NOBLOCKMAP unclear description

[Kzer-Za]

Could somebody please explain what exactly does this flag mean? "This object is excluded from passive collision detection. Nothing else can run into a NOBLOCKMAP object but the object itself can run into others." I can't make any sense of it. Does it mean that an actor with this flag collides with another actor only if another actor has greater velocity? How is it determined which of them ran into which? Can it collide with level architecture?

[phantombeta]

Collision in the Doom engine is done per actor, by adding the `vel` vector to the actor's position, then checking for collisions against any actors in that new position*.
This is done in each actor's `Tick` virtual. The "blockmap" referred to in the flag's name is a grid of fixed-size squares in the XY axis which contains the actors that overlap that block. This is used to find collidable actors to speed up collision checking, and setting that flag means the actor won't be added to it, so other actors won't be able to check for collision against it after moving.
But an actor doesn't need to be in the blockmap to check for collision against others, so the actor with NOBLOCKMAP will still collide with others when it's performing its own movement.

Hopefully this explanation is clear enough.

* (It actually splits the movement into "steps" first to avoid "tunneling" through walls and other actors, but the way it works is still the same)

[Kzer-Za]

Thanks!

[determin1st]

watched this, to get some info.. it's that G thing..