How does having multiple Speeds interact with Difficult Terrain?

The rules on "Using Different Speeds" state (emphasis mine):

[…] Whenever you switch, subtract the distance you’ve already moved from the new speed. The result determines how much farther you can move. If the result is 0 or less, you can’t use the new speed during the current move. […]

However, I’m unsure what the rules mean when they say "the distance you’ve already moved" especially when difficult terrain is thrown into the mix. The very next section is on "Difficult Terrain" and it states:

[…] Every foot of movement in difficult terrain costs 1 extra foot. This rule is true even if multiple things in a space count as difficult terrain. […]

So what happens if a creature with a Walking Speed of 30 and a Climbing Speed of 20 begins their turn by walking through 15 feet of difficult terrain to the nearest wall (which is not difficult terrain)? Their remaining Walking movement will be 0, but will their remaining Climbing movement be 5 or 0?

Do parties with rangers dispense with navigation checks while in their favored terrain in Tomb of Annihilation?

In Tomb of Annihilation the prospect of getting lost/turned around in the jungle is written explicitly into the rules: “The Land of Chult” and its “Navigation” section have the party navigator making a check every day, and they may become lost if the check goes awry.

But my party has a ranger, favored terrain of forest. Whose group, due to Natural Explorer, “can’t become lost except by magical means.”

Nothing suggests that difficulty finding one’s way in the lion’s Tyrannosaurus’ share of Chultan jungles is magical. It’s just mundane bushwhacking-sometimes-goes-awry.

So do groups playing Tomb of Annihilation who include a (forest) ranger simply dispose of the whole “Navigation” section while in the jungle?

(We’ve ruled that forest==jungle for Natural Explorer’s purposes. As does Jeremy Crawford. If you disagree hold your ire and imagine we were talking about coast, or swamp, or mountain; all of these are Natural Explorer candidates and might be where navigation checks would be indicated in ToA.)

Difficult terrain while flying

I am playing a very specialized character. The goal is to use Precipice Strike with arcane trickster surprise spells feature. Since being on the ground naturally puts me at greater risk, I was wondering what ways were open to me to count as being in difficult terrain, hopefully ones that can be quickly turned on/off.

For more information I am in a gestalt game where I am a wizard rogue who then levels up in arcane trickster and magus. This links to questions Ive asked in the past about sneak attack damage on spells.

Are there specific rules for pushing enemies into/over difficult terrain?

In previous editions of Dungeons and Dragons there was typically a reasonable assurance of being able to resist/avoid forced movement, or a way to prevent being harshly penalized for it (saving throw to drop prone rather than going over a cliff in 4e).

In 5e while the number of ways to move enemies around has been reduced from 4e it is still very much present. The Warlock’s repelling blast being the best example in that all it requires is a hit to push an enemy 10′, which is positively deadly near a cliff side. Are there any rules I am missing for this? Obviously it isn’t balance critical as it is terrain specific, and honestly the idea behind it is likely to be seen as fun by most players.

Still it seems very out of place from a numbers perspective in 5e. With typically lower monster HP’s and party damage outputs, and falling damage remaining the same the cliffs have already become deadlier. Even simple environmental damage from some spikes is far more valuable to combat than it used to be. So while I am willing to accept there is nothing to do once an ability has moved someone into a dangerous spot besides watching them suffer, I feel like I might just be missing a rule somewhere.

Does Water Walk include ice as a terrain it affects?

Water Walk in DnD 3.5e specifies ice among the surfaces Water Walk lets you traverse as if it were solid ground, eliminating the difficult terrain penalties ice normally imposes:

The transmuted creatures can tread on any liquid as if it were firm ground. Mud, oil, snow, quicksand, running water, ice, and even lava can be traversed easily, since the subjects’ feet hover an inch or two above the surface. (Creatures crossing molten lava still take damage from the heat because they are near it.) The subjects can walk, run, charge, or otherwise move across the surface as if it were normal ground.

If the spell is cast underwater (or while the subjects are partially or wholly submerged in whatever liquid they are in), the subjects are borne toward the surface at 60 feet per round until they can stand on it.

Water Walk in DnD 5e omits ice from the list of affected terrain:

This spell grants the ability to move across any liquid surface—such as water, acid, mud, snow, quicksand, or lava—as if it were harmless solid ground (creatures crossing molten lava can still take damage from the heat). Up to ten willing creatures you can see within range gain this ability for the duration.

If you target a creature submerged in a liquid, the spell carries the target to the surface of the liquid at a rate of 60 feet per round.

Has there been any clarification that allows ice?

Please cite source sites.

What kind of terrain is allowed for Mirage Arcane?

The spell mirage arcane is problematic in that much is left undefined, or unclear. It is a 7th level spell, so its powers are unexpectedly great. Nonetheless, there must be some limitations to the spell. I’m specifically interested in the type of terrain that the local one can be changed to.

Jeremy Crawford once unofficially tweeted that targets could drown in an imaginary lake, or fall off an imaginary cliff, so we can assume that spawning a lake in the middle of nowhere isn’t too far-fetched.

The mirage arcane spell gives you tremendous latitude in how you make the affected terrain look and feel. The altered terrain can even hurt someone. You could drown in the spell’s illusory lake, for example, or fall off an illusory cliff.

1. Can extraplanar terrain be used? Places like the Elemental Plane of Fire are inherently dangerous to humans. Could you use Mirage Arcane to summon typical terrain from that plane, like a sea of fire (for example)?

2. Do acid lakes, toxic bogs, and active volcano calderas count as terrain? I’m inclined to say that if one can drown in an illusory lake, then surely one can melt in an illusory acid lake. The toxic bog though, it’s more debatable if its terrain components are the deadly ones. And I’m not sure if a big lake of lava is terrain.

3. Does the allowed terrain depend on location? Perhaps summoning a lake of lava in the middle of a calm meadow is a bit extreme (then again, perhaps not for a 7th level spell). However, surely if one were to use this spell in a volcanic landscape, then it would be appropriate?

Ray casting through terrain mesh + Octree ( Mesh collision )

I’m currently in development of a terrain editor which i will use for my game. One thing that is partially stopping my development is that I don’t have collision detection implemented between mouse ray cast and the terrain mesh.

The terrain is consisted of triangles and can be lowered and raised.Also the x and y values of the mesh don’t change. Of course what i need to do is ray cast through my mouse positions and get intersected triangle. I would get the point on the triangle using Barycentric coordinates. Just I’m not quite sure how does octree come into play here. As far as I understand it it works like a 3d quadtree, splitting the box into smaller box that the ray intersects. But which box do i choose ? How do i know which box is the closest one to the terrain, if either raised or lowered terrain ? How many subdivisions would i have? I was thinking on having as much as i have tiles( basically the last level of subdivisions would correspond to a x^3 box).

Right now i’m just doing collision with the 0 Z-value plane, and it works okay, but if the terrain is a bit raised then this can give faulty results.

Other two methods that i found is to iterate through the mesh, which of course is insanely inefficient; the other is to project the terrain in reverse and then i guess do some checking with the mouse coordinates (think it’s called z-buffer checking or something).

If anyone has any other ideas i’m all ears. Just note that the algorithm should be an efficient one (of course this depends on the implementation) and accurate.

Thanks !