[RPG] Can Major Image and forced perspective make illusions appear larger

dnd-5eillusion

In the game I'm running, the PCs are about to embark on a wilderness trek during which they will be harried by an opponent who has access to the major image spell. This opponent seeks to distract, mislead, and delay them, forcing them to spend more time and resources reaching their objective than they would otherwise.

I am trying to determine what is possible within the constraints of the major image spell description. One thing that occurred to me is that, while the spell's effects are confined to a 20' cube, they might seem to be spread over a much greater area by tricks of forced perspective. For example, an illusory, 20' tall castle suspended in mid-air between the party and a distant hill might appear to be a full-size castle settled on the hill. Or a 20' wide illusion of a wall of flames might seem to be consuming a vast span of the horizon.

Or alternatively, an illusory pit might seem to be much deeper than 20' by "drawing" its walls and bottom in such a way as to make it seem deeper.

Can forced perspective be used with major image in this way?

Are there potential drawbacks to allowing this? Would any modifiers be appropriate for investigation checks to "disbelieve" them?

Best Answer

Disclaimer: I have no experience with this in a gaming context, but I do work in a visualization research lab that gives me some insight into what this entails.

Binocular (i.e., two-eyed) perspective and depth perception only works in the first place because the world is highly consistent in its geometry. The human visual system, from the pupil, through the retina, and up multiple sections of the brain absolutely requires these consistencies to interpret what it is seeing. When I move ten feet forward, the pattern of light striking my eyes from different objects changes in highly consistent ways that let me infer-- with very high confidence-- what the positions of those objects are.

I mention this because one of the weaknesses of forced perspective illusions is that they in some sense depend on the "accident" of a person standing at just the right spot to create the illusion, and the farther you get from that spot, the less effective the illusion becomes until it just falls apart. (Note that I have seen this with my own eyes using virtual reality equipment. It doesn't take very many steps away for perspectives to get messed up, even when forced perspective illusions are not the subject being modeled. I have seen people in shared environments argue over the size of virtual objects purely because they were drawn precisely for the perspective of Subject A, while Subject B was standing five or ten feet away.)

And as soon as the brain detects these anomalies, it will almost involuntarily focus attention on them until the anomaly is understood. Usually this will involve moving the head, craning the neck, walking around something, etc. Human brains don't like inconsistent visual data.

So if we think about this in terms of virtual reality, that brings to mind the following very probable limitations:

  1. If the image cannot move to respond to the movement of a viewer, it will almost certainly fail (or at least grant some significant mechanical bonus to the viewer on their Investigation check.) But this is not automatically insurmountable, because the image can move... as long as the caster is within 120 feet.

  2. For this type of application specifically, affecting more than one creature at one time will be difficult. It is very hard to convey in words how rapidly perspective anomalies add up, but they do, even over a few paces. I would interpret this as mounting bonuses with additional numbers of viewers.

  3. The more freedom someone has to walk around the illusion, or the more it depends on great distance, the faster it will break down. This makes things like "The floating castle above a hill" or "a wall a flame across a broad plain" are really hard to pull off-- when the subjects move, the illusion will inevitably break down.

  4. As a corollary, in enclosed spaces, this technique might be highly effective. The illusion of a bottomless pit passes my smell test, for instance (subject to 2 above.) The illusion of a vast cave containing a great dragon at the end of a passage, likewise. Possibly even the castle on a hill or wall of flame if seen from inside a building through a single window.

  5. Do not try this against ettins, Tiamat, or other multi-headed monsters. They will eat you.

The real answer to what's possible depends on a bunch of trigonometry for each application. But the five points above, I think, are a useful set of guidelines for what's possible, and what will keep your players or your GM from crying foul.

(Side note: The acoustic interactions, if any, will suffer the same basic problems but, for humans, much less severe. I would not assign a penalty to an illusion that incorporates an acoustic effect, because it would not be more difficult for the caster and probably would not help the subject pierce the illusion.)