard to see in this picture. But from an angled side view, the black hearts suddenly appear.
Clicking on the picture will get you the bigger version.