Npc dog/Playing fetch with Dog

D0G's AI is largely dedicated to picking up physics objects and playing fetch with a target (usually, the player).

As shown in Half-Life 2, D0G can grab objects with his built-in Gravity Gun, toss them over an arcing trajectory, wait for it to be thrown back at him, and either repeat that cycle, or search a new object dynamically.

Valve went all-in with their scripting...

... but you don't have to.
(prop is the TV. SetThrowTarget isn't necessary to play with the player; included for completeness).
The !self in the schedule refers to its position in the world, not D0G's.

While the logics setup used in Eli's junkyard (d1_eli_02) might seem frighteningly complex, a large portion of it has to do with spawning new crates dynamically, and tracking their fate (enabling D0G to act disappointed or excited), and then switching to play with his deactivated Rollermine. As well as a handful of scripts related to Alyx.

The actual setup to play fetch with D0G is not that complicated:

• Use npc_dog in his most basic 'configuration', that is - not distracted by combat, scripted sequences, or follow behavior; all of them usually make his AI switch away from the fetch game.
• Have prop physics available to him.
• Now use the following Inputs to make him play fetch:
  1. SetThrowTarget <name of the target> (leave blank to use the player);
  2. StartCatchThrowBehavior <name of the prop> to switch to his play AI (leave blank to search for an object dynamically);
  3. Make D0G walk or run to where you want him to be, by using aiscripted_schedule, scripted_sequence or a choreo scene (see Known Issues for explanation).

That should be enough for the basic routine. Other inputs can modify or stop it:

• SetPickupTarget <name of the prop> overrides his preferred prop;
• StopCatchThrowBehavior for him to stop altogether;
• SetThrowArcModifier <float> to alter the trajectory;
• StartWaitAndCatch
  Todo: Explain
• StopWaitAndCatch
  Todo: Explain

Note:If you don't care which objects D0G will be throwing at you, and there's suitable props available to him (see below about what kind of props he can use), you can even just use StartCatchThrowBehavior (blank) and make him move. That can be enough.

Known issues

Bug:There appears to be a bug where D0G won't start playing fetch even when everything is in place (all the inputs have been fired, there's valid nodegraph, etc.), until he makes a step.  [todo tested in ?]

It may have gone unnoticed in Half-Life 2, because he always runs toward his dumpster, and therefore 'wakes up'. To fix it, you can either stage it like HL2 - have him run toward a target to start playing - or place a aiscripted_schedule entity near him to make him walk toward it; just a short distance should be sufficient.

If D0G waits too long, he'll give up and search for another prop. His heuristics isn't perfect, and it may lead to unindended consequences and even get him stuck, if he tries to grab a prop that's lodged in scenery, tied to constraints and otherwise not meant for him. There doesn't seem to be a way to filter out what prop he can grab without additional programming, so you'd have to plan for it by not having any unintended physics lying around, or disabling their motion.

D0G won't attempt to grab objects that are:

• Flagged as debris or interactive debris;
• Held by the player;
• Have their motion disabled;
• Are heavier than 250kg (this figure is hardcoded);
• Aren't visible to him;
• Are placed above his eye position.

Bug:While D0G is capable of playing fetch with a target other than the player (even another D0G),^[1] he'll always turn face the player before throwing.  [todo tested in ?]

References

1.   Two npc_dogs playing fetch with each other on YouTube