Facts
A fact is a symbolic representation of a truth concerning the environment. A fact is a vector composed of several fields such as:
Subject: the entity on which the property applies (e.g.: Red_Mug, Human1, Pr2, Human_Right_Hand).
Property: the property linked to the Subject (e.g.: IsOn, IsFull, IsMoving, IsPointing, canSee).
Target: the property may link the entity-subject with an entity-target. As an example if an entity BOOK is on an entity TABLE, BOOK will be the subject while TABLE will be the target.
PropertyType: this parameter defines the category in which the property falls (e.g.: position, state, motion, posture, affordance). Using this parameter, if an external module adds a fact, it is still possible to know which kind of property it is.
Value: a property may have a value linked to it. As an example, the property IsFull or IsMoving may have the value TRUE or FALSE (if we want to have a close world representation). As another example, if we represent the distance between joints, such as robot's hand to human head, the Value parameter could be set to DANGER, CLOSE or FAR, or even holds a numerical value.
In some situations, to represent a lack of knowledge on a property and the awareness of this lack, the fact's value can also be set to unknown.
Confidence: this figure between 0 and 1 represents the reliability of the fact. This can be linked to sensor reliability or to the property computation itself.
Time: the time at which the fact was computed.
FactObservability: the probability that a human would acquire the awareness of the fact if he sees the Subject of the fact.
As an example, the vector:
Subject = Bob_Right_Hand, Property = IsMovingToward, Target = Red_Book, PropertyType = motion, Confidence = 0.8, time = 145571646570, FactObservability = 0.7
represents the fact that, at the given time, the hand (Bob_Right_Hand) of Bob is going toward a book (Red_Book) with a confidence of 0.8.
The following sections describe spatial and temporal reasoning components that generate these kind of facts.
The fact computation is detailed in the page of each module. To sum up, we give below a table which sums up where facts are computed.
