I thought I understood thac0
Ryuuka
Member Posts: 20
Hello everyone. Last night I finally had some time to fire up BG2EE ans embark on my journey.
I fiddled around a bit with the options and turned on hit rolls. I then noticed that when Minsc hit with his twohanded sword his roll was +5. I checked his thac0 and it was 13 iirc. Why then is his roll only +5? Shouldn't be something like +7and then some for strength and specilization?
Cheers everyone!
I fiddled around a bit with the options and turned on hit rolls. I then noticed that when Minsc hit with his twohanded sword his roll was +5. I checked his thac0 and it was 13 iirc. Why then is his roll only +5? Shouldn't be something like +7and then some for strength and specilization?
Cheers everyone!
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In the case of Minsc from your example, he will hit when his to hit roll + the AC adjustment for the target is 13 or higher.
That was your first mistake.
The bonus or penalty on the hit roll you see is just your targets AC affecting the roll.
I'll take an example from BG2:EE, since I'm playing that right now.
In RE: Compiled Character Tables, Table 38, Player's Handbook. The pre-calculated THAC0 for a level 7 warrior class, which is what Minsc is, should be 14.
That's what it is in-game. All bonuses are then subtracted from that.
In RE: Compiled Character Tables, Table 1, Player's Handbook. Strength totals +2 for a character with 18/51 to 18/99, which leaves him at 12.
In RE: Proficiency from in-game. Proficiency is then subtracted, which is +1 for two slots. That puts him on 11.
Since most of the Mephits are AC0, AC4 with a bonus of +4 from from Dexterity. You need to roll 11 or higher (I'm not sure if this is actually a bug or not, but it still seems to be determined by the pre-bonus score of 14; I've rolled several 12s and 13s and missed in a test I just did).
The AC for the standard goblin in dungeon at the start of the game is 7. So if you assume there is a bug, then it is 14-7 (Base THAC0 - AC of creature) = 7. Roll 7 or greater on a 1d20 and you hit.
The character's modifiers are -1 (Pale Green Ioun Stone), -2 (Dex; it is a ranged weapon), -2 (bonus from 10 levels of Swashbuckler), -1 (+1 weapon). If the game was displaying a number that represented this character's combined modifiers I'd expect to see +6, which is neither +5 nor -3.
What this means quite simply is this number is what you need to roll (the higher the better) on 1-20 in order to hit your opponent “If their AC is ZERO”. So a generic fighter with a THAC0 of 15 (No STR Bonus, specialization or magical weapons) means that if he is fighting a monster who has an Armor Class of Zero needs to roll a 15 or higher on Die 20. Now, that same fighter gets hit with a Strength spell (raising their Strength to 18/50) they get +1 to their Role, thus REDUCING their THAC0 to 14 (15-1 for the STR bonus). They also pick up a +1 Sword and their THAC0 goes down another making it 13 (15-1 for STR and -1 for Magical sword). Basically any Plus Adjustment to your THAC0 ‘Reduces’ the base role you need to hit Armor Class zero. This is because those plusses are ADDED to your base role and reduces what that base role needs to be to actually hit that 15 on 1-20 (13 roll, plus 1 for strength, plus another 1 for magic sword equals the 15 you need THAC0). Your need to hit 15 doesn’t change, but what the base unadjusted roll needs does go down with every bonus you pick up.
What you see on your character spreadsheet is the unadjusted roll you need. The Adjustments (being plusses to whatever you roll) are ADDED by the computer to determine if you hit that magical 15 that you need to hit an armor class of Zero. Thus an unadjusted roll of 13 for this Fighter (that you might see on your character sheet when equipped with the magic sword and the STR bonus from the spell) gets a PLUS 1 for STR and a Plus 1 for Magical sword, bringing the total to 15 and making the roll a HIT. Remove those influences and your THAC0 will go back to 15.
Now, this can be confusing if your opponent doesn’t have an Armor Class of Zero. That same fighter (with a THAC0 of 15) who happens to face a goblin with an Armor Class of 4 will hit on a roll of 11 or higher. This represents the fact that their armor class is actually 4 points worse than 0 (0 + 4 = 4) and so you hit them 4 points more easily. Likewise, if you are facing a Dragon with an Armor class of -5, that same Fighter will need a roll of 20 (15 THAC0 + 5 because the armor is +5 better than zero) or higher on 1-20. Be aware that if that same Dragon had an Armor Class of -6, the fighter would NOT need a roll of 21 because a roll of 20 hits anything regardless of your skill.
Hope this helps people.
(* It will be equal to the unadjusted value in the special circumstance where a character has no to hit adjustments to apply, but this is a case of a +/- 0 adjustment rather than being actually unadjusted.)
It actually makes things more confusing that way because in the example I gave, the Unadjusted THAC0 for my Fighter is 15. The unadjusted roll that hits is 13. The difference between the two is the adjustments, but depending on your point of view, they are either positive or negative. An unadjusted throw of 13 needs adjustments of +2 (in whatever form) in order to hit an unadjusted THAC0 of 15.
I'm going to stop now or I will confuse myself now. But thanks for bringing that up.
@Mathsorcerer - Same character attacking with same weapon against two other enemies:
Goblin Commando: Attack roll (d20) + 8
Kobold Commando: Attack roll (d20) + 6
Both attacks were performed at extreme range (ie the character had to walk forward to attack, and then stopped and fired as soon as the distance decreased to maximum). These attacks were with a +1 bolt (everything else was as before). The modifier total displayed on the character record is -7, which doesn't match either the 8 or the 6 seen in the attack feedback text.
By the Nine. Thank the gods for 3E and above.
Can you upload a screenshot of your character record? If so, we can look at the modifiers which are adding to the -7 you are seeing. The frustrating thing is this: on the character record page the thac0 number will start at the base (which depends upon the character's level and class) and then all the modifiers, whether due to strength, dexterity, class, kit, specialization/mastery, or weapon bonuses will *subtract* from the base to arrive at the final modified thac0. When you attack something, all those bonuses--the ones which are displayed as negative numbers on the record page--will be *added* to the random roll. No, I didn't design it that way but it is internally self-consistent.
Consider. I just started a new game and chose the default level 7 fighter. His base thac0 is 14, then -1 for strength, -1 for extra strength, and -3 because he has grandmastery in long sword; this brings his modified thac0 down to a 9. When he attacks the jailkeep golem--which never fights back, by the way--he rolls to hit and it shows up as "Abdel: Attack Roll 11 + 5 = 16: Hit". The +5 is coming from the very same strength, extra strength, and grandmastery which were listed as -5 on the record page.
Which weapon were you using? With that, I can probably figure out the discrepancy between the -7 on the record page and the +6 you are seeing when you attack the kobold commando.
Modified thac0 - AC = the d20 roll needed to hit an opponent
(base thac0 - modifiers) - AC = the d20 roll needed to hit an opponent
If the random roll on a d20 >= the roll needed to hit then you hit the opponent.
random d20 roll >= (base thac0 - modifiers) - AC
random d20 roll + modifiers + AC >= base thac0 --> as long as this is a true statement you will hit the opponent.
You won't normally see the AC of your opponent but you will see the random d20 roll and your modifiers. In my example, the jailkeep golem has an AC of 7 so as long as d20 + 5 + 7 >= 14 the default fighter will hit, which reduces to d20 >= 2, so he will miss only on a 1. Of course, his only longsword is normal right now so he'll never do any damage.....
It is unfortunate that this is how the attack feedback text works, because it effectively conveys no information to the player. The only way to reliably interpret the number displayed is to already know all the factors affecting the chance to hit, including the enemy's AC versus your attack type. (If you don't know that AC then you can't calculate it with certainty, even if you think you know all the modifiers affecting the roll, because there will remain the possibility that some effect is applying a modifier you don't know about.) If it worked the way I thought it did then you'd be able to derive the enemy's AC easily, and it would be possible to determine which damage type an opponent was most vulnerable to without other factors like weapon proficiency or strength obfuscating matters.
Oh well.
Thanks for helping me to see that I was wrong.
It makes sense for me because I look at the roll versus the THAC0. What I mean by this is, I roll the dice and whatever I get receives a bonus for STR or Magic or Specialization or whatever. That Roll PLUS the bonuses are then compared to the THAC0 (adjusted up or down for the various armor values) and Hey, presto. So it is Roll plus bonuses. When I think about it that way it is simple. Ok, so maybe I am weird.
@the_spyder don't take it personally--I agreed that you are weird. That simply means you are in good company here.
I am simply as old as dirt and got used to 2nd edition thac0 back in 1982.
But then I have a huge problem with the whole 'Add points to stats as you level up which, as a human, can go above and beyond Titan strength'. It's a whole thing. Or maybe I'm old.