'natural AC' vs. 'effective AC' in .CRE files

[subtledoctor]

Anyone know what the deal is with these values? They seem to be identical in a handful of creatures I inspected, but i hate messing with values that I don't understand.

It sounds like "effective" AC might be the final values after considering adjustments, but that doesn't seem to be the case - adjustments for equipment etc. seem to be applied afterward, in-game. I'm just afraid that setting this value could preempt in-game adjustments from being applied, or something.

As usual, the IESDP is unhelpful.

(EDIT - sorry, don't mean to trash-talk the IESDP too much - overall it is immensely useful and it is often present in one of my browser tabs. Just, on specific inquiries like this it often presents some kind of pat language that sounds informative but actually is not, and that can be frustrating.)

Edited March 16, 2023 by subtledoctor

[jmerry]

Natural AC isn't too hard to understand. It's the base AC before equipped items, spells, stat bonuses, and everything in general modifies AC.

On "Effective AC", my working hypothesis is that it does nothing at all and should be completely ignored. I just checked a save - the epic Dragon Disciple I looked at had the "effective AC" in the creature file still at 10.

[lynx]

I'd guess it's a design leftover. Could be an optimisation, but I doubt that, since the games keep many copies of stat values already.

[argent77]

From my own tests it looks like only "Effective AC" is used by the game. Even characters with a natural AC progression (like monks) modify only effective AC. Natural AC doesn't appear to be used by the game. I'd say the term "Effective AC" is somewhat misleading, since it doesn't include AC bonuses from stats, items or spell effects.

[kjeron]

Effective AC is the characters inherent base AC value - the per-character default the game considers when determining the characters "best/lowest" base AC value, as opposed to using a hardcoded value of "10" for everyone.

Edited March 16, 2023 by kjeron

[subtledoctor]

So, "effective AC" means natural AC. Got it.   

[Bubb]

The engine names the two fields as:

• .CRE [+0x46] - m_armorClass
• .CRE [+0x48] - m_armorClassBase

I see no evidence that m_armorClass is used in any meaningful way. It is accessed by:

• op74 (State: Blindness) — duration type = 1 — modifies m_armorClass.
• The LoB upgrade / downgrade routines — modifies m_armorClass.
• Action 370 (ChangeStat) — Stat = 2 — modifies m_armorClass.

Notice how the engine never actually uses it.

[subtledoctor]

Ah so I was reading the word "effective" as meaning something like the end result after adjustments (e.g. "when wearing black and standing in the shadows, the thief was effectively invisible")

But maybe it really means "effective" as simply "not ineffective."

Edited March 16, 2023 by subtledoctor