1-\prod_{I=0}^{3}\left(\operatorname{sign}\left(\left(\max\left(\left|\left(3^{I}\left(x-.5\right)\right)-\operatorname{floor}\left(3^{I}\left(x-.5\right)\right)-.5\right|,\left|\left(3^{I}\left(y-.5\right)\right)-\operatorname{floor}\left(3^{I}\left(y-.5\right)\right)-.5\right|\right)-\frac{1}{6}\right)\right)+0\right)\ge0\left\{-.5<x<.5\right\}\left\{-.5<y<.5\right\}