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Old Stealth Mechanics (versions up to 1.7.2)
- Always approach monsters (except for fliers and spiders) diagonally - this makes your apparent distance to them higher and thus their stealth checks more difficult.
- If you don't want to wake up a monster, don't stand in its LOS -- and if you have to, keep your distance, keep still, and minimize the time spent there. (Wandering monsters can notice you from outside your LOS if they stand on your scent, however stealth increasing checks still help!)
- Monsters that move at double speed make twice as many checks to notice you (hello, vampire bats!)
- Packs wake up more readily than lone monsters, since they make independent stealth checks.
- Being in darkness helps substantially (blue > white > yellow) - absolutely nothing else about the terrain (whether you're on 'grass', whether you're levitating or not) matters.
- Remember that tall grass breaks LOS, preventing stealth checks from sleeping monsters!
- Nothing else produces noise or wakes monsters up, so anything that you can think of to save moves will help. Blink towards a monster, beckon it towards you, polymorph it, teleport it, quaff haste...
- You can force a stealth roll by crossing terrain that the monster cannot cross, by blinking to where it can no longer see you, or by entrancing it. When a monster snaps out of entrancement, it has to try to notice you again.
A monster wakes up immediately if
- it takes damage from any source
- you read Aggravate Monsters
- another member of its band wakes up for any reason
Every wandering or sleeping monster in your FOV gets a 1-in-X chance to notice you when it takes a movement turn, where:
X = D * (1.1^B) (rounded down)
One chance in X is the same as a probability p = 1/X. Intuitively this means that the monster must successfully make one roll to notice you at your current distance and another roll to notice you with your current stealth bonus (as the power of 1.1). The two are independent, and you can think about them separately.
D represents distance, but in a peculiar way:
- If the monster flies, is waterbound, is swimming or spins webs and you're in LOS, its king's move distance * 3.
- Else it uses the scent map - as you walk around, you throw your 'scent' onto every tile in LOS (on your tile, fresh; on every other tile, fresh - 2*(longer distance) - 1*(shorter distance)) and your scent grows weaker (by 3 points) every turn.
What this means is that if the monster is landbound, approaching it diagonally makes D larger and the monster less likely to wake up, as the scent map approximates euclidean distance.
Stealth Bonus (B)
B is your stealth bonus.
- +1 for every level of any Rings of Stealth you are wearing
- +5 for being in super darkness (@ is blue)
- +3 if the monster is asleep
- +1 and double everything before this if you stood still this turn
- -4 if the monster is hunting you
- +1 and double everything before this for being in darkness (@ is white or blue)
You just stepped to one tile away diagonally from an asleep ogre with no rings of stealth. You are in white light level.
D = 2*1 + 1 = 3 (if you weren't at a diagonal, it would merely be 2!) B = ((0 + 3 + 0) + 0 +1) * 2 = 8 X = 3 * 1.1^8 = 6.43 (rounding down) = 6
Thus, the ogre has a 1 in 6 (16.667%) chance of waking up each turn.
Note that since any member of a band waking up wakes the rest up, bands (such as vampire bats) are harder to sneak up on.
Losing Track of You (scentThreshold)
A monster that is hunting will go back to wandering if X becomes larger than the monster's scentThreshold - thus, stealthiness and being in darkness will help you throw off monsters' tracks, but not at random.
- Each monster type has its own scent threshold. Refer to its page.
- Since it will be using scent not sight by this point, make sure you're using the scent calculations and the concept of scent lingering on tiles to determine when this will happen. (Yes, strangely, moving into darkness will make monsters suddenly lose your scent...)
This article is related to version 1.7.2 and may be out of date.