I come back with some more time to write out the ReIg approach. I call the spell
ReIg 20 Invisibility by Transparency to Light (R: Touch, D: Sun, T: Ind)
The main result of the spell is to make its object - a person, animal or thing - invisible.
It targets the light around that object - thus limits the effect to a height of about 6 ft and an area of 3 paces diameter (see Circle of Encompassing Flame, ArM5 p.140).
The light is just made to pass through the object, thereby carrying any species it carried before. That makes it slightly unnatural (like fire not burning a specific person) and magical, hence detectable as such by Hermetic magic.
(Base 4, R: Touch, D: Sun, T: Ind, +1 object can move)
The different spells to simulate invisibility are getting away from the question, and relying on assumptions that are vey very much saga-dependent. I'd suggest you discuss those in a different topic.
And IMHO, any Art combination other than PeIm should result in a more difficult spell than Veil of Invisibility, or with some sort of bigger different drawback/limit. PeIm should remain the best way to achieve invisibility -- otherwise you are shifting game balance.
The magical light of Invisibility by Transparency to Light, that passes through an object to make it invisible, interacts of course with MR.
A maga not suppressing her MR thus resists that light. If successful, the object's shadow is seen by her and cast on her robe, to be also seen there by a shield grog.
A magus turning himself invisible by IbTtL also has to overcome or suppress his own MR for the time of that spell working.
So even for an Ignem-Flambeau IbTtL comes with a trade-off.
How is the target individual? It's the light around the person. Shouldn't that make it group?
I consider a good rule of thumb when working out targeting, is to ask the question, "Is the target of the form?" If the human is made of fire or light, individual works. Otherwise I'd consider the target should be group.
A spell using a creative unorthodox technique or form to achieve a result should be harder to cast, and the target being group would achieve that.
It certainly shows, that the size of an individual is enough for the Ignem light spell IbTtL.
If you also wish to see, how T: Ind is sufficient for spells controlling a Form around an object of another Form for some time, see e.g.:
ArM5 p.124 ReAq 5 (R: Touch, D: Sun, T: Ind) Cloak of the Duck's Feathers
and many other Rego spells in the core book.
It does not appear to be a problem, that in CotDF the water around the object changes over the time of the spell effect.
If you wish to elsewhere continue the discussion started by @TimOB in your thread here, feel free to start that different topic. I so far just answer to questions and objections posed here.
