Guest Spotlight: Constie!

Some of you will remember my “EoL is totally batty right now” comment from the Group Healing Q&A; I’m sure some of you have even shared the sentiment. Turns out, I was wrong. Constie, a regular priest forumer, author of the WotLK Lux et Umbra and long time theorycrafter, was kind enough to correct me and write up this lovely post in response to my question:

Just how the eff does Echo of Light work?, aka:

Why doesn’t Echo of Light stack?

Let me start this out by making it clear that I have tricked you; Echo of Light does in fact stack as it should. “But Constie,” I hear you exclaim, “I watched my combat text very closely during the raid last night, and I’m 100% sure that when I cast Greater Heal and followed it up with a Heal a few seconds later, the Echo of Light ticks got smaller, not larger like I expected!” Well, good news: you were, in this particular instance, not hallucinating. So, how can I claim that it’s stacking like it should while that’s also true? Let’s find out!

What Echo of Light does is, of course, to add a periodic heal over 6 seconds for [Mastery*1.25]% of the total value of any of your direct healing spells when they hit a target, overhealing included. As long as we don’t heal someone a second time during those 6 seconds, nothing strange or counter-intuitive goes on; each tick simply heals for ⅙ of [Mastery*1.25]% of the original heal.

When we do cast another heal and refresh the duration of Echo of Light, things sometimes look a little strange. If at 10 Mastery we were to get a Greater Heal in for 20000 healing, then a Flash Heal for 10000 3 seconds later, this would happen: Echo of Light would heal for 416⅔ for 3 ticks and then ~357 for another 7* ticks. The ticks got smaller! But that’s not the whole story. If we sum up all the Echo of Light heals, we get a total of ~3750 healing done – which does happen to be [10*1.25]% of 20000+10000. The ticks got smaller, yes, but the total healing done is nonetheless the same as advertised. What exactly is happening is that the game takes the total Echo of Light healing from the initial application (12.5% of 20000, or 2500), subtracts the healing already done by the ticks that have occurred (416⅔*3, or 1250), adds the healing from the refreshing spell (12.5% of 10000, or 1250), and then divides that by 7 [(1250+1250)/7= ~357]. In this way, the HPS output of Echo of Light is as even and consistent as possible.

Now, there are a couple of small notes to be made here. These are things that have such insignificant effects that they won’t affect your performance or gameplay in any way, and I mention them here only for the sake of completeness. Firstly, Echo of Light doesn’t actually heal for exactly as much as expected when refreshed – it’s not known whether this is a latency-related bug, something wrong with how the game calculates the size of the ticks, or a bit of both, but it’s off by about 1-10%, depending on what you’re casting and how. However, it quite often heals for more than it ought to instead of less, and it is a very small difference anyway, so it’s nothing to worry about. Secondly, the tick recalculation mechanic does mean that your healing from Echo of Light will get a tiny little bit staggered – in the example in the above paragraph, it would take one extra second to get all the Echo of Light healing from the initial Greater Heal than it would if Flash Heal hadn’t been cast. This, too, is such a small difference for such a short time that it’s guaranteed not to be worth worrying about.

And there you have it. Echo of Light functions just fine, and (possible overhealing and heavy Renew usage aside) you can safely take your Mastery tooltip’s word on what it does. If you found this class mechanics information sexy and exciting, Lux et Umbra is a forum thread just for you.

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Come on, die young.

* Since 4.0, when you refresh periodic spells, the refreshed duration will be a little bit longer than the original duration. This is to prevent them from getting “clipped”. Previously it was possible to, say, cast Renew over and over on a target and never have the first tick occur because it would always take place 3 seconds after the last application of the spell, but what happens now is that the current tick is undisturbed while the extra duration is added directly after it – the length of a refreshed periodic spell is the original plus the time left before the current tick. In our Echo of Light example above, the first 4 ticks are from its original application, and the last 6 are added by the refresh.