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Wearing heavy body armour provides you with an amount of [[guaranteed damage reduction]] in melee combat. This is a percentage of an enemy's maximum possible damage output that you ignore from each of its hits. Because enemies rarely hit for their max damage, the GDR on some of the heaviest armours often reduces enemy damage to 0, and those hits that do get through are greatly reduced.
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{{version030}}
  
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Your GDR% = (14*(Body Armour Base AC-2)^(1/2))% (see chart below for exact figures with each armour). However, the amount of damage you can ignore this way is capped at one half of your total AC. While equipment in other slots, enchantments, and the Armour skill will raise your AC and thus raise the cap, your GDR% is only determined by the body armour you wear. So long as enemy attacks aren't hitting the cap, however, you will see them deal 0 damage per hit GDR% of the time.
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'''Guaranteed damage reduction (GDR)''' gives players a layer of protection in melee and ranged combat. Normally, each attack that hits you is reduced by a random amount up to your AC. Against physical attacks, your minimum AC reduction is equal to GDR% of the attack's damage, unless this would be higher than <code>1/2 * AC</code>. Therefore, GDR is a form of insurance against poor AC rolls.
  
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==What It Works Against==
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==What GDR Affects==
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GDR works against one thing: physical damage from monsters bashing you in melee (including via a [[reaching]] brand). It does not work against any sort of ranged attack, be it from a physical launcher or a [[Conjurations]] spell that deals physical damage. Neither does it work against melee attacks that are non-physical, such as an [[ice beast]]'s cold damage. However, most such attacks need to do damage to you in the first place to trigger their added effects, so high GDR will make them happen less often.
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GDR only works against physical damage, either in melee, with [[reaching]], or with a ranged/throwing weapon.
  
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==GDR of Various Armours==
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It does ''not'' apply against any sort of spell or ability. Therefore, GDR does not apply to a [[Conjurations]] spell that does physical damage. Neither does it apply against "physical" abilities like [[Slug Dart]] or [[Harpoon Shot]]. It also doesn't work against elemental melee damage, such as an [[ice beast]]'s additional cold damage.
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==Calculating GDR==
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Your GDR is calculated by the following formula: <code>GDR% = AC^(1/4) * 16</code>. Any means of obtaining AC counts towards your GDR.
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===GDR of Various AC Values===
 
{| class="wikitable"
 
{| class="wikitable"
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! Armour || Base AC || GDR
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! Base AC || GDR
 
|-
 
|-
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| [[Robe]], [[animal skin]] || 2 || 0%
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| 2 ([[Robe]]) || 19.0%
 
|-
 
|-
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| [[Leather armour]] || 3 || 14%
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| 6 ([[Scale mail]]) || 25.0%
 
|-
 
|-
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| [[Troll leather armour]] || 4 || 19.79%
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| 10 ([[Plate armour]]) || 28.5%
 
|-
 
|-
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| [[Ring mail]], [[steam dragon armour]] || 5 || 24.25%
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| 16 (+2 [[Robe]], aux) || 32%
 
|-
 
|-
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| [[Mottled dragon armour]], [[scale mail]] || 6 || 28%
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| 32 (+10 [[Plate armour]], aux) || 38.1%
 
|-
 
|-
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| [[Chain mail]], [[swamp dragon armour]] || 7 || 31.30%
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| 40 (+14 [[Crystal plate armour|Crystal plate]], aux) ||40.2%
 
|-
 
|-
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| [[Splint mail]], [[fire dragon armour]] || 8 || 34.29%
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| 50 || 42.5%
 
|-
 
|-
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| [[Ice dragon armour]] || 9 || 37.04%
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| 100 || 50.6%
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|-
 
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| [[Plate mail]], [[pearl dragon armour]], [[storm dragon armour]] || 10 || 39.60%
 
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|-
 
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| [[Gold dragon armour]] || 12 || 44.27%
 
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|-
 
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| [[Crystal plate armour]] || 14 || 48.50%
 
 
|-
 
|-
 
|}
 
|}
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Auxiliary slots include a +2 [[Helmet]], +2 [[Cloak]], +2 [[Gloves]], and +2 [[Boots]]. Percents on this table are rounded to the nearest tenth.
  
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==Strategy==
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==Damage Reduction==
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GDR is a very desirable property to have in your armour. It is a major reason for casters to consider wearing leather armour over a robe: 14% is a significant amount of guaranteed damage reduction. On the other hand, the powerful [[robe of the Archmagi|archmagi]] and [[Resistance (ego)|resistance]] brands are only found on robes, so the decision is not always clear-cut.
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GDR is '''not''' a form of damage reduction independent from AC. If you could have excellent GDR and terrible AC, it would do you little good. It is effectively a minimum AC roll, or a ''guaranteed'' AC roll. It is capped at <code>GDR% * damage taken</code> or <code>1/2 * AC</code>, whichever is lower.
  
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For fighter types, GDR is the main reason to go for heavier and heavier armours: If you aren't casting and are planning on training Armour, you should wear the first plate mail you find: the GDR is well worth the accuracy penalties. Later on, you might want to upgrade to a crystal plate mail or a gold dragon armour, though a branded, enchanted plate mail is a worthy ascension kit item as well.
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Let's say you take a 50 damage attack, but have 48% GDR. First, [[AC]] rolls. If AC roll is a 1, then GDR will raise the reduction to (50 * .48) = 24, meaning you actually lose 26 HP. But if your AC roll is 36, then GDR will not do anything, as you are above the guaranteed reduction of 24.  
  
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==Example==
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If you took said 50 damage attack with 16 AC and 32% GDR, then the AC/2 limit applies instead: you'll always reduce damage by at least (16 / 2) = 8 over the expected (50 * .32) = 16.
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30 AC with a +6 chain mail is worse than 28 AC with a +0 plate mail.  
 
  
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The graph below lists the average damage taken after GDR calculations for a 28 AC plate vs a 30 AC chain. The plate has 40% damage reduction while the chain has 28% damage reduction. Note this omits the AC damage reduction check that can be greater than the GDR value.
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==Strategy==
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GDR is tied to your [[AC]]; any form of extra AC will also increase GDR. This comes with the same costs and benefits; are you willing to sacrifice [[EV]] and spellcasting potential in order to become more bulky? Characters who want to avoid melee will still want to avoid melee as much as possible. However, GDR is a large reason why AC is more consistent in stopping damage.
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[[File:GDR_Example.png]]
 
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This chart illustrates an important aspect of GDR: GDR will help reduce damage from small and numerous attacks. With monsters that have a low max damage, you will take less damage with the higher GDR plate mail. For example, for a monster that has a max damage of 15, you will take on average 4.2 damage with the chain mail and 3.0 damage with the plate mail. So while you have a higher AC with the chain mail, you will be taking ''40% more'' damage, on average, than if you wore the plate mail.
 
  
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The higher AC only starts to overcome the better GDR when the max damage surpasses 50, but by then the difference in reduction is negligible.
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For most characters, GDR will slightly reduce the maximum physical damage enemies can deal.
  
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==Spell-Based GDR==
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==History==
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There are two [[Transmutations]] spells which allow you to have GDR in spite of the fact that they effectively disable your body armour: [[Dragon Form]] (34%) and [[Statue Form]] (39%).
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*Prior to [[0.27]], GDR was more complex. Each set of body [[armour]] had a defined GDR, calculated by <code>(14 * (Body Armour Base AC - 2)^(1/2))%</code>, no matter what your actual AC was. Certain [[transmutation]]s also had a set GDR. [[Gargoyle]]s increased base GDR, while [[Draconian]]s (being unable to wear body armour) didn't have any. For more information, see [http://crawl.chaosforge.org/index.php?title=Guaranteed_damage_reduction&oldid=52698 this past revision].
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*Prior to [[0.8]], GDR was equal to <code>(Body Armour Base AC) * (13 + Armour) / 17</code>.
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*Prior to [[0.5]], GDR was higher.
  
 
[[Category:Defence]][[Category:Game mechanics]]
 
[[Category:Defence]][[Category:Game mechanics]]

Latest revision as of 06:02, 17 December 2023

Version 0.30: This article may not be up to date for the latest stable release of Crawl.

Guaranteed damage reduction (GDR) gives players a layer of protection in melee and ranged combat. Normally, each attack that hits you is reduced by a random amount up to your AC. Against physical attacks, your minimum AC reduction is equal to GDR% of the attack's damage, unless this would be higher than 1/2 * AC. Therefore, GDR is a form of insurance against poor AC rolls.

What GDR Affects

GDR only works against physical damage, either in melee, with reaching, or with a ranged/throwing weapon.

It does not apply against any sort of spell or ability. Therefore, GDR does not apply to a Conjurations spell that does physical damage. Neither does it apply against "physical" abilities like Slug Dart or Harpoon Shot. It also doesn't work against elemental melee damage, such as an ice beast's additional cold damage.

Calculating GDR

Your GDR is calculated by the following formula: GDR% = AC^(1/4) * 16. Any means of obtaining AC counts towards your GDR.

GDR of Various AC Values

Base AC GDR
2 (Robe) 19.0%
6 (Scale mail) 25.0%
10 (Plate armour) 28.5%
16 (+2 Robe, aux) 32%
32 (+10 Plate armour, aux) 38.1%
40 (+14 Crystal plate, aux) 40.2%
50 42.5%
100 50.6%

Auxiliary slots include a +2 Helmet, +2 Cloak, +2 Gloves, and +2 Boots. Percents on this table are rounded to the nearest tenth.

Damage Reduction

GDR is not a form of damage reduction independent from AC. If you could have excellent GDR and terrible AC, it would do you little good. It is effectively a minimum AC roll, or a guaranteed AC roll. It is capped at GDR% * damage taken or 1/2 * AC, whichever is lower.

Let's say you take a 50 damage attack, but have 48% GDR. First, AC rolls. If AC roll is a 1, then GDR will raise the reduction to (50 * .48) = 24, meaning you actually lose 26 HP. But if your AC roll is 36, then GDR will not do anything, as you are above the guaranteed reduction of 24.

If you took said 50 damage attack with 16 AC and 32% GDR, then the AC/2 limit applies instead: you'll always reduce damage by at least (16 / 2) = 8 over the expected (50 * .32) = 16.

Strategy

GDR is tied to your AC; any form of extra AC will also increase GDR. This comes with the same costs and benefits; are you willing to sacrifice EV and spellcasting potential in order to become more bulky? Characters who want to avoid melee will still want to avoid melee as much as possible. However, GDR is a large reason why AC is more consistent in stopping damage.

For most characters, GDR will slightly reduce the maximum physical damage enemies can deal.

History

  • Prior to 0.27, GDR was more complex. Each set of body armour had a defined GDR, calculated by (14 * (Body Armour Base AC - 2)^(1/2))%, no matter what your actual AC was. Certain transmutations also had a set GDR. Gargoyles increased base GDR, while Draconians (being unable to wear body armour) didn't have any. For more information, see this past revision.
  • Prior to 0.8, GDR was equal to (Body Armour Base AC) * (13 + Armour) / 17.
  • Prior to 0.5, GDR was higher.
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