slot.md

Slot

Slot {
  SFString type     ""     # any string
  SFNode   endPoint NULL   # {node, PROTO}
}

Description

Slot nodes always works with pairs, only a second Slot can be added in the endPoint field of a Slot before to be able to add any node in the endPoint field of the second Slot. Furthermore, the second Slot can be added only if it has the same type as the first one. The Slot node is particularly useful with PROTO nodes, it allows the user to constrain the type of nodes that can be added in an extension field of the PROTO. Imagine for example that you have an armed robot in which you can plug different kinds of hands. In order to do so you will put the hand as an extension field of your robot, you will then be able to add all the different PROTO models of hand that you have made. But nothing prevents you from adding a PROTO of table in the hand extension field. The Slot is made for preventing this kind of problems. By encapsulating your extension field in a Slot and using the Slot node as base node for all your hands PROTO nodes and defining the same type for the field Slot and the PROTO Slot, only hands can be inserted in the extension field. This is illustrated in the example section.

Field Summary

• type: defines the type of the Slot. Two Slot nodes can be connected only if their types match. It is possible to specify a gender by ending the string with a '+' or a '-'. In this case, two Slot nodes can be connected only if they are of opposite gender (e.g. a Slot with a type ending with '+' can only be connected to a Slot with the same type, except that it ends with '-' instead of '+'). The default empty type matches any type.

• endPoint: The node inserted in the endPoint of a Slot should be another Slot if this Slot is not already connected to another Slot (i.e., its parent is a Slot). If the pair of Slot nodes is already connected, any node that can usually be inserted in a children field can be inserted in the endPoint field of the second Slot.

Example

If you want to write a proto of a robot called MyRobot that accepts only hands in its field handExtension, you have to set the field handExtension to be the endPoint of a Slot.

PROTO MyRobot [
  field SFNode handExtension NULL
]
Robot {
  children [
    Slot {
      type "robotHand"
      endPoint IS handExtension
    }
    ...
  ]
}

Then any PROTO of a hand needs to use the Slot as base node and the type of this Slot should match the one in MyRobot.

PROTO RobotHand [
]
{
  Slot {
    type "robotHand"
    endPoint Solid {
      ...
    }
  }
}