Illusions, lighting, and reflectance

[toucanbuzz]

This is how I've played illusions for the last 20 years, and it seems consistent with 3rd edition's 4-part series on illusions and 5th.

1. Illusions aren't real and don't create real, physical objects, so they don't and can't "block" anything, including light. They are figments of your imagination, tied to magical beacons that keep trying to fool you even when disbelieved (hence the sensory residue that persists until the spell ends).
2. Illusions are only as good as the victim's mind (see mirror discussion below).

For example, an illusionary tree in a desert won't protect you from sunburns or allow you a cool place to rest. If you see one, you might try a short rest underneath it. Your mind tells you trees provide shade, ergo you can rest under it. With illusions, your mind is tricking you into believing the illusion operates with the same physics as the rest of the world. But, your mind cannot make the sun stop burning your skin, nor the heat to stop searing your lungs, and so on.

Keep this in mind and we'll go along.

#1-3. By default, the light is not "blocked" because there is nothing real and physical blocking the light. So if Alice is in a dungeon and around the corner there's an illusionary wall blocking a torch, Alice doesn't know the wall is there. It isn't real. It cannot block torch light, nor heat or crackle. Because her mind isn't being fooled (yet), she actually might see torchlight from up ahead and come around the corner to investigate. When she arrives, the area is still illuminated (because the wall isn't real, it doesn't stop anything), yet she sees a wall. She might get suspicious, or she might believe a wizard put up magic to illuminate the area. Who knows.

But, a good illusionist will make sure he doesn't screw up and give the victim tips there's something awry.

Now, let's try something a bit trickier. Let's say Alice sees the Illusionist in front of the torch and his disco ball. He declares "goodbye Alice. I'm sealing you into this corridor for eternity with my wall of stone!" Then he casts the illusionary wall between her and the light. Alice's own mind is now her enemy. She knows walls block light. So her mind tells her she's now sealed into a corridor of darkness. Until she interacts with the illusion, she'll continue to believe the illusion in this way. If you were to teleport a stranger into the situation who didn't know the illusionary wall was cast to "seal her in darkness," he'd see a wall and illumination as above, and he'd see Alice fumbling around as if she were in the dark.

It's all in your head.

#4-6. Yes, you can make an illusion of a mirror, but remember, it's not real. It's only as good as the viewer's mind and doesn't actually reflect anything because...it's not there. It's all in your head.

So, the mirror only works as good as the target believes. If you approach the mirror with your torch, you know mirrors reflect light so you'd see yourself with a torch. Your mind would create the illusion of yourself in the mirror even though it's not really there, and light isn't being reflected and so on. If you failed a save and used a hand mirror to look around a corner, it would be blank (unless you'd been around that corner). Why? Because your mind fills in the reality, and only what you know. If you were using the mirror and a friend quietly walked in, you wouldn't see them in the mirror until you became aware of who they are and what they're wearing. Your mind makes it up and fills in the blanks.

And, if you're from a culture or a creature that has never seen a mirror, you'd see nothing but blank glass because your mind can't fill in those blanks. So mirrors generally make really bad illusions, but no, you can't create an illusionary mirror for your pal Jake to peek around the corner with. You could create an illusionary mirror, however, to shove in the face of Ms. Medusa and possibly make her believe she turned herself to stone, assuming you convince your DM that Ms. Medusa isn't stupid and knows exactly what a mirror is and does.

#7. Each illusion spell specifically says how it is defeated. So unless it says "being told" causes the spell to fizzle, nothing happens.

 

[Ovinomancer]

I think if you have an illusion and it doesn’t behave nearly identical in all ways to the regular object then it’s not really a magical illusion. I think There can be no consistent scientific Phenomenon that can be observed or not observed to indicate if something is an illusion. However each illusion has small flaws that when carefully observed can indicate its an illusion. If those flaws were consistent then all a character would need to do is look for such phenomenon and it would indicate you are viewing an illusion. That clearly isn’t the case.

So illusory mirrors reflect. Illusory walls block light. Etc. once you know it’s an illusion the magic is broken and the light passes through and the reflections stop etc.

This leads to really weird situations, like a hallway where some people see it fully lit while others can't see because it's dark and still others are using darkvision. Or even a Drow not suffering from daylight sensitivity because there's an illusory ceiling overhead. Strangeness!

My rule of thumb is that illusions can only add, they can never subtract. Well, except for light, they can't add light.

 

[FrogReaver]

This leads to really weird situations, like a hallway where some people see it fully lit while others can't see because it's dark and still others are using darkvision. Or even a Drow not suffering from daylight sensitivity because there's an illusory ceiling overhead. Strangeness!

My rule of thumb is that illusions can only add, they can never subtract. Well, except for light, they can't add light.

I don't find any of those things strange. What I find strange is trying to make sense of investigation checks on illusions to disbelieve them if illusion magic involves scientifically repeatable phenomenon that always occur that would make them so easy to spot there would be no doubt that you could spot it.

 

[Ovinomancer]

I don't find any of those things strange. What I find strange is trying to make sense of investigation checks on illusions to disbelieve them if illusion magic involves scientifically repeatable phenomenon that always occur that would make them so easy to spot there would be no doubt that you could spot it.

Riiight... Did I miss something where that's a thing, or something? I mean, other that something you mentioned without preamble above but didn't follow up on?

 

[FrogReaver]

Riiight... Did I miss something where that's a thing, or something? I mean, other that something you mentioned without preamble above but didn't follow up on?

Huh? You going to have to be a little more specific cause I got no idea what you are talking about.

 

[Ovinomancer]

Huh? You going to have to be a little more specific cause I got no idea what you are talking about.

Can't help, must be one of those illusion things.

 

[FrogReaver]

1. Alice is standing in a sealed corridor with a light on the other end. Bob the Illusionist, standing at the other end of the corridor, casts a Minor Illusion (or Silent Image, or something) to create the image of a wall that covers the entire cross-section of the corridor. Does this mean that Alice is now standing in darkness because the wall blocks the light?

The spell let's you create an image of a wall and allows an action with an investigation check to disbelieve the illusion. My take is that this means the illusion appears to be real in every way imaginable unless you can pass your investigation check. That check is what determines if something is unusual about the illusion which can be narrated as any number of phenomenon or inconsistencies with the illusion. So if you ask me as the DM a bunch of questions trying to figure out if it's an illusion before making an investigation check then every one of those questions will be answered in whatever way would be consistent with that object actually being there. After you pass your investigation check I will come up with some other phenomenon to describe as finally indicating to you that this particular object was an illusion.

To Alice, she would be in the dark, to Bob who knows it's an illusion he would see Alice in the light. So if you ask Alice or anyone that doesn't know it's an illusion if light passes through the answer is no. If you ask Bob or anyone that does the answer is yes.

2. If not, then consider a more normal case where Alice is looking at Bob in a large room and Bob casts a minor illusion of a wall in front of him to give him cover. Presumably in this case the illusion WOULD block the light that's reflected from the light source off of Bob (if the illusion generated its own light but didn't also stop the light reflected off of Bob, then Alice would just see the wall superimposed on Bob (and the background), since she's seeing both the light from the illusion and the light reflected off of Bob. So does that mean that the illusion can "tell the difference" between light reflected off of Bob and light coming directly from the light source, so it will block one but not the other?

If Alice hasn't yet passed her investigation check then everything would indicate to alice that there was a wall actually present there. Presumably Bob knows it's an illusion and everything will appear to him as if there is no wall there.

3. If so, consider scenario (1) again but this time, the whole corridor is lit by a light source that is far away, and whose light is reflected off, say, a disco ball at Bob's end. Thus every ray of light that gets to Alice's end of the corridor (without the illusion) was reflected off of some surface at Bob's end. Does this change the answer to question (1)?

no.

4. Can you make an illusion of a mirror, and will it reflect light normally? So you could make an illusionary mirror to peer around corners?

Kind of. As long as you believe it's an illusion then it will behave exactly like a mirror. Personally I assume the caster knows his spell is an illusion and thus he could never get such a benefit from such an illusionary mirror, however other players in the party could (at least till someone tried to grab said mirror).

5. If illusionary objects don't reflect light normally, then the only way illusions could work at all is if they *gave off* light that matched whatever a viewer would actually see when looking at it if it were real. Under that interpretation an illusionary mirror placed across a corner would still show what's around the corner.

Whether the illusion actually reflects light or emits it's own isn't important unless there is a way for a player to observe the difference. I know of no way so I can't say which the illusionary mirror that's believed to be an illusion actually does. However, if there was a way to know if the illusionary mirror reflected or emited it's own light then it would behave exactly as the a real mirror until it's known to be an illusion.

6. Another interpretation that I've seen is that the illusionist decides what the mirror shows - i.e. the illusionist is creating the image. This would imply effectively that (even in a normal case) the illusionist is figuring out what the illusion should look like given the ambient lighting and creating an illusion that shows that. (In other words, the illusionist has repurposed part of his or her brain as a GPU.) Note that this would mean that the illusion would still need to change as ambient light changed (and the illusionist would have to be initially aware of the positions of any light sources, especially if the object the illusion was being made of exhibited high levels of specular reflection).

Presumably the illusionist could decide and the magic can take care of the details.

7. If you know that an illusion is not real (e.g. because the illusionist who made it told you, or because you saw something pass through it) but you didn't actually spend the action to make the check to examine it, is it still faint/translucent for you? (What if someone who claims to be the illusionist tells you it's an illusion, and it actually is an illusion, but the person who told you isn't the illusionist? If this counts as knowing, it would be easy to get around illusions by having your friend "claim to be the illusionist" and "tell you it's an illusion" every time you see something that might be an illusion.)

The rules only provide 2 ways to determine something is an illusion. Simply believing it's not because someone told you is apparently not adequate for you to cease to be effected by the illusion. You must either see something pass through it or pass an investigation check regarding it.

 

[Imaculata]

How I've always ruled illusions, is that they 'seem' to behave like a real object. An illusory wall doesn't block light, but it 'seems' to block light as if it were a real wall. Similarly, if you throw a rock at an illusory wall, it doesn't bounce back, but it 'seems' to bounce back. To the person affected, there is no difference with a real wall. But to those unaffected, the rock passes clean through the illusion, as does the light.

As long as the illusion affects you, you can throw a rock down an illusory abyss and see it fall all the way to the bottom, even though in reality the rock falls on the ground in front of you. It becomes more difficult for the spell to fool its victims when they interact with the illusion, which is why in most editions the person in question gets an extra check when this happens. What happens for example when you try to jump down into an illusory pit and fail to disbelieve the illusion? How I've ruled it, is that the victim almost falls down, but finds themselves clinging to the side in a panic, as dirt falls down below them. They find themselves unable to make that jump, because it just looks too real.

 

[Ovinomancer]

How I've always ruled illusions, is that they 'seem' to behave like a real object. An illusory wall doesn't block light, but it 'seems' to block light as if it were a real wall. Similarly, if you throw a rock at an illusory wall, it doesn't bounce back, but it 'seems' to bounce back. To the person affected, there is no difference with a real wall. But to those unaffected, the rock passes clean through the illusion, as does the light.

As long as the illusion affects you, you can throw a rock down an illusory abyss and see it fall all the way to the bottom, even though in reality the rock falls on the ground in front of you. It becomes more difficult for the spell to fool its victims when they interact with the illusion, which is why in most editions the person in question gets an extra check when this happens. What happens for example when you try to jump down into an illusory pit and fail to disbelieve the illusion? How I've ruled it, is that the victim almost falls down, but finds themselves clinging to the side in a panic, as dirt falls down below them. They find themselves unable to make that jump, because it just looks too real.

Again, I strongly disagree that illusions are able to remove things from reality. An illusory wall can't block light, so will instead have a feature that appears to emit the light.

As for illusory pits, well, they are illusions. If you jump into one, you get a surprise and some confusion as you appear to stand on empty air. Probably get advantage on the INT check. There's no mechanic that says you can't walk through an illusionary wall or stand where there's an illusion of a pit. Illusions are already powerful enough that they don't need strange mechanics like this. You just can't drop a silent image of a pit and have it foil all attenpts to cross it without a successful save. A badly placed illusion is like a badly placed fireball -- it doesn't work as well as you hope. If your opponent tosses a rope onto the illusory pit, it lies on the ground because the illusion can't make it disappear. You still got the round or two of delay, which is okay.

 

[Dausuul]

Dang, that is one hell of a first post. Well done.

So, to the questions you pose:

1) I would have to say that Alice thinks she is in darkness. The illusion does not actually change whether there is light or not; Carol, standing next to Alice, successfully made an Investigation check to see through the illusion, and Carol can see fine. But the illusion is causing Alice's brain to not perceive the light.

2) N/A.

3) Not exactly. Alice's brain is having the reflections off the disco ball edited out. But it also has new reflections, off the illusory wall, being edited in! So Alice can still see.

4) Yow. This is a tricky one. I originally said you couldn't trust the reflection in the mirror... but I think I have to change that answer (due to the difference between illusions and phantasms). Yes, the illusory mirror will work. The illusion is telling your brain what to see, and while you are not positioned to see around the corner, the illusion is.

5) N/A.

6) I don't like this interpretation, mostly because I can't conceive of a human brain doing that much conscious computing work in real time, but also because it gives too much control to the illusionist. The illusion itself is the GPU.

7) This is a more straightforward rules question, and I think the intent of the spell is reasonably clear: If you see something physically pass through the illusion, you see through the illusion and no check is required. Otherwise, you must make an Investigation check. If a friend sees through the illusion, they can either tell you "Hey, that's not real" (essentially telling you "Hey, make an Investigation check"), or they can just lob a rock at it.

The question then becomes, if the illusion can edit your brain to erase light passing through it, why can it not do the same for a rock? My answer is that the illusion "editor" lacks memory; it's editing the real world, not simulating an entire alternative reality. It can hide what is behind the illusion at any given moment, but once the rock passes through, it no longer considers the rock as a thing to be edited out. Your brain recognizes the discrepancy and becomes able to distinguish the false input from reality. Making an Investigation check means looking for similar failures of the "illusion GPU."

Note that some spells, like phantasmal force, co-opt the subject's brain to help with the editing process. If the wall were a phantasmal force, Alice's own mind would be compelled to figure out a reason why the rock had just appeared.