I'm an idiot.
Ho Ho's comment to my post on haste raised a question in my mind about how dots work. I have, for some time now, and without any apparent empirical evidence to support my view, been operating under the assumption that the first tick of a dot happened instantly.
If you are nodding encouragingly at this point and waiting for me to get to the punchline, you too are an idiot.
What Ho Ho knows, and what every self-respecting shadow priest should know (especially the ones who blog about it) is that the first tick of damage occurs only three seconds after application. This is true of shadow word: pain, vampiric touch and devouring plague. All three spells go on to tick every three seconds till they expire, the final damage occurring at the exact moment of expiry.
Even mind flay, which ticks every second, takes a full second to kick in.
If I'd stopped to think about it, I might have worked it out. But of course, I didn't: I'm an idiot.
What this means is that I have to completely re-evaluate my approach to overlapping spells.
When I talk about overlapping, I don't mean spamming the spells every time you get the chance. The issue is about how accurately we can maintain 100% uptime of our dots. In an ideal scenario, your shadow word: pain would tick every three seconds for the entire duration of a fight. But there are so many variables that make this difficult, if not impossible. Lag, movement, pushback, resists and the sheer mind-boggling complexity of juggling spells all contribute to an imperfect spell rotation. Sometimes, you're faced with a choice between reapplying a dot just before it finishes, thereby 'clipping' the end of the existing dot, or taking a different action and allowing a short period of down time.
Take a look at this first chart. This, in my ignorance, is what I thought happened when you clipped. The pink line is the first dot, the green is the second. Each vertical line is a tick, and the black bit is what's clipped.
Omg, what a tit. You see what happens there? Without realising it, I had created a fantasy world in which clipping actually improved dps — if a dot ticked instantly on application, you could manufacture shorter tick times every application at the expense of a little mana (more casting over time).
If I had ever stopped to think about this, I would have realised how unlikely it was.
So, as you probably know, this is what really happens:
Just as Ho Ho said, you could go five seconds without a tick, instead of three.
The consequences are horrendous. I've even done some maths to prove it. For shadow word: pain first, over a five minute fight, if you consistently clipped one second off the spell, you would loose eight ticks or 8% of damage.
Vampiric touch is worse, as it needs casting more often, potentially resulting in more clipped dots. In the same fight, once again consistently clipping one second off the spell, you would loose 14 ticks or 14% damage.
Perversely, if you're gonna clip at all, it's better to clip more time than less (up to just under three seconds), as this results in a smaller gap between ticks. But this would use up more mana and it still eats into your dps.
The conclusion, if you haven't reached it already, is to be perfect. And if you can't manage that, it's preferable to let your dots expire than overlap. I'm going to have to go back and edit my haste post, (although I don't see how this affects my overall scepticism of haste for shadow priests).
There's promise of some relief from this situation in Wrath. A tier 9 talent, pain and suffering, currently enables mind flay to refresh the duration of shadow word: pain. Notice it says refresh, not reapply, so we would no longer need to worry about timings. Mind flay would simply ensure the spell kept ticking along nicely and indefinitely. With one less dot to worry about, spell rotations could get a whole lot more manageable. We'll have to wait and see.
Huge thanks to Ho Ho for putting me straight on this.
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