              // to [+1, +1] behaves like regular clamp-to-edge.
                // Other three quadrants should be mirrored.
                checkPixel(signX * 0.125, signY * 0.125, black);
                checkPixel(signX * 0.375, signY * 0.125, red);
                checkPixel(signX * 0.750, signY * 0.125, red);
                checkPixel(signX * 0.125, signY * 0.375, green);
                checkPixel(signX * 0.125, signY * 0.750, green);
                checkPixel(signX * 0.375, signY * 0.375, blue);
 