I knitted this scarf for my wife. I created the pattern in Microsoft Excel. Shadow or Illusion knitting is a process of knitting or purling stitches of two contrasting colors to create a hidden image. The basics of shadow knitting can be found in the instructable I created here. The basic idea is that each pattern row is four knit rows.

Row # a - Color 1 - Knit across
Row # b - Color 1 - Knit on the white pattern squares, Purl on the dark squares
Row # c - Color 2 - Knit Across
Row # d - Color 2 - Purl on the white pattern squares, Knit on the dark squares

Move to the next row in the pattern and repeat.

I used Lion Brand Wool-Ease worsted weight yarn. The whole scarf is about 5 feet in length and took me about two weeks of evenings to knit. My wife (a music teacher) looked at it initially and asked, "What is it?" "A piano," I replied. She said what I hoped she'd say... "It doesn't look like a piano to me." So I held it up and said, "That's because you need to look at it from a different perspective." She spent the next five minutes raising and lowering it to see the pattern come in and out of focus.