More Fixes for AI Ramming into Stationary Targets

[wookieejedi]

TLDR: This is a long description, but it boils down to exposing 2 hardcoded AI values and updating 1 value in the optional 'better collision avoidance' behavior allows mods to substantially fix fighters ramming into capital ships they are attacking (for one example, in tests ramming went from 2-3 times a minute with 8 attacking ships to 0).

AI accidentally ramming into big ships while trying to evade fire from a hostile turret or a dogfight makes some sense given a chaotic combat situation. What does not seem good though is AI not being able to attack a large stationary target that is not shooting back and has no fighters around. This is especially evident in mods that use the new strafing AI fields, where AI will fly to a distant point then turn around to do an attack run, then once close enough pick another strafing run to conduct. In these situations, AI will commonly not pull up from the attack run in time, especially for mods that use different speeds or AI damp values compared to retail.

Fortunately, there exposing two values to the AI profiles allows modders to tune these values, the strafe retreat time and strafe retreat distance. By default these values are 2 seconds and 100 meters, which is commonly far too short for mods with higher speeds or larger damp values. Exposing these values allows modders to make the AI far more effective and virtually eliminate ramming into stationary ships they are attacking. For example, on current 26.0 with FotG testing we register 2-4 rams of a stationary target per minute with 8 attackers. Using the new exposed values in this branch eliminates the ramming (after 10 minutes of firing no ramming occurs across multiple runs of this and other test missions).

Furthermore, this PR helps better_collision_avoidance_triggered function (enabled by using better collision avoidance in the ai_profiles.tbl fix an edge case with predicting collisions. On current master the delta_time value used in this function was 0, which was only ever used eventually downstream in the will_collide_with_big_ship function call. This will_collide_with_big_ship function had an early return on line 7620:

int will_collide_with_big_ship(object *objp, vec3d *goal_point, object *big_objp, vec3d *collision_point, float delta_time)
{
	float		radius;
	vec3d	end_pos;

	radius = big_objp->radius + delta_time * objp->phys_info.speed;

	if (vm_vec_dist_quick(&big_objp->pos, &objp->pos) > radius) {
		return 0;
	}

This early return and the passing of 0 to the delta_time from better_collision_avoidance_triggered meant that the early out radius distance was only the big object's radius, and did not project any forward time. In other words, better collision avoidance only actually checked or triggered when the small ship was within the radius distance of the large ship. Thus, with ships that had geometry reach right to the edge of the object radius, it could result in the small ship not having enough time to react to avoid a collision. Notably, all other calls to will_collide_with_big_ship use a delta_time of 5-10. As such, this PR also fixes that edge case by ensuring the better collision avoidance also incorporates some lead travel time by passing a non-zero value for delta time. Given better_collision_avoidance_triggered aggression factor is 3.5 by default, it fit well in testing to pass this value to delta_time. That aggression factor is already tuneable for mods, so also using it here provides extra adaptability.

Overall these values and updates were tested and substantially remove collision accidents, especially with ships with high speeds and/or higher damps (as one result shows, ramming went from 2-3 times a minute with 8 attacking ships to 0). The PR boils down to exposing two values to the AI profiles table and updating values in the optional 'better collision avoidance' flag, so it will not affect retail. Happy to discuss as well, thanks!

[Baezon]

Looks good. Using the avoidance aggression as the delta time is a good idea too.