Allow bird wings to glide (redux)

[worm-girl]

Summary

Features "Allow bird wings to glide"

Purpose of change

Continued from #70443 - github desktop decided to flip out on me

We have bird mutants. The bird mutants have wings. We have Z levels. The wings should, logically, have some interesting interaction with this feature.

There are a number of factors standing in the way of self-powered flight for a mutant survivor, the main one being that keeping a human body in the air would require something like a seven meter wingspan, to say nothing of other physiological changes involved, and at the very least would completely cost you the use of your arms. Playing a survivor with no arms would at present lock you out of most of the content in the game, so it doesn't seem worth implementing. On top of that it would be pretty difficult (for me) to code realistic flight as opposed to just walking around in the sky.

Bird wings already allowed players to negate all fall damage, but they were doing so by falling down in a straight line. This is not how wings work. This PR addresses both issues by instead making wings into a tool for gliding, which will allow unburdened survivors with free hands to glide forward a short distance as they fall.

Describe the solution

THE BIRD WINGS MUTATION

• Bird Wings now replace your arms, rather than being a new set of limbs growing out of your back. Real-life birds' wings are their arms, they don't have six limbs
• Bird Wings are now described as a halfway point between arms and wings, something akin to microraptor with nearly vestigial fingers so they can still craft, hold items, shoot guns, and whatnot at a hefty penalty
• Bird Wings replace your arms and remove your hands, with grip, manipulation, lift, balance, block, swim, and crawl all on the same two limbs
• Bird Wings have lost Wing Stubs as a prerequisite, and instead have Hollow Bones and Feathered Arms
• Rather than negating fall damage completely, bird wings will reduce it by two Z levels if you are at less than 50% burden, you aren't immobilized, and you have two working arms

THE BIRD WING LIMB

• Bird wings are inferior to human arms+hands in all ways except balance, where they have a substantial bonus, just as birds IRL are much better at balancing than humans. The overall effect of this is that you're worse at wielding melee weapons, unarmed combat, and shooting guns, but testing shows that your beak and talons are still extremely effective. The biggest issues with guns are aim speed (slightly slowed) and reload speed (greatly slowed), but your massively improved mobility and somewhat improved dodging should offset this. You can't slap a new STANAG in your m4 or line up a good shot as quickly as a human, but you can simply glide to safety and take all the time you need from there
• Bird wings are slightly larger than human arms (longer and skinnier) and have around 20% less HP, but are harder to hit (most of their area is just feathers, it may be difficult for enemies to land a good hit on them)
• Bird wings impose a -2 bash damage penalty to unarmed. At 8 strength and low skill, this can actually prevent you from doing damage with them. This is offset by talons, toe talons, and beak/woodpecker beak
• The mutant body part limb HP UI isn't properly set up yet, but as wings are simply replacing arms, they use the arm HP bars

GLIDING

• Gliding is done by attempting to move off a ledge. You will be prompted to climb down, jump, or whatever else as normal, but a new option appears, glide
• Gliding requires you be carrying less than half your weight. This is based on bald eagles weighing around 6 kg and being able to easily carry about 3kg in flight (they can carry more if they're moving quickly, but you are not a bald eagle). It also requires that your hands are free, that you have more than 4 strength, and that you have two unbroken arms which are not covered by any equipment
• Gliding moves you at a base rate of five tiles forward, then one tile down
• If the wind is at or above 12 mph (19 kph) you will additionally be bonused or penalized by one tile if you have a tailwind or headwind within 45 degrees of your heading
• Colliding with an obstacle, such as a wall, will terminate your glide, causing you to fall the remaining distance
• Colliding with a corporeal enemy of medium size or larger will cause a dodge check. Should you both fail, you and the enemy will be knocked out of the air. Huge enemies and enemies which fly via unconventional means (IE those who are immune to the downed condition) will not fall to the ground
• Being grabbed or slammed will terminate your glide. Be careful up there!
• Bird Wings will require the Light Bones mutation. Thanks @Vaskritaya

THE INSECT WINGS MUTATION

• These have been broken for a very long time now. They were always on, for free, and for some reason they made you sleepy and thirsty while providing no additional benefit. As such they've been reworked
• They now burn stamina fairly quickly when active. This will vary based on your fitness, but activating your wings and running will usually get you 20-40 steps before you reach 50% stamina. This is still incredibly useful in the short term
• They still provide a 25% bonus to move speed while active. They also apply a bonus to your effective strength when driving muscle-powered vehicles. The closest IRL analogue, the 4MM jetpack, actually works better on muscle-powered vehicles than on legs: https://youtu.be/HdnAx7-q8so?feature=shared
• These should be a limb someday, but that ought to wait for the HP UI to display extra limbs, so for now they are not
• They now restrict gear, requiring OVERSIZE items on the torso. This isn't a huge deal for committed insect mutants as they're already chitin'd up, but it makes them a little more awkward to grab on a dip
• If you're at less than 50% burden and not immobile, insect wings reduce fall damage by one Z level, whether they are active or not

THE FEATHERED ARMS MUTATION

• This is now a prerequisite for Bird Wings
• The encumbrance on arm feathers was way too high for how they were described. This has been reduced
• Arm Feathers will reduce fall damage by 1 Z-level if you aren't immobilized, are at less than 50% burden, and don't have any broken or missing arms
• Overall this is a substantial buff to Raptor, which was one of the worst mutations. It is still not good, but hey it's something

STAGGER

• An existing problem with Z level traversal is that we have some tiles which are unpassable but do not create a floor tile above them. In the case of trees and cars this is probably something we will fix someday, and some tiles like fences are climbable so that's fine, but there will always be edge cases that the player really shouldn't be able to stand in
• stagger() is a new function in character.cpp which looks for an adjacent free tile which is not enclosed in a vehicle and goes there if possible, similar to a zombie stumble. There's already a function in do_turn which does this for monsters who are stuck in walls, I'm not sure why characters were exempt
• So if your glide finishes with you inside of a tree, you will be shunted out automatically
• It's of scope for now, but this could also be a useful effect for effects (ie drunkenness, via a stagger_chance) where characters might make individual missteps, as opposed to being stunned which randomizes movement only when the character steps

MISC

• I attempted to use the limb regeneration EOC system to regrow your arms if you mutate wings, however the system isn't working properly and you are still without arms if this happens, so for now being armless will simply prevent you from growing wings. A project for another day
• I also removed insect arms and arachnid arms from the (currently not properly functioning) list of mutations which will heal severed arms, as these mutations have nothing to do with your original human arms, they are an extra handless pair that grow out of your back and sides and will likely not have directly analagous functions when limbified. Additionally, I intend for molting to regenerate limbs, like in nature, so it's not really needed here

Describe alternatives you've considered

I have plans for bat and butterfly wings which are beyond the scope of this PR. That being the case, they'll be left as-is for now.

I'm wondering if we can apply this code to implement parachutes. My thinking is that a parachute would require a very high altitude to work, maybe our maximum z level and one or two below that, and you'd move forward a lot less and get blown around by the wind quite a bit more.

Testing

• Tested mutating bird wings, saw that they require hollow bones and feathered arms (which they replace), destroying gear and replacing limbs as intended
• Playtested the new wing limbs to try to eyeball whether the penalties felt like a reasonable simulation of what is described. This may need tweaking but it feels pretty good to me.
• Tested gliding with low strength, no stamina, a broken arm, heavy burden etc. to ensure that gliding only works when it should
• [Mind over Matter] Tested levitation to ensure that it was not impacted by these changes, other than the minor fix I rolled in
• Tested fall damage, mixing and matching traits to ensure that reductions were stacking properly, including in cases where some should be disabled (IE a tiny character with insect wings and a heavy burden)
• Tested gliding into solid objects, saw that I fell out of the sky
• Tested gliding through enemies, saw that dodge rolls were made when appropriate
• Tested landing on monsters. This works as it already did in game::vertical_move(), with the player being displaced and an appropriate message given
• Tested landing on a car. As with falling on a car, this allows you to get into vehicles if you hit them at just the right angle. I wouldn't consider this a new issue, it merely highlights the existing problem of roofed vehicles not creating floors on z+1
• Tested riding a bike and a skateboard, turned on insect wings, saw my top speed go up
• Tested gliding into various monsters, saw that collisions happened at an appropriate rate. Saw that huge monsters and those immune to being downed did not fall out of the air
• Saw that a zombie wrestler at z-1 was able to use its reach attack to grab me out of the air, inflicting fall damage in the process (can't fly while grabbed)

Additional context

MOD AND FUTURE DEV SUPPORT

• While all forms of gliding currently require wings (via the json flag WING_GLIDE), I added a flag called GLIDE. WING_GLIDE requires >4 Strength, <50% burden, and some stamina to do its thing. GLIDE has none of these restrictions and makes no mention of wings in any of its messaging.
• LEVITATION had an issue where it was impossible to choose to fall from a ledge. You would move to the empty air tlle but stay on the same Z level. I corrected this. Now LEVITATION works as before, but if you examine an open air tile and choose to fall, you will safely descend to the ground.
• This is all in JSON_FLAGS.MD, but just for reference: WINGS_1 reduces fall damage by 1 Z level. WINGS_2 reduces fall damage by 2 Z levels. ARM_WINGS checks for unbroken arms when gliding or using WINGS1/2. WING_GLIDE glides as bird wings. GLIDE glides without physical exertion or body parts
• MUSCLE_VEH_BOOST is a json flag, so you can use it if you want to make a spell where, for example, the wind pushes you along and helps you ride your bicycle, or you technomance it into working better or something

Here's a brief demo of glide being run on a custom mutant type. Pretend this dude is a bird:
https://www.youtube.com/watch?v=3_5ygmuel9A

I'm interested in hearing from modders about anything you might want out of the core gliding framework. I'd like it to be something that could easily be carried over to MoM or Magiclysm so you can have levitation and flight spells, and I'll try to make sure that functionality is available to you in the most open-ended way I can manage.

[Venera3]

You can add an encumbrance_threshold to the wing, turn the flag conditional and require the check to have two wing flags to work - that means you can filter out encumbered wings and one disabled wing better while allowing for multiwings later.

The scores feel a bit generous but that's always tweakable down the line - a lot of things need limbing yet so the gulf between normal bits and weird new ones will only get larger.

[worm-girl]

You can add an encumbrance_threshold to the wing, turn the flag conditional and require the check to have two wing flags to work - that means you can filter out encumbered wings and one disabled wing better while allowing for multiwings later.

The scores feel a bit generous but that's always tweakable down the line - a lot of things need limbing yet so the gulf between normal bits and weird new ones will only get larger.

I'll give that a shot, thanks, and yeah the scores now are only a sketch really, so anyone who works on them will know what I was going for.

[worm-girl]

You can add an encumbrance_threshold to the wing, turn the flag conditional and require the check to have two wing flags to work - that means you can filter out encumbered wings and one disabled wing better while allowing for multiwings later.

I gave this a try and was dismayed to discover that mutation limbs don't properly get encumbered. This includes gastropod foot, so I can't really test it as you have described. I recommend we leave it as-is until that's in place. The current check for two unbroken "arms" works, at least.

I did put the conditional flags on the limbs, but damaging them to 0 wasn't preventing me from flying, even with one has_conditional_flag for each side in can_fly().

[Venera3]

That's a concern. I haven't gotten around to testing the current state of armor/limb interactions but if the new flexibilits only comes from not throwing an error but without actually doing the armor bits that will be a problem.

I'll finish up the mod and then I can look into what broke with conditionals, that is a bit I know used to work.

[Venera3]

Sorry, I mixed things up - you want encumbrance and health limit not threshold to limit flags/scores. Who came up with these names....

[worm-girl]

Sorry, I mixed things up - you want encumbrance and health limit not threshold to limit flags/scores. Who came up with these names....

No, I mean, limbs do not become encumbered at all. I can put on an XL heavy survivor suit and my gastropod foot and bird wings stay at 0 encumbrance.

[DeciusBrutus]

What is the expected behavior when a player is in a vehicle that drives off of a tall cliff?

[I-am-Erk]

Forgive me if I missed it in here. Going forward I'd like to see athletics play a role here, where gliding is an athletic activity (grants xp) and higher athletics improves your ability to glide, especially encumbered. Unfortunately that will probably mean nerfing "base level" gliding a bit, so that best level isn't too good; what you've described here sounds to me like a fairly athletic glider but there's room for improvement in a top tier athlete.

[worm-girl]

Forgive me if I missed it in here. Going forward I'd like to see athletics play a role here, where gliding is an athletic activity (grants xp) and higher athletics improves your ability to glide, especially encumbered. Unfortunately that will probably mean nerfing "base level" gliding a bit, so that best level isn't too good; what you've described here sounds to me like a fairly athletic glider but there's room for improvement in a top tier athlete.

Good thinking. I don't think that'll be too hard to implement either, as glide_distance is determined in iexamine where it'll be easy to check skills.

[worm-girl]

What is the expected behavior when a player is in a vehicle that drives off of a tall cliff?

Death probably.

[DeciusBrutus]

What is the expected behavior when a player is in a vehicle that drives off of a tall cliff?

Death probably.

I feel like someone with the ability to glide riding a skateboard off a cliff should glide rather than stick to the skateboard. Something with a roof should be different, but differentiating convertible cars from enclosed ones seems hard.

[worm-girl]

I feel like someone with the ability to glide riding a skateboard off a cliff should glide rather than stick to the skateboard. Something with a roof should be different, but differentiating convertible cars from enclosed ones seems hard.

Sounds reasonable but I really don't want to be the one to have to try and code that. Vehicles are hell.