@staticmethod
def index_corners(x, y, cell_size):
    """x, y is the index of a cell (bottom left)"""
    x0 = x
    y0 = y
    x1 = (x + 90 + cell_size) % 180 - 90
    y1 = y0
    x2 = x1
    y2 = (y + 180 + cell_size) % 360 - 180
    x3 = x0
    y3 = y2
    return x0, y0, x1, y1, x2, y2, x3, y3
