Prereq Chains for Improvised Magic Perk

[haligon]

I'm hoping to get clarity on how the Improvised Magic Perk from Thaumatology: Magical Styles (or Spell Defaults in general from Thaumatology) calculates penalties for spells that have a prerequisite like "6 Plant Spells" or "4 Water Spells"

Is every spell in the college considered part of the prereq chain, or is the caster at the maximum penalty no matter how many other spells in the college they know?

Opinions? I tried hunting the Forums to see if this had been discussed, but no luck.

[Say, it isn't that bad!]

Originally Posted by haligon

I'm hoping to get clarity on how the Improvised Magic Perk from Thaumatology: Magical Styles (or Spell Defaults in general from Thaumatology) calculates penalties for spells that have a prerequisite like "6 Plant Spells" or "4 Water Spells"

Is every spell in the college considered part of the prereq chain, or is the caster at the maximum penalty no matter how many other spells in the college they know?

Opinions? I tried hunting the Forums to see if this had been discussed, but no luck.

Alright, to give a theoretical example, let's look at the following (made-up) spells:

Sense Weebles
You can sense weebles.

Push Weeble
Prereq: Air Jet.
You may substitute your level in this skill for your ST when pushing a weeble.

Create Weeble
Prereq: Sense Weeble, Push Weeble.
You may create a weeble. This costs $200.

Destroy Weeble
Prereq: Create Weeble.
You may destroy a weeble. This automatically and immediately switches your alignment to Devil Bunny.

You wish to destroy a weeble, but you don't know Destroy Weeble. However, you have the Improvised Magic perk for the Weeble college, and can cast Destroy Weeble at a skill level equal to your highest known spell in the Weeble college, which is Push Weeble 30-, with a penalty equal to -4 (or 26-).

Edit: My math is incorrect. See Anaraxes' post below this one, and Planes' post on the correct prereq chain for Air Jet.

You also have put at least one point into Push Weeble, and know all of its *prerequisites. Push Weeble has a prerequisite of Air Jet. You also know Shape Air, Create Air, Seek Air, and Purify Air as the prerequisites for Air Jet.

Edit: The correct prereq chain is Shape Air, Create Air, and (Seek Air or Purify Air).

So Push Weeble has 5 prerequisites. You add each of its five prerequisites as a +1 bonus to your Destroy Weeble skill, giving you 30 - 4 + 5 = 31- to destroy the Weeble.

You monster.

* You don't actually technically have to know its prerequisites, but you really should have to.

[Anaraxes]

The prereq count is the sum of everything in the whole tree of prereqs-of-prereqsl when there's more than one path, it's the shortest of those paths (the lowest count). There's an official prereq count in the table starting on M223, but it's known to have a lot of errors. So you can decide whether you're going with the published table, because rules, or whether you want to do all the work of counting up those trees yourself. (Or trust someone else's work that's already done so.)

The Perk can never work out to a net bonus. The penalty is a base -4, minus the entire prereq count of the spell you're trying to cast. (In the OP's example, it would be -4 -6 for "6 Plant spells", or -4 -4 for "4 Water spells". Ideally, you'd find the minimum prereq set. But I've always been lazy and assumed that in such cases there always were 6 Plant spells (for example) that had no other prereqs. That might not actually be true in some particular case for a narrow and tall College.)

If (and only if) the known spell you chose to base the default skill from happens to be a prereq of the unknown spell you're improvising, you add a bonus of the known spell's prereq count. Since all prereqs of the known spell are also prereqs of the to-be-improvised spell, this bonus can never be larger than the penalty of the unknown prereq count. (Since the known spell is also a prereq, but isn't included in its own prereq count, the best you can do is a net -1, -4 for the base use of improvisation, for a total of -5. If the unknown spell has more than one immediate prereq, it will be at -1 for each of them, -4.)

In Say's example, the calculation should be:

Air Jet: 4
Sense Weeble: 0 {no prereqs}
Push Weeble: (Air Jet) + 1 {for Air Jet itself)
Create Weeble: (Sense Weeble) + (Push Weeble) + 2 = 7
Destroy Weeble: (Create Weeble) + 1 = 8

So, the skill for Push Weeble, 30, -4, -8 (Destroy Weeble's count), +5 (Push Weeble's count), for a net 23, rather than 31.

(Check the math; it's late here :))

[Say, it isn't that bad!]

Originally Posted by Anaraxes

(Check the math; it's late here :))

(Your math looks correct here, too; but it's kinda late here)

[haligon]

I don't disagree with the math, but I don't think it's covering the situation. Using those poor weebles as an example...

Sense Weebles - no Prereq
Push Weeble - Prereq: Air Jet.
Create Weeble - Prereq: Sense Weeble, Push Weeble.
Destroy Weeble - Prereq: *3 Weeble Spells*

Presuming our proto Devil Bunny knows Create Weeble at 20 and wants to finally cross to the dark side and destroy one, what is the ultimate Penalty?

Improvised Destroy Weeble - Base: 20-4-3=13 (Create Weeble-4-3 for prereqs)
Modifier for known Spells: +?
Is it +0? (No 'named' spells in prereqs)
+2? (Based on Create Weeble, there are 2 Weeble Spells in the Prereqs...and Create Weeble is effectively 'named' as part of the set "3 Weeble Spells")

Not a large difference to wipe out a Weeble, but if for instance the spell involved is Plant Form (Prereq: 6 Plant Spells), and the spell being used as a default is Essential Wood (also Prereq: 6 Plant Spells) the question becomes is Plant Form defaulting to
(Essential Wood -4 -6)
or
(Essential Wood -4 -6 +6)?
From Improvised Magic:
"However, if the known spell is in the unknown spell’s prerequisite chain, add a bonus equal to the known spell’s standard prerequisite count"

Is Essential Wood (or Create Weeble) "in the unknown spell’s prerequisite chain"?

[Plane]

Originally Posted by Say, it isn't that bad!

you have the Improvised Magic perk for the Weeble college, and can cast Destroy Weeble at a skill level equal to your highest known spell in the Weeble college, which is Push Weeble 30-, with a penalty equal to -4 (or 26-).

You also have put at least one point into Push Weeble, and know all of its *prerequisites. Push Weeble has a prerequisite of Air Jet. You also know Shape Air, Create Air, Seek Air, and Purify Air as the prerequisites for Air Jet.

So Push Weeble has 5 prerequisites. You add each of its five prerequisites as a +1 bonus to your Destroy Weeble skill, giving you 30 - 4 + 5 = 31- to destroy the Weeble.

You monster.

I don't think you can count BOTH the "Purify Air" and "Seek Air" though, since it's an OR as to which is required for Create Air.

"at -4, plus a further penalty equal to the unknown spell’s standard prerequisite count.
However, if the known spell is in the unknown spell’s prerequisite chain, add a bonus equal to the known spell’s standard prerequisite count"

You forgot the further penalty. Let's tally prereqs here, consulting M23-M24..

Sense Weebles (0)
Seek (or Purify) Air (0)
Create Air (1, not 2)
Push Air (2)
Air Jet (3)
Push Weeble (4)
Create Weeble (6)
Destroy Weeble (7)

30-4=26 is the starting point for spells defaulting from a 30-point skill, yes... but then you need to look at the spell you're trying to default...
since Destroy Weeble has 7 prereqs, you also apply another -7 bringing it to 19
The known spell you are defaulting from (in this case Push Weeble) has a prerequisite count of 4 so you only get a +4 bonus, for knowing the prereqs, so the default is only +23

[Anaraxes]

Originally Posted by haligon

if for instance the spell involved is Plant Form

In the cases where there's more than one possible prereq path (Spell A OR Spell B, or more commonly "any N spells"), then the prereq cost is the minimum possible path. In this case, both Plant Form and Essential Wood have the same prereq count, 6. When building your druid, you could pick up one spell and count it as one of the 6 prereqs for the other*. But either way, the base spell has a "known spell in my tree" count of 6 prereqs + itself = 7.

The thing not to do is try to find the maximal path and count spells multiple times. So, it would be incorrect to calculate as "Plant Form is based on five spells plus Essential Wood, so that's six; Essential Wood also needs six, so that's twelve, plus the one for Plant Form is +13." That's making the spells harder than they need to be.

GURPS Magic Spell Charts is a free download. It might be useful to take a look at these. They're basically the spell prereq chains drawn as a "graph" (in the math sense, a bunch of boxes and lines connecting them), so it's easier to see the various alternative ways to get to some particular spell. You can just count along a path and find the shortest one. Or try drawing your own picture. (It's fun! It's like drawing dungeons... kinda.)

--
* this example is weird for a different reason. If you think of learning spells in order, this mage has to know 8 Plant spells total. 6 + Plant Form, then Essential Wood (prereqs already in place); or 6 + Essential Wood, then Plant Form (prereqs already in place). I can imagine someone wanting to count their Plant Form as a prereq for Essential Wood, plus five other spells, and also count Essential Wood as a prereq for Plant Form, plus five other spells, so only 7 spells total. I wouldn't allow that sort of loop. That's a pool of six that leads out by two paths, one to EW and the other to PF, each of total cost 7.

[Plane]

Originally Posted by Anaraxes

In the cases where there's more than one possible prereq path (Spell A OR Spell B, or more commonly "any N spells"), then the prereq cost is the minimum possible path.

Yeah otherwise it's broken and you'd get bonuses. Example:

Spell A (prereq 0)
Spell B (prereq 1: A)
Spell C (prereq 2: A+B)
Spell D (prereq 3: A+B+C)
Spell E (prereq 4: A+B+C+D)
Spell F (prereq 5: A+B+C+D+E)

If you want to use F at default it's -4 base -5 prereqs = -9 total penalty.

If you default it from E, due to E having 4 prereqs it is +4 = -5 net modifier.

If however we counted FOUR of the FIVE spells in the prereq chain that have prereq counts (Spell A is useless, it has no prereq count) we would get +1+2+3+4 = +10 making the net modifier +1

That's broken since it allows you to use the spell at default at a higher skill level than you would with 1 point in it.