With the USB port at the bottom, stick Circuit Playground to the back of your screen, facing the wall behind. Blue poster-tack or double-stick foam tape can work…or sometimes even regular clear sticky tape across the face of the board. But DO NOT stick Circuit Playground directly on the back of a metal-bodied device, like an aluminum MacBook, or you’ll get an electrical short…cover the back of the board or use a mounting method that leaves a little space between board and screen.
The Processing sketch clones the contents of your screen and sends this data to the Circuit Playground, which then updates the LEDs. Try moving windows around, or play a movie or a game.
The Processing code doesn’t work with all graphics cards and modes…full-screen video, for example, may be accessing the graphics hardware directly and bypassing the normal frame buffer that Processing works with. (In one case, it works fine with full-screen movies on my NVIDIA-based iMac, but not my Intel Graphics-based MacBook Air…in the latter case, I have to keep the movie in a windowed mode, with the menu bar visible. Ah well.)
There are compatible third-party apps that may work better with full-screen video. The original Adalight guide mentions a couple of options (Lightpack and Boblight) that may fare better in this regard, though they’ll require some initial set-up to define the pixel placement as described above.