What is base damage?

Hey guys.

I was playing around with the ice shot skill, but I don't understand how some things work. I also am a little confused as to how the various damage multipliers are used.

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For the sake of simplicity, lets just talk about physical damage here. Lets consider two kinds of passives:
Projectile damage increases,
and physical damage increases that work with bows.

So how are these used to calculate the default attack damage?

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Now, in the context of ice shot:

1. What is the base damage?
Is it calculated the same way as our default attack damage?

2. When are the modifiers applied?
For instance, considering the bow above with no passive skill modifiers, ice shot would deal 60 phys, 40 cold on hit and 100 cold on the blast. Now if we add modifiers, how will the damage be calculated? Do the modifiers apply to the attack of the bow itself, or to the damage the skill does?

Lets say we have 0% increased projectile damage, but 50% increased phys damage with bows. If the first scenario is true (if the modifier applies to the attack of the bow before the damage of the skill is calculated), this would mean that we do more cold damage with the skill by having increased physical damage, if the latter was true, our cold damage output would stay the same, and only our physical damage output would increase. This doesn't seem right to me though, since this would mean that we would do less damage with the blast as well, since the blast converts every bit of physical damage into cold, so I'm not saying I think this is the case, I'm just trying to find a good way to make the question clear.

With ice shot, depending on how the modifiers are applied, will the blast still take into consideration the projectile damage increases that served to increase the physical damage of the shot?

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Thank you in advance for the help.

in short, these are the basic rules:

* all applicable "increased" damage modifiers stack into one multiplier
* converted damage benefits from original type mods (phys in this case) as well as from the new type (cold)

lets say you use Ice shot with phys bow

phys part will be affected by: phys (with bows), projectile, attack damage
cold part will be affected by: all the above + elemental/cold (with weapons)

cold explosion (which is btw also phys/cold) also has an AoE property

Just want to add to what people have already said by mentioning scaling some damage scales better than others and the calculations involved in that are not all ways intuitive and as a side issue the tool tips are not to be trusted and tend only to give numbers based on a single instance of what ever skill you are using.

So scaling

Some types of damage scale better than others for example physical damage can be boosted my lots of different things(flat damage, % damage, weapon damage,crit damage and class damage(like one handed damage) etc.So are said to scale better than say elemental damage which is a lot harder to boost and has a lot fewer options for improvement.

Read this(see under) its a bit complex and in some places outdated but it gives you a solid understanding.

http://pathofexile.gamepedia.com/Damage
and this
http://pathofexile.gamepedia.com/Physical_damage