Allow projectiles to penetrate through ships, dealing damage multiple times

[Amazinite]

Feature: This PR implements the feature request detailed and discussed in issue #424/part of EndlessSkyCommunity#58.

Feature Details

This PR adds a new weapon mechanic that allows projectiles to persist after impacting a ship, allowing them to either impact a single ship multiple times or impact multiple ships over the projectile's lifetime.

• "penetration count": the number of hits that a projectile is capable of making before dying. For example, the default penetration count of 1 means that the projectile will die after a single hit, but a penetration of 2 means that the projectile would be able to hit the first target, damaging it, then go on to either hit the same target again on a later frame or a second target on the same frame for a total of two hits. Both ships and asteroids will consume penetrations as projectiles impact them.

Given a penetrating projectile with a low velocity, the most likely scenario for such a projectile is to impact the same ship multiple times over multiple frames. The following image shows an Energy Blaster with penetration firing at an Emerald Sword. Observe the multiple hit effects being created and the projectiles exiting out the other side of the ship, meaning that they still have available hits.

But firing at the ship from a different angle the projectiles no longer exit the other side of the ship, as they run out of penetrations while traveling through the ship.

Penetrating projectiles can also impact multiple ships on the same frame, particularly useful for low-lifetime/high-velocity projectiles such as lasers. The following image shows an Emerald Sword firing its Dragonflame through a hostile World-Ship (observe the hit effects on the World-Ship) and into a Cruiser on the other side (observe the health on the targeted Cruiser).

Penetration will also apply to a projectile's trigger radius, allowing it to trigger multiple times over its lifetime, although a trigger radius projectile will only ever trigger at most once per frame.

Usage Examples

"penetration count" is a weapon attribute:

outfit "Energy Blaster"
	...
	weapon
		"shield damage" 9.6
		"hull damage" 6
		"penetration count" 10

Testing Done

Tested the following after the refactor:

• Ships can still be damaged.
• Minables can still be damaged.
• Shooting normal asteroids doesn't break the game.
• Ship explosions still damage nearby ships.
• Trigger radius still works.
• Phasing weapons still work.
• Projectiles with no defined penetration can only damage a single ship before dying.
• Projectiles with a defined penetration of 0 will penetrate through an effectively infinite number of ships.
• Projectiles with a defined penetration >1 will penetrate through the specified number of ships before dying.
• Projectiles with multiple penetrations can impact multiple ships on the same frame.
• Projectiles can penetrate through asteroids, minables, and ships all at once.
• If a projectile with no penetration fires at a ship that is hiding behind an asteroid, the asteroid blocks the damage.
• Projectiles with damage dropoff still work.
• Lasers with damage dropoff still work.
• Damage dropoff and penetration can be combined.

Performance Impact

Non-penetrating projectiles shouldn't have any noticeable performance change.
Penetrating projectiles won't short-circuit collision detection, although this would only potentially cause issues with a large number of very fast (laser-like) projectiles.

[Just-Existing]

The projectiles should not penetrate a shielded ship. Isn't that the point of the shield?

[Amazinite]

We're limited by the 2D perspective of the game, but despite the name, projectiles with penetration might not necessarily (in-universe) be boring through the ship they're impacting (which one would expect to deal damage directly to the hull, but without "piercing" that is not the case). Perhaps penetrating but non-piercing projectiles skim across the shields of a ship, not being completely disintegrated by the shields upon first impact but instead impacting it multiple times and therefore dealing damage to the shields multiple times. The "penetration" keyword still seems accurate though given the 2D perspective; the projectile is penetrating through the ship's collision mask while normally it would just be stopped at the mask's edge, regardless of whether that means it's boring through the ship or skimming the shields. Those are just in-universe explanations for how a penetrating projectile does what it does, but from a gameplay perspective it's all a matter of allowing projectiles to pass through collision masks.

[Terin]

Are projectiles being unable to pierce asteroids a technical thing? Since it'd be very strange if like, the Dragonflame Cannon could blast through multiple Kar Ik Vots, but be stopped by a dinky space rock. Especially since there are systems very dense with them.

[Amazinite]

Are projectiles being unable to pierce asteroids a technical thing?

No, it's a design choice. However imperfect they are at this role, asteroids are intended to be able to be used as cover, so outside of "phasing" weapons which are entirely intended for passing through asteroids (and other ships without damaging them), I wouldn't want any projectiles passing through asteroids to hit ships on the other side.
Although, I also think it wouldn't be very straightforward to allow penetrating projectiles to go through asteroids given the tiled nature of asteroids.

[Terin]

Asteroids provide cover by taking stacks of penetration off. If a weapon is balanced around hitting multiple times for multiple instances of damage, if it hits asteroids on the way, it'll do a lot less damage.

If a weapon's balanced on just quickly piercing through and guaranteeing to hit a target, then it'll probably do less damage than a traditional projectile. Asteroids provide value by making a player consider if they should take a higher yield weapon that they can play well to make hit, or choosing a better arena, or if it'd be worth it to go for the guaranteed shots.

Just universally saying "No, asteroids are completely impermeable rocks" as part of the design takes a pretty big factor out of that consideration. On the tech end, though, I completely understand if that's a whole other undertaking. I'd just want the option for asteroid piercing to be open in a future PR.

[EjoThims]

Phasing and penetration both would make a projectile that only hits the target ship, but can still hit it multiple times, yes?

[Amazinite]

Yes.

[yafflehk]

Asteroids are solid rock whereas a ship is essentially a series of voids separated by thin walls - It makes sense that an asteroid would stop weapons designed for use against relatively fragile ships.

[Amazinite]

Turns out that since the asteroid tile size is 4096x4096, you'd need a projectile with at least a velocity of 4096 in order to even have a chance of hitting the same asteroid twice in the same frame in two different tiles, but that velocity is so high that it would cause issues (#4225) and should really never be used in the first place, so we can ignore the case of a projectile hitting the same asteroid twice in a frame, therefore greatly simplifying what I thought would need to be done with projectiles penetrating asteroids.

[quyykk]

Would it make sense to allow a weapon to have unlimited penetrations as well? Maybe with penetrations -1?

[Amazinite]

Spotted a bug in this PR and in #9079 when combining projectile penetration with damage dropoff. Also trying to combine Rising's CollisionSet changes with the projectile penetration behavior from this PR. Both issues are proving to be more difficult than I expected. This is going to require a decently large refactor of how collisions are handled.

I'm planning to create a new Collision class that does some of the work that DoCollision is currently doing. The responsibility of DoCollision will be to take every collision that occurred for a given projectile in a given frame into a vector of Collisions sorted by increasing range at which the Collisions occurred. It will then iterate through this list of Collisions until the projectile has died or there are no more Collisions left to calculate. This will require additional refactoring of CollisionSet (closestHit becomes pointless to keep track of) and AsteroidField (AsteroidField::Collide should return all impacted bodies, and shouldn't have the responsibility of dealing damage to Minables).

[Amazinite]

Requesting testers to make sure I didn't break the game in a horrible manner. Basically everything projectile collision related needs tested. I added a checklist to the PR description of the stuff I plan to look at, but more eyes would be helpful.

[RisingLeaf]

like it, need it, NOW