def largest_product_horizontally(matrix):
    """
    Computes the largest product horizontally (line by line) on a given matrix
    """
    largest_product = 1
    for line in range(0, matrix.shape[0]):
        for column in range(0, matrix.shape[1]-3):
            product = int(matrix[line, column] *
                          matrix[line, column+1] *
                          matrix[line, column+2] *
                          matrix[line, column+3])
            if product > largest_product:
                largest_product = product
    return largest_product