Attack Rolls - chance to hit, trouble with understanding all factors

Slavo

I don't know if this is right place for this, but anyway...
When i enable attack rolls in feedback i see something like this:

attack roll x+y=z :hit/miss

i figured that:

x is pure d20 roll
y is some kind of modifier, it changes when "z" reaches certain value, char dependent ? or lvl ? or smth. else
- don't know why, also it will go into negatives if you hit critical roll 20 or 19
z as a sum is then subtracted form your base THAC0, and then checked against enemy AC

B.THAC0-z =< enemy AC :hit
B.THAC0-z > enemy AC: miss

Now, what exactly "y" is ?
It must have something to do with base and actual THAC0, and with all the other bonuses like proficiency and such, but its too high to be just that (i think so), because lvl 4 Imoen with 1 * in short swords using her short sword has y=+6 against my dwarf (full plate) and it's not changed even if i remove armor.

Maybe i'm over thinking this... but still, i would like to know how this game calculates hit chance.

sry for my bad English

TJ_Hooker

Y should be your total bonus to hit. In your example with Imoen, does your dwarf have a ranged weapon equipped? Because if you melee attack a character who has a ranged weapon equipped, you get a bonus to hit (I believe +4). Also you have a tiny error in your formulas; in the second line (B.THAC0-z >= enemy AC: miss), it should be greater than, instead of greater than or equal to.

p.s. your english is great, I would never have guessed it's not your first language.

Slavo

Thanks for your answer, no dwarf is using 2 long swords (+1 and normal) one thing i did noticed is that Imoens "y" will drop to +3 if Dwarf is wearing Elves' bane belt (-3 piercing weapon)
and thanks for pointing that mistake in formula, already fixed.

TJ_Hooker

Slavo said:

Thanks for your answer, no dwarf is using 2 long swords (+1 and normal) one thing i did noticed is that Imoens "y" will drop to +3 if Dwarf is wearing Elves' bane belt (-3 piercing weapon)
and thanks for pointing that mistake in formula, already fixed.

Huh, that's weird. After watching a few hit rolls, the y value for each of my characters appears to be constant. Not sure why your y values would be changing, or why Imoen's would be so high in the first place. Hopefully someone else will come along and be able to figure this out.

Slavo

TJ_Hooker said:

Huh, that's weird. After watching a few hit rolls, the y value for each of my characters appears to be constant. Not sure why your y values would be changing, or why Imoen's would be so high in the first place. Hopefully someone else will come along and be able to figure this out.

For most part they are constant, but check critical rolls, or those that are close to 20
I'll try to capture some screen shots tomorrow, other characters included. Someone may get some ideas from that, but i think numbers are there somewhere, i simply know to little about AD&D rules to see them.

MoobooMagoo

Imoen isn't using that +2 short sword you get pretty early in the game, is she? That might explain why she has such a high bonus.

The reason why the bonus drops to +3 when you wear the belt is because short swords do piercing damage. I would have thought the game would add that bonus to your AC but apparently the game calculates it by subtracting it from the attacker's role.

But anyway, your formula is a little off. 2nd edition rules are a little backasswards before you get used to them.

Basically THAC0 is how much you have to role to hit someone with an armor class of 0. For every 1 armor class the enemy has you subtract one point from your THAC0 to see what you have to role after bonuses.

EXAMPLE:
You have a THAC0 of 17 with no bonuses and are attacking someone with an AC of 0. If you role a 17 or higher then you hit.

Now if you are attacking someone with an AC of -6, you would have to roll a 23 which is impossible. You can still hit them with a role of 20 because that always hits (and 1 always misses).

Now back to the person with the AC of 0. If you equip a +2 weapon you still have to have a 17 to hit, but your bonus of +2 is added to the role so you only have to role a 15.

Now if you are attacking someone with your +2 weapon who has an AC of 8 you will only have to role a 7. Technically speaking, you would have to role your characters THAC8 (to hit armor class 8), which would be 9, but it is cumbersome to list out every value like that, which is why THAC0 is universally used and adjusted by the defender's AC.

So basically, just subtract the enemy's AC AND your bonus to hit from your THAC0 and that is the number you have to roll in order to hit the enemy.

I can't really speak for how the game calculates this. It could be that your characters armor class above 0 is being added to her attack role or something. Regardless of how the game physically calculates everything it works out the same as this.

Hope that helped!