Hi all!
Good morninnnng 
I have some question.
I wanna invent some spell dispelling the imaginem (its, for mainly purpose, for dispelling the invisibility).
I create my spell like that:
base general 4
+2 voice
= spell leve 10.
It can dispell spell less than (20+stress die) levels.
Let's say that a magus cast a PeIm to himself for invisibility.
Do i need to penetrate his parma imaginem?
There is no such thing as a "Parma Imaginem", but you do have to penetrate any Magic Resistance that the target has, which might include a Parma Magica.
(And, again - list the Technique and Form so others can follow/find the Guidelines more easily!)
(I'm not sure what "Base General 4" you are talking about, and am not going to dig to find it. I do know there is a General PeVi that would work, vs a "specific effect".)
HUm, i guess you don't know dispelling like being PerdoVim... my mistake.
I assumed that with the spell's name (in title) and "dispelling" you have had an idea of what i was talking about.
And i know for the parma magica, but "parma imaginem" is a short way to say "parma magica + form imaginem". Speed writing, you see...
Anyway.
And for the PeVi in case, its a general guideline: dispell a spell less than (level +4 magnitudes +stress die) level from any one Form (hermetic one or other)
And in the case of my spell, I fixe the base level at 4.
+2 magnitudes for voice
so the spell level is 10.
Then i can dispell up to 20+stress die imaginem spells.
Ok. My question is now: does the dispelling spell targets the magus who casted the spell i want to dispell (and then, needs to penetrate) or the spell cast (or his effects), and then, no penetration needed, because its not the magus who's thargeted, and a spell hasn't magic resistance.
Do you see, now?
If he only wants to put things in his right place, he can counter PeIm with CrIm and ReVi with an other ReVi spell to put "species in their right place". That would not destroy the previous spell, though, only counter its effects.
To destroythe other spell, yes, you need a PeVi spell. Quite useful in fact. Unravelling the (something) of Form (imaginem, in this case) it is called.
You can specialize vs imaginem effects, but that would only have an impact against spells that try to destroy or alter your own species. Like making you invisible 8when you do not want to) or preventing you from emiting sounds. It is not the most common specialization of Parma for sure, but it is not useless 
it will not help you to see people that have turned themselves invisible, nor detect illusions of any sort, though :9
Cheers,
Xavi
Xavi, read the title: Unravelling the fabric of imaginem
So, I knew for the spell.
My only question is: does it need to penetrate or not?
My first instinct is yes (and both Xavi and CH have implied or stated the same). It’s probably what I would go with were I SGing because it just seems logical.
However, I can see an argument for the opposite case. If your target is technically the spell (and not the person who that spell targets), then you might not have to Penetrate since spells have no magic resistance. I don’t particularly advocate this point of view, but I could see support for it.
Let's say:
PeIm on a species of house : house is there but not visible. I cast PeVi on the "not visible" house. I haven't to penetrate the parma magica of the caster. Right?
PeIm on a species of myself. Why my enemy would need to penetrate my Parma? My species is not me.
It depends on the interpretation of the your SG/Saga. I could see both views as valid to different people.
To me, your question seems simple:
When there is a magical effect on a person/creature, you need to penetrate the MR of that person to dispel it.
Who cast the spell to be dispelled, doesn't matter.
E.G.:Maga A, Mage B, Grog G.
Maga A cast invisibility on all three.
You need to penetrate Maga A's MR to dispell her invisibility spell. (MR = Parma+Vim, if using PeVi, CrIm would need Parma+Imaginem for MR, etc)
You need to penetrate Mage B's MR to dispell the invisibility spell active on him.
Grog G doesn't have any MR, so no need to penetrate to dispell "his" invisibility.
Notice you need to somehow sense your target if you want to cast a spell at it/him.
Yes, it's a pain to dispell invisibility... In our saga, we add +2 magnitudes to PeVi Guidelines.
So a character has an Imaginem effect cast on him (that makes him invisible).
You want to dispel that Imaginem effect using a PeVi effect.
Then, yes you need to penetrate his Magic Resistance, because the character is affected by your spell. If he was a (normal) magus then the relevant magic resistance would be 5*Parma + his Vim score --- your spell is a PeVi effect.
Also as Iudicium notes, you would also need to sense the character somehow, which could be problematic if he was invisible.
I believe you’re asking, “Is it correct to say that I do not have to penetrate the caster of the invisibility spell’s MR in order to dispel the invisibility on this house?†If so, then you are right, although I’m not sure why you’d even need to ask. Your target is probably the house (but some could interpret it as being the spell itself). It is definitely not the caster of the spell. Either way, the caster’s MR has absolutely nothing to do with it.
This is what we’re debating. The more I think about it, the more I’m pretty sure the species you emit are enough of a part of “you†that Parma protects them in the same way it protects clothes and armor.
I think this is the crux of the argument. It's pretty much accepted that Parma protects against having your armor or clothes destroyed, so it should also protect against having spells on you destroyed as well.
(Is it clear i don't agree ? )
Let's say, a magus cast a rego imaginem no his species, to let it in the front of me, when he is walking away.
Then, i cast "invisibility" on this species (basically a PeIm on it). Do i have to penetrate? I think not because its only a species.
I'll discuss it with my ASG but i don't think that "the image you emit" is you.
And in fact, that would be very good for me, if he agrees.