- idx < width * height || // low Z plane
- idx % width == 0 || // low X plane
- (idx / (width * height)) == depth - 1 || // high Z plane
- idx % width == width - 1 || // high X plane
- (idx / width) % height == 0 || // low Y plane
- (idx / width) % height == height - 1; // high Y plane
+ idx < side * side || // low Z plane
+ idx % side == 0 || // low X plane
+ (idx / (side * side)) == side - 1 || // high Z plane
+ idx % side == side - 1 || // high X plane
+ (idx / side) % side == 0 || // low Y plane
+ (idx / side) % side == side - 1; // high Y plane