def averaging(grid, numGrid, numPix):
    """
    resize 2d pixel grid with numGrid to numPix and averages over the pixels
    :param grid: higher resolution pixel grid
    :param numGrid: number of pixels per axis in the high resolution input image
    :param numPix: lower number of pixels per axis in the output image (numGrid/numPix is integer number)
    :return:
    """

    Nbig = numGrid
    Nsmall = numPix
    small = grid.reshape([int(Nsmall), int(Nbig/Nsmall), int(Nsmall), int(Nbig/Nsmall)]).mean(3).mean(1)
    return small