Behavior Trees#

Behavior tree codelets are one of the mechanisms to control the flow of tasks in GXF. They follow the same general behavior as classical behavior trees, with some useful additions for robotics applications. This document gives an overview of the general concept, the available behavior tree node types, and some examples of how to use them individually or in conjunction with each other.

General Concept#

Behavior trees consist of n-ary trees of entities that can have zero or more children. The conditional execution of parent entity is based on the status of execution of the children. A behavior tree is graphically represented as a directed tree in which the nodes are classified as root, control flow nodes, or execution nodes (tasks). For each pair of connected nodes, the outgoing node is called parent and the incoming node is called child.

The execution of a behavior tree starts from the root which sends ticks with a certain frequency to its child. When the execution of a node in the behavior tree is allowed, it returns to the parent a status running if its execution has not finished yet, success if it has achieved its goal, or failure otherwise. The behavior tree also uses a controller component for controlling the entity’s termination policy and the execution status. One of the controller behaviors currently implemented for Behavior Tree is EntityCountFailureRepeatController, which repeats the entity on failure up to repeat_count times before deactivating it.

GXF supports several behavior tree codelets which are explained in the following section.

Behavior Tree Codelets#

Each behavior tree codelet can have a set of parameters defining how it should behave. Note that in all the examples given below, the naming convention for configuring the children parameter for root codelets is [child_codelet_name\child_codelet_scheduling_term].

Constant Behavior#

After each tick period, switches its own status to the configured desired constant status.

Parameter

Description

s_term

scheduling term used for scheduling the entity itself

constant_status

The desired status to switch to during each tick time.

An example diagram depicting Constant behavior used in conjunction with a Sequence behavior defined for root entity is shown below

../../../../_images/constant_behavior.png

Here, the child1 is configured to return a constant status of success (GXF_BEHAVIOR_SUCCESS) and child2 returns failure (GXF_BEHAVIOR_FAILURE), resulting into the root node (configured to exhibit sequence behavior) returning GXF_BEHAVIOR_FAILURE.

The controller for each child can be configured to repeat the execution on failure. A code snippet of configuring the example described is shown below.

name: root
components:
- name: root_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 0
- name: root_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: true
- name: root_codelet
  type: nvidia::gxf::SequenceBehavior
  parameters:
    children: [ child1/child1_st, child2/child2_st ]
    s_term: root_st
---
 name: child2
 components:
 - name: child2_controller
   type: nvidia::gxf::EntityCountFailureRepeatController
   parameters:
     max_repeat_count: 3
     return_behavior_running_if_failure_repeat: true
 - name: child2_st
   type: nvidia::gxf::BTSchedulingTerm
   parameters:
     is_root: false
 - name: child2_codelet
   type: nvidia::gxf::ConstantBehavior
   parameters:
     s_term: child2_st
     constant_status: 1

Parallel Behavior#

Runs its child nodes in parallel. By default, succeeds when all child nodes succeed, and fails when all child nodes fail. This behavior can be customized using the parameters below.

Parameter

Description

s_term

scheduling term used for scheduling the entity itself

children

Child entities

success_threshold

Number of successful children required for success. A value of -1 means all children must succeed for this node to succeed.

failure_threshold

Number of failed children required for failure. A value of -1 means all children must fail for this node to fail.

The diagram below shows a graphical representation of a parallel behavior configured with failure_threshold configured as -1. Hence, the root node returns GXF_BEHAVIOR_SUCCESS even if one child returns a failure status.

../../../../_images/parallel_behavior.png

A code snippet to configure the example described is shown below.

name: root
components:
- name: root_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 0
- name: root_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: true
- name: root_codelet
  type: nvidia::gxf::ParallelBehavior
  parameters:
    children: [ child1/child1_st, child2/child2_st ]
    s_term: root_st
    success_threshold: 1
    failure_threshold: -1

Repeat Behavior#

Repeats its only child entity. By default, won’t repeat when the child entity fails. This can be customized using the parameters below.

Parameter

Description

s_term

scheduling term used for scheduling the entity itself

repeat_after_failure

Denotes whether to repeat the child after it has failed.

The diagram below shows a graphical representation of a repeat behavior. The root entity can be configured to repeat the only child to repeat after failure. It succeeds when the child entity succeeds.

../../../../_images/repeat_behavior.png

A code snippet to configure a repeat behavior is as shown below -

name: repeat_knock
components:
- name: repeat_knock_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 0
- name: repeat_knock_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: false
- name: repeat_codelet
  type: nvidia::gxf::RepeatBehavior
  parameters:
    s_term: repeat_knock_st
    children: [ knock_on_door/knock_on_door_st ]
    repeat_after_failure: true
---

Selector Behavior#

Runs all child entities in sequence until one succeeds, then reports success. If all child entities fail (or no child entities are present), this codelet fails.

Parameter

Description

s_term

scheduling term used for scheduling the entity itself

children

Child entities

The diagram below shows a graphical representation of a Selector behavior. The root entity starts child_1, child_2 and child_3 in a sequence. Although child_1 and child_2 fail, the root entity will return success since child_3 returns successfully.

../../../../_images/selector_behavior.png

A code snippet to configure a selector behavior is as shown below -

name: root
components:
- name: root_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 0
- name: root_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: true
- name: root_sel_codelet
  type: nvidia::gxf::SelectorBehavior
  parameters:
    children: [ door_distance/door_distance_st, door_detected/door_detected_st, knock/knock_st ]
    s_term: root_st
---
name: door_distance
components:
- name: door_distance_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 0
- name: door_distance_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: false
- name: door_dist
  type: nvidia::gxf::SequenceBehavior
  parameters:
    children: []
    s_term: door_distance_st
---

Sequence Behavior#

Runs its child entities in sequence, in the order in which they are defined. Succeeds when all child entities succeed or fails as soon as one child entity fails.

Parameter

Description

s_term

scheduling term used for scheduling the entity itself

children

Child entities

The diagram below shows a graphical representation of a Sequence behavior. The root entity starts child_1, child_2 and child_3 in a sequence. Although child_1 and child_2 pass, the root entity will return failure since child_3 returns failure.

../../../../_images/sequence_behavior.png

A code snippet to configure a sequence behavior is as shown below -

name: root
components:
- name: root_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 0
- name: root_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: true
- name: root_codelet
  type: nvidia::gxf::SequenceBehavior
  parameters:
    children: [ child1/child1_st, child2/child2_st ]
    s_term: root_st

Switch Behavior#

Runs the child entity with the index defined as desired_behavior.

Parameter

Description

s_term

scheduling term used for scheduling the entity itself

children

Child entities

desired_behavior

The index of child entity to switch to when this entity runs

In the code snippet shown below, the desired behavior of the root entity is designated to be the the child at index 1. (scene). Hence, that is the entity that is run.

name: root
components:
- name: root_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 0
- name: root_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: true
- name: root_switch_codelet
  type: nvidia::gxf::SwitchBehavior
  parameters:
    children: [ scene/scene_st, ref/ref_st ]
    s_term: root_st
    desired_behavior: 0
---
name: scene
components:
- name: scene_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 0
- name: scene_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: false
- name: scene_seq
  type: nvidia::gxf::SequenceBehavior
  parameters:
    children: [ pose/pose_st, det/det_st, seg/seg_st ]
    s_term: scene_st
---

Timer Behavior#

Waits for a specified amount of time delay and switches to the configured result switch_status afterwards.

Parameter

Description

s_term

scheduling term used for scheduling the entity itself

clock

Clock

switch_status

Configured result to switch to after the specified delay

delay

Configured delay

In the diagram shown below, the child entity switches to failure after a configured delay period. The root entity hence returns failure.

../../../../_images/timer_behavior.png

A code snippet for the same shown below -

name: knock_on_door
components:
- name: knock_on_door_controller
  type: nvidia::gxf::EntityCountFailureRepeatController
  parameters:
    max_repeat_count: 10
- name: knock_on_door_st
  type: nvidia::gxf::BTSchedulingTerm
  parameters:
    is_root: false
- name: knock
  type: nvidia::gxf::TimerBehavior
  parameters:
    switch_status: 1
    clock: sched/clock
    delay: 1
    s_term: knock_on_door_st
 ---
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