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 thrown back at him, and either repeat that cycle, or search a new object dynamically.

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. 1. SetPickupTarget <name of the prop>;
   2. SetThrowTarget <name of the target (leave blank to use the player);
   3. Make D0G walk or run to where you want him to be (see Known Issues for explanation);
   4. StartCatchThrowBehavior <name of the prop> to start the game.

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

• StopCatchThrowBehavior for him to stop altogether;
• SetThrowArcModifier <float> to alter the trajectory;
• StartWaitAndCatch
  Todo: Explain
• StopWaitAndCatch
  Todo: Explain

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