Pat Ashforth and Steve Plummer create knitted tapestries with hidden images in them that only become visible from the correct angle, like this image of the Mona Lisa.

In 2009 we suddenly took an interest in illusion knitting. It was very different from the mathematical work that can be found on the main part of the Woolly Thoughts site. As we searched the Internet we became more and more confused by the different descriptions and decided there had to be a better way so we looked at the technique with a mathematical, logical approach. Since then we have gone on to develop our own method of charting and, by doing so, we can now create far more complex designs than anything we had seen before.

[via susannacarolina]