Act 3 Map Tells: the full AI report

How D2R builds its maps

D2R uses the same map generator as classic Diablo II, so the classic generator can be run on any game seed and its maps measured. Five facts explain almost every rule below:

1. One seed per game. All five acts are built from the same game seed.
2. Every act draws the same first numbers. The rolls that link the outdoor levels of Acts 1, 3, 4 and 5 are identical in every act, so one roll places a level in Act 1 and another in Act 5. That is where the cross-act rules come from.
3. Each level gets its own seed (the act's start seed plus the level number), so neighbouring levels get neighbouring seeds.
4. The random number generator is weak. Two levels with nearby numbers start almost in step, so their first choices are linked. That is why Spider Forest predicts Flayer Jungle and some dungeons seem to copy each other.
5. Mazes pick special rooms from the newest dead ends. Stairs, waypoints and quest rooms are always dead ends. Each level rolls one door direction and turns it a quarter turn per special room, and the newest rooms sit at the edge of the maze, so the stairs are almost always the farthest fitting dead end.

Which way is the exit, level by level

3,000 generated maps per level, measured from where you arrive. Directions are on your monitor (up = top of the screen); a level's edges are top-left, top-right, bottom-right and bottom-left, its corners top, right, bottom and left. In mazes, left / straight / right is relative to the way you face when you arrive. One screen is the default 1080p view. "Follow the open ground" means the target is within 45° of the way the open floor runs around you. "Matched real saves" = the generated layout was checked against explored maps from real D2R single-player saves.

Rules that cross acts

Cold Plains: the Stony Field exit edge decides four later maps

The same roll puts Cold Plains on one side of Stony Field and sets these later maps. Every row held in all games it covers, in Normal, Nightmare and Hell. Runners already found the Lower Kurast and Frigid / Tundra part by hand in 2019.

Kurast Bazaar's exit is then in the opposite corner, as already known. In Act 4 the same roll only shifts the Outer Steppes a bit along the edge you enter by.

The edge you entered Cold Plains by

A second shared roll places Plains of Despair next to the Outer Steppes and sets Arreat Plateau's orientation. In Act 1 it is only the first try for Cold Plains and Black Marsh, and Act 1 sometimes rejects that try, so the Act 1 view shows it most of the time, not always.

• Add Dark Wood: after a top-left entry into Cold Plains, a Black Marsh exit on Dark Wood's bottom-left or bottom-right edge means Plains of Despair on the top-right edge; a top-right one means bottom-left (both 100%). Overall 88% instead of 82%.
• Arreat Plateau from Act 4: Plains of Despair exit on the Outer Steppes' bottom-right edge means Crystalline Passage up-left; bottom-left edge means up-right (100%, found by runners in 2019). After a top-right Plains exit the Cold Plains entry edge settles it: top-left entry = up-left, bottom-right entry = up-right (both 100%), top-right entry = up-right (75%). Overall 94%.

What is not linked

• The real Tal Rasha tomb: nothing in Act 1 predicts it (best guess 14.5%, pure chance 14.3%).
• The old hint that the Stony Field entrance decides Lut Gholein's exit does not hold (51%).
• Lut Gholein's exit says little about Act 4, and the City of the Damned exit edge stays a guess (54% even when you know the Plains exit).

Maze levels: the stairs are a dead end, and you know which way its door faces

In every maze level (Jail, Catacombs, Tower Cellar, Halls of the Dead, Maggot Lair, Flayer Dungeon, Durance of Hate, the Act 5 ice caves, Worldstone Keep) the stairs, the waypoint and the quest rooms are always dead ends with one door. X is the door of the room you arrive in; "+ 1 turn" is a quarter turn clockwise on screen. These held in 100% of 6,020 games.

And the stairs are the farthest dead end with that door, seen from where you arrived:

How to use it: on the automap, ignore every dead end whose opening faces the wrong way, then head for the farthest of the rest. Where the door direction is random (Catacombs 1 and 3, Worldstone Keep 1 and 3), the stairs are still the farthest dead end of their own door type: once you find a far dead end that is not the stairs, its door type is ruled out for every dead end nearer than it.

• Catacombs 1: you arrive in the middle of a 4-way room. Stairs = farthest of its door type 93%, farthest dead end of any kind 40%, one of the two farthest 68%.
• Catacombs 2: you arrive in a straight corridor. Stairs door = waypoint door turned a quarter turn counter-clockwise (100%), farthest such dead end 88%. That is the root of the old "from the Catacombs 2 waypoint, go right" advice.
• Catacombs 3: random door direction, stairs = farthest of their door type 84%. Catacombs 4 is fixed.
• Worldstone Keep 2: from the waypoint, the level 3 stairs door is the waypoint door turned a quarter turn counter-clockwise (100%), farthest such dead end 95%. No other level predicts the Worldstone Keep stairs.
• The room just before a dead end tells you nothing. Its look (torches, braziers, decoration) is picked independently of whether the dead end is the right one, so the "3 torches versus 2 torches" idea does not hold.

Fixed rules (always true)

• Spider Cavern: always the same loop of four rooms. You arrive in the right-hand room (the only one with the way back to Spider Forest). Khalim's Eye and Sszark are always in the bottom room, so take the bottom-left exit; the top-left exit goes the long way round (top, left, then bottom). The chest sits in the left half of the bottom room.
• Arachnid Lair: the same loop, but only a consolation chest, in the left room.
• Palace Cellar: the same four-room loop. Cellar 1: the waypoint is always in the bottom room. Cellar 3: the Arcane Sanctuary portal is always in the top room.
• Chaos Sanctuary seals: each arm is one of two layouts, and you can tell which from the wall shape as you walk in (also on the automap). Infector (bottom-right arm): always the seal nearest the left of the screen. Grand Vizier (top-left arm): the upper-right seal in one layout, the lower-left in the other. De Seis (top-right arm): one seal at the far end, left side in one layout, right side in the other.
• Canyon of the Magi: fixed map, the real tomb is random (1 in 7). Use the quest-log symbol.

More on the jungle clearings

• Each jungle has 2 or 3 clearings. The three map pieces are dealt at random: waypoint (plus Arachnid Lair / Swampy Pit), quest cave, and an empty decoy. With only 2 clearings there is no decoy. The clearing's name (Webby W, Pygmy NS, ...) only says where its gaps are.
• Out of every multi-piece map spot on the route, only these 30 jungle clearings decide right versus wrong by their look. Tomb entrances, Kurast sewer and temple entrances, the River of Flame entrance, Worldstone Keep pairs and the Chaos bridges all do the same job in every variant.
• Cocoons: the two cocoon types use the same HD model (only the idle animation differs), so swapping one for the other is no tell in HD. A cocoon starts already broken 1 game in 14.
• HD-only tells (webbed corpses, HD ponds, torch decoration, the log bridge) do not exist in legacy graphics. The pond, torch, cocoon and altar tells work in both.

The weak ones: the decoration at the foot of the entrance torch (HD only, and it even swaps between clearings):

Arcane Sanctuary

The Summoner's arm is picked after the whole sanctuary is laid out, and nothing you can see before that depends on it. Tested against hundreds of visible features in all five acts and against every possible arrival view: no clue, as the classic runners already said. The arm layout (which arm has the teleport pads, stairs, walkway) echoes the Halls of the Dead 2 entrance room 91% of the time, but you see the layout on arrival anyway, so it saves nothing.

How this was made, and the catch

• Game data: D2R 3.1.2; the map pieces are unchanged in 3.3. Nothing was tested in the running game: it all comes from the map files and from running the classic map generator on random seeds (3,000 maps per level for directions, 6,020 full games for the links).
• D2R builds the same maps as classic from the same seed: the generated layouts matched explored automaps from real D2R saves in all five acts. The jungle rule and the maze rules inside Catacombs, Durance and Worldstone Keep were not checked that way.
• "100%" rules follow from how the generator works and held in every generated game. Other percentages are measured, give or take 1-2 points.
• The screenshots are brighter than the game; lit torches and braziers only show as small glows, and monsters and animations are not drawn.
• Credit: some cross-act links (Stony Field to Lower Kurast and Frigid / Tundra, Outer Steppes to Arreat, Halls of the Dead to the Arcane layout, and "no Act 1 clue for the Summoner") were found by hand by d2lod runners in 2019.

Open questions for runners

• Can you spot the small HD corpses (Webby W / E, Pygmy NS / SW) at game brightness and speed?
• Do the Pygmy W / N ponds show on the automap? Then the pond tell works before you even see it.
• Underground Passage 1: on real runs, is the exit "straight" from the first tile (guides) or left / right?
• Clearings with two gaps: which side you arrive from depends on the river. Worth noting on runs.