Creating Custom Protobuf Messages

Unity and Python communicate by sending protobuf messages to and from each other. You can create custom protobuf messages if you want to exchange structured data beyond what is included by default.

Implementing a Custom Message

Whenever you change the fields of a custom message, you must follow the directions in this file to create C# and Python files corresponding to the new message and re-install the mlagents Python package.

Custom Message Types

There are three custom message types currently supported - Custom Actions, Custom Reset Parameters, and Custom Observations. In each case, env is an instance of a UnityEnvironment in Python.

Custom Actions

By default, the Python API sends actions to Unity in the form of a floating point list and an optional string-valued text action for each agent.

You can define a custom action type, to either replace or augment the default, by adding fields to the CustomAction message, which you can do by editing the file protobuf-definitions/proto/mlagents/envs/communicator_objects/custom_action.proto.

Instances of custom actions are set via the custom_action parameter of the env.step. An agent receives a custom action by defining a method with the signature:

public virtual void AgentAction(float[] vectorAction, string textAction, CommunicatorObjects.CustomAction customAction)

Below is an example of creating a custom action that instructs an agent to choose a cardinal direction to walk in and how far to walk.

The custom_action.proto file looks like:

syntax = "proto3";

option csharp_namespace = "MLAgents.CommunicatorObjects";
package communicator_objects;

message CustomAction {
    enum Direction {
        NORTH=0;
        SOUTH=1;
        EAST=2;
        WEST=3;
    }
    float walkAmount = 1;    
    Direction direction = 2;
}

The Python instance of the custom action looks like:

from mlagents.envs.communicator_objects import CustomAction
env = mlagents.envs.UnityEnvironment(...)
...
action = CustomAction(direction=CustomAction.NORTH, walkAmount=2.0)
env.step(custom_action=action)

And the agent code looks like:

...
using MLAgents;
using MLAgents.CommunicatorObjects;

class MyAgent : Agent {
    ...
    override public void AgentAction(float[] vectorAction, string textAction, CustomAction customAction) {
        switch(customAction.Direction) {
            case CustomAction.Types.Direction.North:
                transform.Translate(0, 0, customAction.WalkAmount);
                break;
            ...
        }
    }
}

Keep in mind that the protobuffer compiler automatically configures the capitalization scheme of the C# version of the custom field names you defined in the CustomAction message to match C# conventions - "NORTH" becomes "North", "walkAmount" becomes "WalkAmount", etc.

Custom Reset Parameters

By default, you can configure an environment env in the Python API by specifying a config parameter that is a dictionary mapping strings to floats.

You can also configure the environment reset using a custom protobuf message. To do this, add fields to the CustomResetParameters protobuf message in custom_reset_parameters.proto, analogously to CustomAction above. Then pass an instance of the message to env.reset via the custom_reset_parameters keyword parameter.

In Unity, you can then access the customResetParameters field of your academy to accesss the values set in your Python script.

In this example, the academy is setting the initial position of a box based on custom reset parameters. The custom_reset_parameters.proto would look like:

message CustomResetParameters {
    message Position {
        float x = 1;
        float y = 2;
        float z = 3;
    }
    message Color {
        float r = 1;
        float g = 2;
        float b = 3;
    }
    Position initialPos = 1;
    Color color = 2;
}

The Python instance of the custom reset parameter looks like

from mlagents.envs.communicator_objects import CustomResetParameters
env = ...
pos = CustomResetParameters.Position(x=1, y=1, z=2)
color = CustomResetParameters.Color(r=.5, g=.1, b=1.0)
params = CustomResetParameters(initialPos=pos, color=color)
env.reset(custom_reset_parameters=params)

The academy looks like

public class MyAcademy : Academy
{
    public GameObject box;  // This would be connected to a game object in your scene in the Unity editor.

    override public void AcademyReset()
    {
        var boxParams = customResetParameters;
        if (boxParams != null)
        {
            var pos = boxParams.InitialPos;
            var color = boxParams.Color;
            box.transform.position = new Vector3(pos.X, pos.Y, pos.Z);
            box.GetComponent<Renderer>().material.color = new Color(color.R, color.G, color.B);
        }
    }
}

Custom Observations

By default, Unity returns observations to Python in the form of a floating-point vector.

You can define a custom observation message to supplement that. To do so, add fields to the CustomObservation protobuf message in custom_observation.proto.

Then in your agent, create an instance of a custom observation via new CommunicatorObjects.CustomObservation. Then in CollectObservations, call SetCustomObservation with the custom observation instance as the parameter.

In Python, the custom observation can be accessed by calling env.step or env.reset and accessing the custom_observations property of the return value. It will contain a list with one CustomObservation instance per agent.

For example, if you have added a field called customField to the CustomObservation message, the agent code looks like:

class MyAgent : Agent {
    override public void CollectObservations() {
        var obs = new CustomObservation();
        obs.CustomField = 1.0;
        SetCustomObservation(obs);
    }    
}

In Python, the custom field would be accessed like:

...
result = env.step(...)
result[brain_name].custom_observations[0].customField

where brain_name is the name of the brain attached to the agent.

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