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For situations like Obscuring Mist and other vision impairment that overcomes Darkvision (and similar abilities), how have you successfully enforced that in Roll20?

My PCs would probably meta-game if they're simply told "you can't see in this area" and it kind of nullifies a major encounter coming up (and would be good to know for the future).

Assume Plus membership to the website (access to Advanced/Fog of War and Dynamic Lighting but not API).

Measurements of success:

  • Players are able to manipulate their token on a battle grid to explore the area of restricted sight
  • The GM can interrupt players when their sensory information changes without significantly interrupting play each round
    • Ideally, you would also avoid constantly moving them back
  • You have some method of providing information to players about what their character perceives
    • Ideally, only the player that "should" have the exact knowledge would have it, and others only what they can convey

Good answer(s) will follow the concept of Good Subjective and cover key points:

  • What has been your experience using the system?
    • i.e. player feedback, how you felt it worked
  • How you were able to restrict sight and also reveal information
    • Did you just tell the one player? Did everybody get to know when Bob was next to the bad guy?

Related question for physical play: For physical tabletop play, how do you restrict PCs' vision in situations like fog?

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  • \$\begingroup\$ To clarify - this is for temporary effects like Obscuring Mist and not more permanent effects like darkness or general fog of war? Further, was this effect present when the characters first entered the area or is it the result of an effect they triggered or was cast on them? Reason being, if they already got a look at the layout of the area, the fog only hides changes to this layout (such as a door opening/closing). Unless a creature is hidden, the characters know where it is, even if they can't see it. So in most cases, hiding this info from them is unnecessary. \$\endgroup\$ – cpcodes Oct 16 '19 at 22:35
  • \$\begingroup\$ The intent is sudden changes in the battlefield such as a spell being cast after they've seen the area. If it also applies to original exploration, that would be good to note. \$\endgroup\$ – Ifusaso Oct 16 '19 at 23:02
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So I have not used Roll20 in a couple of years (or play D&D over those years unfortunately), but here is what I used to do for my players in situations where they were in the dark without a light (or otherwise had very low visibility). I would turn on Dynamic Lighting but turn off Global Illumination. Then for each of their tokens I would go to the advanced setting, make them emit light out to 5 ft and unselect All Players See Light. That way each player could see what was right next to them but would not see the other players if they were far apart. I also asked my players to not move too quickly (especially outside of combat when they are not taking turns) and they were pretty good at stopping or asking questions when they butted up against something or someone. Also you might try to incentives them asking questions in ways that do not reveal information to other players. "What am I seeing next to me?" instead of "IS THIS HIDEOUS CREATURE HOSTILE!?". In 5E they have ways to reward players for role playing well which could be adapted for this.

The disadvantage of this approach in your case is that if you only want small sections of the map not visible you will have to create a grid of light sources across the map and then turn off the ones where the Obscuring Mist should be.

That is what I have done in the past. Looking at the current offerings I think Advanced Fog of War is what you want. First of all, you can set the sight for each token (under the token's advanced settings):

tokens that emit light, or have a set view distance, clear fog dynamically as they're moved across the Page.

And they can only see what is in view for their token:

The fog cells that a token has revealed is saved for each token on a Page, so each player potentially can have a unique view of the same Page they are all currently exploring.

My suggestion would be to turn on advanced fog of war, set each token's sight to 5ft or however far you think they should be able to discern, use the Reveal Areas tool to reveal the whole map, and then use Hide Areas when something like Obscuring Mist is cast:

If you choose to reset the fog or manually hide areas, the areas within the Token's set sight or vision range can not be hidden from view.

Lastly, if the players need more than what they can see from the map when they uncover an area, I would "whisper" to that player. To help me be able to whisper fast enough I type up (in a text editor or word processor) whispers for things I know I want to share at some point. Then I can cut and paste them to the appropriate players. Of course the players are always going to ask about something that I have not prepared before hand, but hopefully these are not too frequent and all the players are patient while I type the answers to these questions.

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    \$\begingroup\$ Upvoted, but this answer could be improved by referencing the features of "permanent darkness" and "explorable darkness" now available. \$\endgroup\$ – Kirt May 14 at 20:16
  • \$\begingroup\$ Thank you for the heads up. I will take a look at Roll20 today and learn about these. \$\endgroup\$ – goryh May 15 at 14:21
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There is two way to achive that:

1.) Reducing the view distance when PC reach the mists

There is an option in the advanced settings of the tokens which called "/ Multiplyer" which you can use to modify the already set light radius. Set this between 0 and 1 to reduce the distance and set back to 1 if they leave the mists. For this option every token should has their own light which can only seen by the owning player.

2.) Create light emitters and give them to the tokens

You can create other light emitters, which has different light, and you can bind them to players token with the group function. For this variant you need to set the tokens sight off, so the players could see only the light emitters. (I never tried this before, but I already planned this in the next month)

Recommendation:

I would recommend the #1) because its mutch more easy to create and organise. And as I already mentioned, I never tried the #2 before, but it should work too. For IE I would recommend to search after the roll20 wiki. I read somewhere there that by IE there are some exception. If you have some more information than you can whisper to the player or just say the others that they should do metagaming (based on the experience of the players).

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Dynamic Lighting, inside and out.

Blocking vision from the outside is simple enough - just trace a boundary around the edge of the obscured area, on the Dynamic Lighting layer.

Blocking vision within the area is going to be more tricky, but it can be done by drawing a grid of lines on the DL layer, inside the obscured area, effectively cutting it up into small cells of viewable areas. It should look a bit like a pizza cut into squares.

This will come with some trade offs:

  • The smaller the grid, the longer to draw (especially if you’re drawing it live for a temporary effect) and the more a player’s icon will cover what they can see within the space. Cells twice the length and width of player icons (i.e. 5’ icon yields 10’ square cells) will give them enough room to move their icon to look around, but too much area makes the obscurement less obscuring.
  • If you want them to be able to move blindly through, you can toggle the Restrict Illumination flag in map options under dynamic lighting. However this will affect all lines, including walls the characters shouldn’t be able to move through. You’ll have to decide whether having to keep them from walking through walls, having to handle blind movement yourself, or having to toggle between the two is the bigger headache.
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  • \$\begingroup\$ I don't personally like this method because sudden Dynamic Lighting can be difficult to set up ahead of time, and time-consuming if it's unexpected. Also the limiting to 2x2 or 3x3 boxes doesn't accurately accomplish the vision restrictions caused by the example effects. \$\endgroup\$ – Ifusaso Oct 31 '19 at 17:46
  • \$\begingroup\$ @Ifusaso 1x1 is also an option, but it does exacerbate the other concerns... \$\endgroup\$ – Stop Being Evil Oct 31 '19 at 17:49
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I'm setting up an encounter for some of my PCs now in Roll20 in a very similar scenario. The idea here is that they are in the blizzard and vision is severely reduced.

What I'm planning to do for this encounter is make the map dark, give all the tokens night vision, and limit the nightvision range to 20 feet or so.

This will avoid the problem of having everyone carry light sources - which would create little 'pools' of light that all PCs could see, and would be easy to tweak during gameplay - a single change to the token characteristics either extends or curtails vision depending on your desires.

Finally - this won't require me to do a bunch of prep setting up barriers to vision on the dynamic lighting layer.

I'm setting this up now, but based on my previous experience with dynamic lighting I think this will work pretty well. I'll repost here later once I can confirm this works properly.

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    \$\begingroup\$ My PCs are currently exploring the glacial rift of the Frost Giant Jarl (G2/Yawning Portal 5e version). They are currently in daylight and are about to venture into the bottom of the rift, where blizzard-like conditions restrict visibility to 30'. Your description is the exact changes that I plan on making before the next session. However, they do not directly address the OP's question of something like obscuring mist, where a small, restricted area on the board means that anyone who moves into it cannot see at all, but they can see if they step out. \$\endgroup\$ – Kirt May 14 at 20:13
  • \$\begingroup\$ Hi John! This is great advice for general visibility limitations but this question is about visibility limitations for a specific area. I haven't used Roll20 for about a year but I don't think your information quite applies in this particular case, unless there's a way to use the general visibility in a more specific way. Maybe you could edit and improve your answer to speak to that? \$\endgroup\$ – Rykara May 14 at 22:05

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