One of the limits is the limit of arcane connections which encompasses the limit of line of effect.
This is why spells need to perceive their Target at the time of casting. Simple example is that as you cast BoF, you need to be able to perceive the target, establishing a line of effect.
My question is, what kind of effect could break the line of effect? Target that property and deny it, thus making all effect's that rely on it, fail automatically.
I see some kinday of Rego, Creo or Perdo Vi that creates a shell or targets all in the room giving everyone the equivalent of short range magic flaw...
'Line of effect' may be from another game. Is it D&D?
But disabling all the senses of a potential spellcaster - perhaps with a Mentem spell or effect targeting A&A p.31 Common Sense - will certainly prevent him from casting most spells.
RoP:TD p.94 The Neriusian Portal creates a small regio, which can hide the caster. It protects her from all effects and beings which cannot penetrate the regio boundary.
Would you allow a magus with no visual (using well without light for exemple) to cast a spell, say at voice range, if he only has normal hearing perception ?
I would with something like eye of the bat, but otherwise I think I wouldn't.
If they can perceive the target yes. So if it is a person talking (or snoring) in a dark room certainly. trying to target rustling clothes or a board creaking in the floor- not unless it is a herbam effect targeting the board itself! Of course if the target is room then you only have to be able to perceive the room.
That's interesting, so if the target is doing the noise directly you would allow it but if he's doing noise indirectly you wouldn't.
Instinctively, I'd probably do the same but I have the feeling I wouldn't use the same rule for visual perception, say someone is invisible but you see the grass moving would you allow a spell ?
You can instead see the plants move while the target animal pushes through shrubbery, or the tracks it leaves in the snow on the ground. And you can also target an animal which you just locate from the noises it makes - like winnowing or beating the ground with its hooves. All that is sensing it.
Note, that MoH p.101 To See as Through a Plethron Distant, which magically recreates species, does not extend the targets for spells. Hence ArM5 p.127 Eyes of the Bat, which also magically creates species, should neither.
I would allow "deductive perception" to work. So, I'd allow you to target a person who's invisible but leaving footprints in the mud; or someone you hear walking on the other side of the wall, even if you hear the sound his boots create from the floor and not his joints creaking. Otherwise, it starts to get very tricky: can you target someone who's swathed in clothing to the point you can't see even a little bit of skin with a PeCo spell without requisites?
Remember that invisible things still cast a shadow! If you can spot the shadow, you can aim the invisible dude who drops it. My troupe had escalated a lot the military race of invisibility. Magi going invisible use to cloud the sky to avoid shadows, magi fighting invisible people under clouded skies use to cast bright lights to create shadows or perdo or rego auram to remove or move the clouds...
grass moving is a poor example in my opinion. Are we talking footprints in short grass, a patch of swaying grasses that are hi high? A large patch where something is crawling through? To me you have to be able to discerne more than "there is something there" footprints- okay, you clearly are dealing with one person. Swaying high grass- is it an invisible person, a small animal...?