It’s a given that the difference between the attacker’s OA and the target’s DA affects chance to hit.
They also affect the chance and magnitude of critical hits.
But defensive ability does more than that.
For a start , here’s the probability to hit equation ;
(offensiveAbilityDV – defensiveAbilityDV + 1000) / 10
There is a second part , which I mistakenly thought applied to chance to hit , but it only applies to criticals.
Anyway , if the probability to hit is over 99% there is a chance to critical.
If the probability to hit is 99% or less you not only get a chance to dodge , but also take less damage from the attacks that that actually hit.
My experiment was done using an attacker of known damage (1268 per hit) and OA (807).
I used a naked skill-less char changing his DA with custom-made rings.
And this is the table showing the actual damage taken.
As you can see , if my DA is up to his OA + 9 , he does full damage.
At that point his chance to hit is 99.1% .
When I hit a DA equal to his OA + 10 I immediately get a 5.52% dmg reduction.
At that point the chance to hit is 99%.
Every point of DA over that offers an extra 0.05% dmg reduction.
So 100 points of DA would give an extra 5% dmg reduction , besides giving the guy a 10% chance to miss.
For example , if your DA is 210 points higher than his OA , he’ll get a 21% chance to miss and do 15.52% less damage even if he hits.
I repeated this test with a much higher damage number to make sure that it’s percentage based reduction.
And I also tried a much higher OA to make sure that it was proportional to the DA-OA difference.
Furthermore , this is NOT the attacker throwing full damage attacks and the target reducing them.
The attacker just throws attacks for less damage.
So this effect comes before everything else (resistances , armor etc).
Also , if the attacker is using a piercing weapon , him doing reduced physical damage will result in equal reduction to his piercing damage.
This only happens because piercing damage is a part of physical , any flat non-physical damage is not affected by OA and DA.
If this is not obvious , this is what happens when you get attacked but also what happens when you attack.
To sum this up ;
If the attacker’s OA is equal to DA-9 or higher ; full damage , 99% or higher chance to hit , chance to critical
If the attacker’s OA is equal to DA-10 or lower ; listed_dmg*(1-5,52%-(DA-OA-10)*0,05%) damage , (DA-OA)/10 chance to miss , no chance to critical.
Let me tie this up with an example ;
Attacker ; 790 OA , 1000 dmg – 30% of which is piercing (spear) , so a total of 700 physical and 300 piercing dmg.
Target ; 799 DA
The attacker does full damage , 700 physical and 300 piercing , which then has to go through resistances -> armor -> dmg absorbtion , in that order.
Target ; 1200 DA
DA is 410 points higher than OA , which gives a 41% chance to miss.
The first 10 points give a 5.52% reduction , the rest give 0.05% per point , so a total of 25.52% reduction.
The attacker does 1000*(1-25.52%) = 745 dmg , 70% is physical = 522 , 30% is piercing = 223.
So 522 physical and 223 piercing damage will then get modified by the target’s defense.
I should repeat that THIS ONLY APPLIES TO MELEE COMBAT AND ONLY TO PHYSICAL DAMAGE.
Piercing damage is affected ONLY as a part of physical , if the attacker has a weapon with a flat piercing damage bonus then that flat bonus will not be reduced.
Oh and another thanks to Whisk33 for confirming this one.
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