Cast When Damage Taken Full Mechanics Explanation

I am trying to compile a full explanation to all the mechanics for "Cast When Damage Taken" gem.

I am tired of googling and forum searching to find one unclearly asked question/answer at a time.

I am going to arrange all the possible setups I can think of below, and ask questions where I don't understand the mechanics, letting people comment, add and explain. I will modify this post as more information shows up in the thread below. If you have the answer to a question, a new setup I missed, or disagree with notes, please comment clearly with responses.

Below, I will refer to the "Cast When Damage Taken" gem as "CWDT"


FACTS I UNDERSTAND:

- CwDT does nothing by itself, it just gives every linked spell gem a damage counter and a damage counter cooldown which triggers it when it reaches the damage threshold.
- Each counter works independently 'per skill gem' and cooldowns work 'per spell'.
- While the spell is on cooldown no other gems of that spell can trigger and their damage counters are paused


POSSIBLE SETUPS:

For each setup, lets assume I get constantly hit by the same invincible enemy (so the proc spells won't kill him, slow him, stun him, ... whatever) for 1 damage each millisecond in rapid succession. Let us also assume that I have all level 20 CWDT gems, which add the 3240 damage quota to each gem.

What I want to know, is the order in which the linked spells will trigger, and at what times.

Setup1:
Item1: CWDT + Firestorm
Notes: --
Timeline:
time == 0 ms : nothing happens/
time == 3240 ms -> Firestorm damage counter reaches 3240 damage , Firestorm procs, Firestorm cooldown begins
time == 3241 ms to 3490 ms -> Firestorm damage counter remains at 0 because of cooldown
time == 3491 ms -> Firestorm damage counter reaches 1

Setup2:
Item1: CWDT + Firestorm + Fireball
Notes: This behaves as two simultaneous independent copies of Setup1
Timeline: 2 simultaneous copies of timeline 1 are running.

Setup3:
Item1: CWDT + Firestorm1 + Firestorm2
Notes: Has two spells of the same type, linked to same CWDT Gem
Each firestorm has it's own damage counter and damage quota. Thus having both firestorms will enable both to cast but on staggered timelines.

Timeline (please correct if I am wrong):
time == 0 ms : nothing happens/
time == 3240 ms -> reach 3240 damage on both gems, Firestorm1 procs, Firestorm2 can't because of 250ms cool down
time == 3241 ms to 3490 ms -> both firestorm gems are now on the 250ms cooldown. Cooldown applies to both gem's damage counters. Firestorm1 counter remains at 0 damage, Firestorm2 remains at 3240 damage for this whole time.
time == 3490 ms -> Firestorm2 procs because it's counter is still at 3240, which equals the quota. Now both gems are on the 250ms cooldown again.
time == 3491 ms to 3740-> Both Firestorm1 and Firestorm2 have 0 damage on their respective counters.
time == 3741 -> Firestorm1 damage counter reaches 1, Firestorm2 damage counter reaches 1.
.
.
repeat

Setup4:
Item1: CWDT1 + Firestorm
Item2: CWDT2 + Fireball
Notes: EXACTLY the same as Setup2.
Timeline: EXACTLY the same as Setup2.

Setup5:
Item1: CWDT1 + Firestorm1
Item2: CWDT2 + Firestorm2
Notes: EXACTLY the same as Setup3.
Timeline: EXACTLY the same as Setup3.

Setup6:
Item1: CWDT1 + Firestorm1 + Fireball1
Item2: CWDT2 + Firestorm2 + Fireball2
Notes: This behaves like 2 simultaneous copies of Setup3, each copy does not interact with the other.
Timeline: See Setup3 timeline, imagine 2 copies of the timeline running simulateously, one for each gem.

Setup7:
Item1: CWDT1 + Firestorm1 + Fireball1
Item2: CWDT2 + Ice Nova + Glacial Cascade
Notes: 4 different spells, 2 different CWDT gems
Timeline: 4 independent simultaneous copies of Setup1 timeline.


BIG REMAINING OPEN QUESTIONS:
1) How does increased/reduced duration support gems / Tree nodes effect the cooldowns inherent to the CWDT gem?
ANSWER: They DO NOT.

2) How do the cool downs stack?
ANSWER: They DO NOT. If you have a gem with it's own cooldown, then the spell's innate cooldown + the CWDT cooldown will both start. The spell's damage counter will remain frozen until the longest of any cooldowns is over. Note that the longer cooldown is likely the spell's innate cooldown, not the 250ms cooldown associated with the CWDT link.

3) Does each supported skill gem have it's own damage quota, or is the damage quota built into the support gem?
ANSWER: Each skill gem does have it's own damage quota, damage counter, and SHARES a cooldown with all other spells of the same name, also linked to a CWDT.

4) (NEW!!) How does the damage counter work when the proc is triggered and thus the cooldown also triggered?
TENTATIVE ANSWER: It is my understanding that the instant a spell is cast, then all the gems of the same name are on cool-down. This means their damage counter's are also on cool-down. So receiving more damage during this time, does not help you cast the spells any faster.

5) (NEW!!) How does the damage counter reset work on cast proc?

In the real game players receive large amounts of damage, and would over the damage quota:

If I have CWDT (level 20) linked to a Firestorm, and I get hit for 5000 damage in 1 hit -> My damage counter reaches 5000, thus the spell will proc, the counter will reduce, and go on cooldown. What is the damage counter now set at?

Is it set to 1760 (== 5000-3240) OR is it set back to zero (thus wasting the extra 1760 damage i just received to go over the quota)?




Special thanks to:
raics
Ludvator

For correcting me where I have been wrong so far. (Please keep telling me where I made mistakes)

Ive read only this part

"

FACTS I UNDERSTAND:

Supported spells of the same name are put on their own Cooldown for 10 milliseconds
Cast when Damage Taken has a cooldown of 250ms
Spells have their own cool downs.

which is completely wrong:)

* CwDT gem itself doesnt have any cooldown
* the supported skills are put on 250ms cooldown when triggered (which applies globaly to all other copies of triggered spell)
* some spells have their own cooldown (enduring cry, cold snap) that cant be bypassed by cwdt
* once the spell is on cooldown, CwDT damage counter is put on hold
* (10ms cd is related to another trigger gem, cast on crit)

EDIT: cooldowns are not increased or reduced by duration modifiers

thats all for now

havent read the rest of your post, dont have much time now. But I can say that searching forums for mechanics info is not a great idea (unless you know, what posters know what they are talking about:)). PoE is a complex game, and lot of people speak lot of bullshit, especially in general discussion. Wiki is a good source of mechanics information, another one is a Mechanics thread you can find on forums

First you should understand this:
- CwDT does nothing by itself, it just gives every linked spell gem a damage counter which triggers it when it reaches the damage threshold.
- Each counter works independently 'per skill gem' and cooldowns work 'per spell'.
- While the spell is on cooldown no other gems of that spell can trigger and their damage counters are paused which is important for spells with long cooldowns..

Setup3:
Like I said, you essentially have two firestorms with the counter of same size, therefore, same hit will top up both counters and both copies of firestorm will trigger. However, only the first one will go off because it will also put firestorm spell on 250ms cooldown when it does, so the second one won't be able to.

Setup4:
Not much philosophy here, each spell has own cooldown and they have zero interaction with each other. If CwDT supports are of same level it's the same as putting both spells on same CwDT gem.

Setup5:
If CwDT gems are of same level it's identical to setup3, if not, each has own counter of different sizes and will go off independently, unless the same hit happens to top up both counters.

Setup6:
Combine 4 and 6.

Questions:
1) You can't affect cooldowns in any way.

2) They don't 'stack', they have no interaction with each other, if CwDT triggers an enduring cry it will have a 4s cooldown, not 4,25s.

3) Answered.

Added questions 4 && 5
Reworked the timelines in the setup section
This is great!
Thanks for the help

You can fix the timeline on setup3 a bit, damage is first applied to all active spell counters, then all spells which are on counters that topped up are triggered in sequence, and after that all counters which topped up are reset (or were reset prior to triggering, both of those aren't terribly important for an end user).

You can also add the case for setup 5 where you have CwDT gems of different level.
Let's use cold snap and make hits large to answer question 4 and 5 at the same time:

Cold snap 1 has a counter of 700
Cold snap 2 has a counter of 1000
...
You receive a hit for 500 damage
...
Counter 1 is on 500
Counter 2 is on 500
...
You receive a hit for 400 damage
...
Counter 1 is triggered and reset
Counter 2 is on 900
...
Cold snap 1 gets cast which puts cold snap on 4s cooldown
...
You receive a hit for 300 damage
...
Counters are paused, nothing happens
...
Cooldown passed
You receive a hit for 200 damage
...
Counter 1 is on 200
Counter 2 is triggered and reset
...
Cold snap 2 gets cast which puts cold snap on 4s cooldown
...
You receive a hit for 400 damage
...
Counters are paused, nothing happens
...
Cooldown passed
You receive a hit for 1200 damage
...
Counter 1 is triggered and reset
Counter 2 is triggered and reset
...
Cold snap 1 gets cast which puts cold snap on 4s cooldown
Cold snap 2 doesn't get cast because of cooldown

@raics

I appreciate the note on order of operations for the reset.

Also -> I like the additional setup you added with different level cast when damage taken gems, but feel I am running out of room in the original post. I will leave it in your response for now.