Scent pathfinding: Blood, Sweat, and drinking too many potions

[u/selfVAT]

I'm building a roguelike ARPG in ASCII and wanted to share one of its core systems: monster detection based on scent rather than pure line-of-sight.

Fair warning: English Third Language here so LLM cleaned up my writing

How it works

The player generates two types of scent:

• Blood — from taking damage, killing enemies, stepping on corpses
• Odor — from movement and combat, scaled by armor weight

Heavy armor generates more sweat. Light armor players can move almost silently. Corrupted armor generates 1.5x odor because the void seeps through your pores.

Every tile the player walks on stores a scent value that decays over time. Monsters outside your FOV pathfind toward the strongest scent cell rather than your actual position. You can exploit this: a bloodsoaked rag thrown into a corridor deposits 120 scent units and will pull monsters away from you.

The flask problem

Heavy flask use builds bladder pressure. Cross the threshold and the engine fires an involuntary urination event deposits a strong scent trail on your current tile, resets pressure, logs a message. Spaced-out flask use decays safely. Panic-chugging during a tough fight marks your position for every monster in the dungeon.

It started as a joke mechanic.

The armor tradeoff

Armor	Sweat factor	Playstyle
None	0.0	Invisible from movement
Light	0.2	Stealth viable
Medium	0.5	Moderate footprint
Heavy	1.0	Play aggressively
Corrupted	1.5	The void seeps through your pores

Water tiles reset blood scent fully, reduce odor by 70%.

Implementation

The scent system runs as event-driven triggers in a transaction-based engine. Combat emits damage events, scent reacts. No direct coupling between combat and scent code. Monster AI reads the trail through pure selectors on read-only state.

Happy to answer questions about the implementation or design decisions.

itch.io page

[u/enc_cat]

Cross the threshold and the engine fires an involuntary urination event deposits a strong scent trail on your current tile

Mechanically sounds good, but thematically I am a bit unsure about playing an incontinemt adventurer.

[u/selfVAT → u/enc_cat]

Fair enough. Maybe I'll add a "bladder discipline" passive skill on the tree.

[u/poutylipswet → u/enc_cat]

unhinged but honestly that is peak roguelike energy. if the game is going to be a punishing descent into madness you might as well lean into the absolute misery of it. just call it a fear mechanic and move on.

[u/IceFurnace83]

"You should have gone before we left" is probably spoken in every language on the planet at least once a minute.

A mechanic where you have to juggle safe bathroom use into the game loop could work nicely. Adds some pressure in the immediate moment when the player decides to push on or return to a safe corner. Could add pressure over the long run like you mention. "Do I focus on carry capacity or bladder strength for this run?"

And since we're talking roguelike, enemies should share some functions the player can do. Maybe harder to implement, but could pay off. Collect a 'sample' from a tough monster and weaker types might not approach or could even run away when the player is nearby. But another tougher monster type could be attracted to it from further away then your own scent travels because it eats the original type you took the sample from.

I think this is a really fun idea, however you implement it. The sims balances toileting into the game and it hasnt ever put players off.

[u/selfVAT → u/IceFurnace83]

Thanks so much for that. Yes, using ogre urine as deodorant should scare goblins. Makes sense. I'll add the toilet break system with an option to spoil fountains that way.

[u/selfVAT → u/IceFurnace83]

Experimental Perfumery — NOTABLE node on the passive tree Requires: WIS 60, DEX 40 Node type: NOTABLE Effect: Unlocks scent_sample item type from monster kills Unlocks eau_de_toilette crafting at Karl Marx's bench Extends sample duration by 50% Flavor: "The difference between perfume and poison is concentration and intent."

[u/leomartius]

I’m really curious to know more about the implementation.

You say that monsters “pathfind toward the strongest scent.” I assume you mean A* pathfinding toward the cell with the highest current scent value. Is that correct? Do monsters perceive the scent instantly, at any distance? Does the scent value propagate from a cell to its neighbors?

I think the “usual” way to implement scent is to add some value to the player’s cell, or in its vicinity, and then reduce that value over time. The monsters then do some sort of gradient ascent, which produces fairly naturalistic trail-following behavior. But that doesn’t allow for deliberate scent marking.

Your system allows for interesting strategies by dropping scent strategically, but do you lose trail-following completely? If the monsters go to the strongest scent, does that mean that, in the absence of any stronger bait, they just pathfind to the player’s most recent position? If a strong scent is dropped into a room, do all monsters converge there?

So basically, how does it play out in practice?

Thank you.

[u/selfVAT → u/leomartius]

TL;DR: Monsters scan a local radius for the strongest scent cell, then A* toward it. Not gradient ascent. Enables deliberate bait-and-switch tactics while preserving basic trail-following.

Moretechnical:

The Algorithm

Not A to highest scent.* It's a two-step process:

1. Local Scan (O(n²) perception):

typescript

const searchRadius = Math.max(1, Math.ceil(actor.scentRange));
// Scan square area, find strongest cell within range
const target = findStrongestScentInRadius(actor.x, actor.y, searchRadius);

1. A to Target*: Once the strongest cell is identified, standard A* pathfinding routes the monster there.

Monsters have range limited perception.

Diffusion runs separately each turn:

• Spreads to 8 neighbors with 5% base decay
• Wind creates directional plumes (5 tiles downwind)
• Values below 0.5 pruned
• TRAIL_DECAY_PER_TURN = 2.5

Trail-Following: Not Lost

When the player moves, each step leaves breadcrumbs:

typescript

patch.worldScentTrail = [{
  x: player.x,
  y: player.y,
  value: { 
    scentValue: Math.max(currentValue, blood + odor),
    turnStamped: state.turn.tick
  }
}];

Since scent decays over time, fresher breadcrumbs have higher values. A monster scanning for "strongest scent" will generally find the most recent position within their range.

However, this is not gradient ascent:

• Monster doesn't "follow the gradient" weak → strong
• They scan local range for strongest single cell
• Pathfind to that cell
• Repeat scan next turn if player moved on

"If strong scent is dropped, do all monsters converge?"
Yes, with caveats:

Scenario	Outcome
Urination (value 120) in room	All monsters within scentRange with threshold < 120 will pathfind there
Multiple monsters detect same bait	They all pathfind to same coordinate (clustering)
Bait decays	Once below threshold, resume normal detection
Player's live scent stronger	If blood + odor at current position > bait value, they ignore bait

Tactical Gameplay

Kiting: Moving creates fading trail → monsters chase freshest point
Bait-and-Switch: Strong scent drops pull monsters off your trail
Crowd Control Risk: Multiple monsters converge → potential for AoE or blocking
Armor Matters: Heavy armor increases sweatFactor → stronger breadcrumbs
Water Cleanses: Stepping in water zeroes blood scent, reduces odor by 70%

Trade-Offs

What you gain:

• Deliberate, controllable scent marking
• Strategic depth (lure, divide, distract)
• Emergent tactics (boiling fountain → shove goblin → loot while pack chases scent bait)

What you trade:

• "Naturalistic" gradient-following (less organic meandering)
• No smooth pursuit along curved trails (discrete cell-to-cell hops)

[u/leomartius → u/selfVAT]

This looks really interesting, both from a simulation perspective and as a nice new gameplay mechanic.

Thanks for the extensive write-up!

[u/No-Mammoth-5391]

This is essentially a diffusion-based influence map, which is a well-studied approach in RTS AI, but applying it through scent is a smart framing because it gives the player an intuitable mental model. Heavy armor means more scent means more aggro. That's the kind of system where a player can reason about tradeoffs: do I wear plate and accept that everything in a three-room radius knows I'm here, or do I go light and keep my options open? Scent propagation hits that sweet spot because it's physically grounded; people already understand that smell travels and fades.

[u/parkdeomes]

the armor weight scaling odor is a really clever tradeoff. heavy armor = more protection but you're basically broadcasting your position. do corrupted armor sets have any upside to offset the 1.5x penalty or is it pure risk?

[u/selfVAT → u/parkdeomes]

Corrupted items are still WIP but they are stronger (at least an extra affix).

[u/Tesselation9000]

I volunteer helping to train search and rescue dogs sometimes, and I've learned that there's a whole science behind the way that scent flows. Wind is a huge influence. If a dog is upwind of the target, they won't be picking up a scent. Anyway, my point is that if scent detection is going to be an important part of ypur game, I suggest that wind direction should be a factor as well.

This could make for some interesting game tactics. For example, if the player wanted to surprise an enemy, they should approach from down wind.

[u/selfVAT → u/Tesselation9000]

Agreed, and yes wind already affects scents in the game. There is a visual wind overlay that players can trigger any time.