Merge branch 'patch-4' into 'master'

Update flatland_2.0.md

See merge request flatland/flatland!148

docs/flatland_2.0.md

@@ -28,6 +28,54 @@ For this early *beta* testing we suggest you have a look at the `sparse_rail_gen
 ### Sparse Rail Generator
 ![Example_Sparse](https://i.imgur.com/DP8sIyx.png)
 
+The idea behind the sparse rail generator is to mimic classic railway structures where dense nodes (cities) are sparsly connected to each other and where you have to manage traffic flow between the nodes efficiently. The cities in this level generator are much simplified in comparison to real city networks but it mimics parts of the problems faced in daily operations of any railway company.
+
+There are a few parameters you can tune to build your own map and test different complexity levels of the levels. *Warning* some combinations of parameters do not go well together and will lead to infeasible level generation. In the worst case, the level generator currently issues a warning when it cannot build the environment according to the parameters provided. This will lead to a crash of the whole env. We are currently working on improvements here and are *happy for any suggestions from your side*.
+
+To build en environment you instantiate a `RailEnv` follows
+
+```
+# Initialize the generator
+RailGenerator = sparse_rail_generator(num_cities=10,                        # Number of cities in map
+                                                   num_intersections=10,    # Number of interesections in map
+                                                   num_trainstations=50,    # Number of possible start/targets on map
+                                                   min_node_dist=6,         # Minimal distance of nodes
+                                                   node_radius=3,           # Proximity of stations to city center
+                                                   num_neighb=3,            # Number of connections to other cities
+                                                   seed=5,                  # Random seed
+                                                   realistic_mode=True      # Ordered distribution of nodes
+                                                   )
+
+# Build the environment
+env = RailEnv(width=50,
+              height=50,
+              rail_generator=RailGenerator,
+              number_of_agents=10,
+              obs_builder_object=TreeObsForRailEnv(max_depth=3,predictor=shortest_path_predictor)
+              )
+```
+
+You can tune the following parameters:
+
+- `num_citeis` is the number of cities on a map. Cities are the only nodes that can host start and end points for agent tasks (Train stations). Here you have to be carefull that the number is not too high as all the cities have to fit on the map. When `realistic_mode=False` you have to be carefull when chosing `min_node_dist` because leves will fails if not all cities (and intersections) can be placed with at least `min_node_dist` between them.
+- `num_intersections` is the number of nodes that don't hold any trainstations. They are also the first priority that a city connects to. We use these to allow for sparse connections between cities.
+- `num_trainstations`defines the *Total* number of trainstations in the network. This also sets the max number of allowed agents in the environment. This is also a delicate parameter as there is only a limitid amount of space available around nodes and thus if the number is too high the level generation will fail.
+- `min_node_dist`is only used if `realistic_mode=False` and represents the minimal distance between two nodes.
+- `node_radius` defines the extent of a city. Each trainstation is placed at a distance to the closes city node that is smaller or equal to this number.
+- `num_neighb`defines the number of neighbouring nodes that connect to each other. Thus this changes the connectivity and thus the amount of alternative routes in the network.
+- `seed` is used to initialize the random generator
+- `realistic_mode` currently only changes how the nodes are distirbuted. If it is set to `True` the nodes are evenly spreas out and cities and intersecitons are set between each other.
+
+If you run into any bugs with sets of parameters please let us know.
+
+Here is a network with `realistic_mode=False`
+
+![sparse_random](https://i.imgur.com/Xg7nifF.png)
+
+and here with `realistic_mode=True`
+
+![sparse_ordered](https://i.imgur.com/jyA7Pt4.png)
+
 ## Add Stochasticity
 
 ## Add different speed profiles
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