I've been threatening to write about it for weeks. Thing is, I'm not too sure who I'm threatening, you or me.
Armor Penetration has been with us in one form or another for quite a while now. There are abilities like Sunder Armor and Expose Armor that lower armor temporarily, of course, and the rogue talent Serrated Blades. My first conscious exposure to the mechanic was the epic weapon Bonereaver's Edge, which dropped off of Ragnaros. Back then, the mechanic was fairly simple. Bonereaver`s Edge would ignore a certain amount of armor with each proc of an on-hit ability, in this case 700 armor. It could stack up to three times, so in a fight that lasted for long enough Bonereaver`s could maintain an effective -2100 armor debuff on a boss that only applied to the person using it.
Effects like this weren`t terribly common in Vanilla WoW. I myself never had a Bonereaver's (Don't cry for me, I did all right on Rag drops if I do constantly brag so myself) and so Armor Pen didn't really impinge on my consciousness. Of course, I was mostly either a tank or an offtank back in the old MC/BWL/AQ/NAXX40 days anyway. Back when you could tank with an arms or fury spec and dinosaurs ruled Un'Goro. (They still do, we just don't go there very often.) So it wasn't until Burning Crusade that I really started to notice ArP.
Back in BC, armor pen didn't have rating yet. Enchants like Executioner read "Permanently enchant a Melee Weapon to occasionally ignore 840 of your enemy's armor. Requires a level 60 or higher item." Gear that had armor pen on it told you how much armor it was going to penetrate. Cataclysm's Edge, for instance, just said "Equip: Your attacks ignore 335 of your opponent's armor." What this meant was, when you collected a whole set of ArP gear, all you had to do was add up how much armor you were ignoring. The plus side of this was, it was very simple to understand. The down side? Well, on bosses or classes with low armor (we're talking those annoying skirt wearers who can take half of your health off in one attack that completely ignores armor, you know the ones) reducing up to, say, 3000 armor at level 70 was pretty dang nasty. So they changed Armor Pen to a rating.
From there, all our troubles began.
Armor Pen as a rating has been fraught with difficulties since Wrath launched. The move to a rating based system and to percentage based ArP (ignores a percentage of target armor rather than just X amount of target armor) was aimed at retaining or even enhancing the stats usefulness against high armor targets while making it less ridiculously lethal against low armor targets. Rather than easily stacking enough ArP to rip a shadow priest's head off while barely even scratching a Bear Druid or Protection Paladin, you'll have the same boost to your damage against each target regardless of their personal armor. The change to a rating was also aimed at falling in line with other stats like hit and crit which require more and more rating to achieve the same results as a character levels.
As a result, between the launch of Wrath and patch 3.1, nobody wanted Armor Penetration on their gear. It was so bad that when I noticed the sheer amount of ArP on Ulduar gear and what that meant for how much armor you could actually ignore, especially with certain Arms talents and chances to Battle Stance, it was (to put it mildly) not well received. The buff to Armor Penetration's effectiveness is what ultimately led me to this conclusion: However, it soon became clear that Armor Penetration, while still a somewhat underwhelming stat for classes that do a large percentage of their damage as magical, had become fairly compelling for warriors as a DPS stat. Initial reports that ArP could in fact negatively reduce target armor at a certain point prompted the stat to be capped so that it could no longer reduce armor below 0, meaning that roughly 1399 Armor Penetration Rating = target armor reduced as low as it is possible to go in the game. In essence, if you had 1000 ArP rating and a Grim Toll equipped, which when it procced would push your ArP rating to 1612 ArP rating, you would be wasting about 200 of the proc's ArP because it's impossible to push your target below 0 armor. This doesn't mean that you will automatically push your target to 0 armor with that much ArP, however, as the 'up to' in the tooltip is something you see pointed out a lot.
I've not found a really coherent listing of how they've changed ratings in 3.2.2 at all. If you have one that takes the new values into account please feel free to post it here for discussion. The best source I have is this Elitist Jerks thread. Taking its values, we're currently looking at 13.99 ArP Rating to reach 1% armor reduction.
If you really, really love math, you can pour over the formula for Armor Penetration, which will allow you to find all the ways in which I've just messed up that example. To allow Ghostcrawler to explain in his own words:
- We didn't want Armor Penetration Rating to be too powerful against low armor targets, like it had been in BC. We also didn't want Armor Penetration Rating to be too powerful against high armor targets.
- So, we decided on a system where there is a cap on how much armor the Armor Penetration Rating can be applied to. So, the first X armor on the target is reduced by the percentage listed in the Armor Penetration Rating tooltip, and all armor past that X is unaffected. Another way of understanding that is we multiply the percentage in the tooltip times the minimum of the two values: the cap, and the amount of armor on the target after all other modifiers.
- Computing the cap is a little tricky unless you are already familiar with how World of Warcraft armor works. There is an armor constant we'll call C. C is derived as follows (in some pseudocode):
For a level 80 target, C=15232.5. For a level 83, C=16635.
The cap for Armor Penetration then is: (armor + C)/3.
A level 80 warrior creature has 9729 armor. C=15232.5. So, the cap is (9729+15232.5)/3=8320.5. Let's say a player has 30% armor penetration from armor penetration rating and no other modifiers that complicate the calculation (talents, Battle Stance, Sunder Armor, etc.). The game chooses the minimum of 8320.5 and 9729, so 8320.5. That is multiplied by 30% = 2496.15, and so that much armor is ignored. The effective armor on the target is 7232.85 (9729-2496.15). From a player point of view, the armor penetration rating didn't ignore the full 30%, but instead ignored 25.66%. (85.5% as effective as expected).
What all of this means is, Armor Pen has not only a cap in how much armor it can possibly reduce (it can't push armor below 0, giving wonky results where your damage goes up because it's being modified by negative armor values, as if the mob you were hitting put on armor with knives in it facing himself so that when you hit him his own armor stabs him) but it is also limited in how much armor it will actually be allowed to ignore, as shown in the above formula. It's therefore never going to ignore all of a target's armor: you can't actually push a target to 0 with sufficient Armor Penetration.
Compare this to the original system (ignores X amount of target armor) and while you may have created an elegant way for Armor Pen to reduce everyone's armor equally while at the same time keeping the stat from slapping around both those targets with extremely low or extremely high armor, you've also created a nightmare stat that has two caps built into it and which requires most players to either simply trust others as to how effective the stat is, or sit down with a calculator and a working knowledge of how armor constants work.
This is why when someone says something to me like "Matt, how much ArP should I have" I run screaming for a nearby copse of trees and begin gnawing on bark. I hate the stat, even as I grab gear with it: I hate how fussy it is, how ridiculously complicated that dual cap formula is, and I especially hate when they keep tweaking it over and over so that I have no idea what the current values are.
Things I feel that I can say safely about ArP: it's a decent DPS stat for warriors because not only does it increase our physical damage (which is the vast majority of our DPS) but it also increases our rage generation, allowing us to pump out more DPS. It increases in effectiveness nicely the more of it you have. If you have one or even two trinkets that proc an ArP effect, then you really don't need to try and shoot for above 50% on gear. If you have that much, then one (or both) of those trinkets proccing will push you to close to the cap, which is all you'll need given the way the system calculates how much armor to ignore.
Another reason I wouldn't try to stack ArP too much past 700 rating (roughly 50%) if you have a proc trinket (and even if you don't, really you don't realistically want to just slap on every piece of ArP gear you can find over all concerns) is because you also need to balance out other gear targets. Based on your spec and gear, you'll need a certain amount of hit and expertise, for starters. You also don't want to have 800 ArP and 22% crit, for instance, or 870 ArP and 2000 Attack Power. (I'm just using these numbers as examples.) Another reason I'm so leery of ArP stacking beyond a certain point is that they keep changing the way the stat works. Frankly, with the Patch 3.2.2 changes, figuring out the current ratings made my head hurt. The previous numbers were 12.31 rating to ignore 1% armor, the current vaues seem to be 13.99 rating for 1% Armor Reduction. The forumulas GC used should still be accurate, because they work with percentages and not ratings.
It's not terribly difficult to get decent ArP gear at this point in the game: the Arms gearing post from two weeks ago listed quite a few options. Just make sure you try and stack other good offensive stats (hit, crit, strength) based on what your spec and talents indicate you need. ArP's two cap system means you don't really want to even try to cap it out, especially if you get lucky on trinkets, but as long as you keep your other stats up as well, ArP is still always better the more you have until you simply can't ignore any more armor.
Next week, we'll hopefully hear more about Warriors in 3.3. If not, ArP on gear and the future.