Item Generation Tutorial

This massive article explains item generation in great detail; far more detail than casual gamers are likely to be interested in. Skim this item generation encyclopedia by the chapter headings if you want one specific question answered. Read it cover to cover if you really want to understand how the game works.

For a similar article discussing item generation in earlier versions of the game, check out the v1.09 Item Generation Tutorial.

1. Basics

The goal of most players in Diablo II is to kill enemies while staying alive. There are three things that largely determine how effective your character is at this goal:

1. Your tactics.
2. The build of your character.
3. The items used by your character.

All three have a great impact on gameplay -- characters die quickly on Hell difficulty if they play without wise tactics and kill slowly with poor skill distribution, but most players find that the essential element is proper use of items. A magic using character will see her effectiveness increase dramatically if she doubles her skill levels, and the effectiveness of a melee fighter or bow user is a direct reflection of the quality of their weapon. As a result, most players spend hours seeking top quality items, and then hours more trying to improve their gear further. At the highest end, most truly dedicated players spend virtually all their game time item hunting.

This guide explains how best to find the items you seek, and explains why this task often seems so difficult.

2. Terminology

• Item Type: The kind of item; a cap, a gem, a short sword, a ring, etc.
• Item Quality: Different qualities of the same item type. These are, in order of increasing scarcity: low quality > normal > superior > magic > rare > set > unique.
• Affixes The magical modifiers on an item; prefixes and suffixes. (Not all item properties are affixes. For example: stuffmods, pre—defined properties of crafted items and some special mods of set and unique items. E.g. "+60 to Mana" on magic ring is affix, "+20% crushing blow" isn't affix.
• Qlvl: Quality level. 1) A level of item type. All these levels are stored in the weapons.txt, armor.txt and misc.txt data files. 2) A level of set or unique item. All this levels are stored in UniqueItems.txt and SetItems.txt data files.
• Ilvl: Item Level. All items in the game have levels. These levels are generated when the item is created. This value is not displayed in the game, so identical items can have very different ilvls, depending on which monster dropped them. Be sure not to confuse Item Level with the item's level requirement -- they are different things.
• Mlvl: Monster Level. All monsters in the game have levels as well as players. These are determined largely by the level upon which a monster spawns. (The same monster type found on different levels will have different mlvls.)
• Area Level: Area Level. All areas in the game also have levels. These determine or at least factor into mlvl, ilvl, and more.
• Alvl: Affix Level. It is a level of prefix or suffix. They are stored in MagicPrefix.txt and MagicSuffix.txt in "Level" column. The name "Affix Level" is used to differ them from other "Levels".
• TC: Treasure Class. All item types in the game are organized into treasure classes. In v1.10 there are 29 weapon and 29 armor Treasure Classes (see those here), with additional TCs covering all the smaller, miscellaneous items such as runes, charms, gems, potions, etc. All TCs are recursive: this means that one treasure class may include another treasure class. E.c. Hell Baal drop all his items from "Baal (H)" treasure class, which consists of "Act 5 (H) Equip B", "Act 5 (H) Junk" and "Act 5 (H) Good" treasure classes. Each of them consists of several other treasure classes, each containing fewer entries and growing more specific.
• Clvl: Character Level. It is one of few levels, that you can actually see in the game.
• Rarity:Rarity is a value assigned to every set and unique item in the game. This comes into play largely when there are more than one unique or set item of the same type, and makes some such items much more common than others. For example, the unique ring Stone of Jordan has a Rarity of 1, while the far more common unique ring Manald Heal has a Rarity of 15.
• SP:Single Player mode.

3. Short Explanation of Drop Process

When you kill a monster or open a chest the game may generate items to reward you. The properties of these items are generated at that moment, even though the item may be unidentified. The game creates dropped items with algorithm of such kind:

1. At the beginning it finds treasure class (TC), that describes all drops of the monster you killed, a chest you opened, a hidden stash you found, etc. All drop sources have treasure classes.
2. The game then makes one or several iterations, that are called "Picks", when it selects one choice from several possibilities.
   1. One of the possibilities is "NoDrop", which does what it sounds like. Many monsters have multiple "picks," so even if one of them is NoDrop, the other picks may overrule it. Most normal monsters have only one pick, though.
   2. If nothing is selected from the first TC, the next one down the list will be consulted, then the next, then the next, until an item or NoDrop is selected.
3. Once an item is selected, the item properties are determined. Since most items have numerous possible types, game's algorithm of quality determination is highly complicated. Items such as keys, runes, and potions are only found in normal quality, but items like rings, short swords, helms, and bucklers have different qualities like magic or unique. Note that Magic Find is checked during this step.
4. For every selected item an ilvl is calculated.
5. If the rarity check hits upon an item of unique or set quality, the game creates the list of all valid item types (determined by their ilvl) and randomly selects one to drop. If there is not a unique of that type, a rare item with triple durability will generate. If there is no set item of the selected type, a magical item with double durability will appear.
6. After the game selects item type and item quality, it generates item affixes if needed. It also generates all other properties of an item, if there are such properties. Items like Runes, mana potions, and identify scrolls doesn't have variable properties, but magic rings, rare kite shields, cracked sashes needs property generation.

A simplified example in action: Mephisto is killed! What does the game do to determine your reward?

1. calculates the number of items to drop
2. selects item types
3. selects item qualities
4. selects certain unique or set items if needed
5. selects item properties

4.Game Data Files

This section discusses the data files that are used in item generation process.

Many parameters of drops are stored in txt files, that are packed to MPQ files when Diablo II is installed. If you have installed ATMA you may find that files unpacked in its data folder. These files are stored in CSV format, a plain text format that may be viewed with Excel or OpenOffice.org. You do not need to know everything about these complicated files to be effective in item hunting, but it will help if you know something about their structure. Truly dedicated item hunters will want to dig deeper into the files, and will find the D2 file guides posted at the Phrozen Keep, of value.

4.1. TreasureClassEx.txt

This is the most important file. It organizes all droppable items into a system of Treasure Classes. Since each source of items in the game (except NPCs) drop items from Treasure Classes, this is crucial to understand.

We'll start with description of all fields of TreasureClassEx.txt table.

• Treasure Class: The name of a TC, one the game usually refers to.
• Group: An identification number of similar TCs. This is used in TC upgrade process according to Mlvl.
• Level: The level of a given TC. It is also used in TC upgrade process.
• Picks: A quantity of iterations of the selection process.
  • Unique: A modification number for chance to drop unique item from given TC (more is better).
  • Set: A modification number for set chance.
  • Rare: A modification number for rare chance.
  • Magic: A modification number for magic chance.
• NoDrop: A rate of skipping drops from the TC (the higher NoDrop, the less items the TC will generate).
• Item1-Item10: A code of item to drop or a name of TC to drop from (item codes are stored in weapons.txt, armor.txt and misc.txt data files).
• Prob1-Prob10: A rate of selecting respective Item1-Item10 (more is better).
• SumItems, TotalProb, DropChance, Term Unimportant fields for item drop routine.

TreasureClassEx.txt file doesn't contains all treasure classes. At run time the game adds quite a few additional TCs. Their names are ArmoXX and WeapXX, where XX is two digit number. The game creates them from the armor.txt and weapons.txt files, by looking at quality levels. The first TC of armors, Armo03, contains all armor items with levels from 1 to 3: Cap, Quilted Armor, Leather Armor, Buckler, Gloves, Boots and Sash. The next armor TC, Armo06 contains all armor items with levels from 4 to 6, and so on. The last armor TC, Armo87, contains all armor items with levels from 85 to 87. Weapons are organized in their TCs, Weap03-Weap87 by the same manner.

In this TCs Prob1-Prob10 values determine how likely the given items in those TCs are to drop. Some items have penalties and are therefore seen less frequently. A few of these, for illustrative purposes:

• Normal items: 3
• Assassin claws: 2
• All other class-specific items: 1
• Wands/Staves/Scepters: 1

These values interact with how many items are found in a given TC to create item shortages and abundances. The given items in a TC with 12 items will be comparatively less common than the items in a TC with 7 items. And if those 7 items include several wands and claws, the normal items in that TC will be highly overrepresented. Breast Plates, for instance.

To further demonstrate, let's look at the process of item selection from a given TC.

All monsters have treasure classes, written in monstats.txt file. When you kill a monster in normal difficulty the game simply uses its TC to drop items. However in Nightmare and Hell difficulty the given TC may be upgraded. When determining the TC for a monster, the game selects the highest level TC in the group that is equal to or less than the Mlvl. If there is no group for TC of this monster, than it doesn't upgrade TC. Treasure classes that are included in the selected TC aren't upgraded.

Selecting from a TC the game makes one or several attempts of dropping according to Picks field. Each pick choses from several possibilities, including NoDrop, and each non-empty entry from Item1-Item10 fields. The chance of every choice is equal to X/Sum, where X is respective value from Prob1-Prob10 fields for Item1-Item10, or NoDrop value for NoDrop choice, and Sum is the sum of all that values (NoDrop+Prob1+Prob2+...+Prob10).

If the game selects another TC during this process it will initiates the same cycle for it, then another cycle for newly selected TC, then another and another until it selects an item or NoDrop.

In multilayer game NoDrop value is reduced.

NewNoDrop=int( ProbSum/(1/((NoDrop/(NoDrop+ProbSum))^N)-1) ), where N=int (1+AdditionalPlayers/2+ClosePartiedPlayers/2), AdditionalPlayers — the number of all additional players in the game, where the monster is killed, ClosePartiedPlayers — the number of players in your party near you (not far than two screens away), ProbSum=Prob1+Prob2+Prob3+...+Prob10, int — operation of truncating fractional part of a number.

In SP the Players X command sets the number of NON-partied players, e.g. /players8 command sets N=4.

For example: a character kills a non-boss Devilkin in The Pit level 1 in Hell in /players8 mode (or in a full game on Battle.net). The Devilkin has Mlvl=85. In monstats.txt file Devilkin is figured under "fallen3" name. Non-unique fallen3 drop items from "Act 1 (H) H2H B" TC in Hell difficulty. In TreasureClassEx.txt file "Act 1 (H) H2H B" TC has group=7 and level=66. Because Mlvl=85 the TC is upgraded to "Act 5 (H) H2H C" TC, which has level=85 (less or equal to mlvl). So Devilkins in The Pit drop items from "Act 5 (H) H2H C" TC.

It has 1 pick from fallowing possibilities:

• NoDrop (100);
• gld (21);
• Act 5 (H) Equip C (16);
• Act 1 (H) Junk (21);
• Act 5 (H) Good (2).

The numbers in (parenthesis) are probabilities.

Lets calculate NewNoDrop:

• ProbSum=21+16+21+2=60.
• NoDrop=100.
• N=4.
• NewNoDrop=int( 60/(1/((100/(100+60))^4)-1) )=11.

So The Pit Devilkin has fallowing chances:

• 11/71 for dropping nothing
• 21/71 for dropping gold
• 16/71 for dropping from Act 5 (H) Equip C
• 21/71 for dropping from Act 1 (H) Junk
• 2/71 for dropping from Act 5 (H) Good

Lets assume, that the game decided to drop from "Act 5 (H) Equip C" TC. Here it is: Picks=1, NoDrop=0, Item1="weap81", Prob1=2, Item2="armo81", Prob2=1, Item3="weap84", Prob3=6, Item4="armo84", Prob4=3, Item5="weap87", Prob5=14, Item6="armo87", Prob6=7, Item7="weap87", Prob7=1, Item8="armo87", Prob8=1, Item9="Act 5 (H) Equip B", Prob9=1530. Note, that weap87 and armo87 TCs are mentioned twice, it is not a error.

So this TC has fallowing chances:

• 2/1565 for dropping from weap81;
• 1/1565 for dropping from armo81;
• 6/1565 for dropping from weap84;
• 3/1565 for dropping from armo84;
• 15/1565 for dropping from weap87;
• 8/1565 for dropping from armo87;
• 1530/1565 for dropping from "Act 5 (H) Equip B".

Lets assume, that the game decided to drop from armo87. This TC has picks=1, NoDrop=0, Item1="Diadem", Prob1=3, Item2="Corona", Prob2=3, Item3="Sacred Armor", Prob3=3, Item4="Ogre Gauntlets", Prob4=3, Item5="Myrmidon Greaves", Prob5=3, Item6="Colossus Girdle", Prob6=3, Item7="Dream Spirit", Prob7=1, Item8="Guardian Crown", Prob8=1, Item9="Vortex Shield", Prob9=1, Item10="Bloodlord Skull", Prob10=1.

So armo87 has fallowing chances:

• 3/22 for Diadem;
• 3/22 for Corona;
• 3/22 for Sacred Armor;
• 3/22 for Ogre Gauntlets;
• 3/22 for Myrmidon Greaves;
• 3/22 for Colossus Girdle;
• 1/22 for Dream Spirit;
• 1/22 for Guardian Crown;
• 1/22 for Vortex Shield;
• 1/22 for Bloodlord Skull.