Each pixel is represented by a number from 0 to 3, where 0 is black, 1 is dark gray, 2 is light gray, and 3 is white. The + sign is represented by the matrix:
\[ \left[\begin{array}{lll}1 & 3 & 1 \\ 3 & 3 & 3 \\ 1 & 3 & 1\end{array}\right] \]
Show more…