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Wearing any body armour with AC greater than 2 provides you with an amount of '''guaranteed damage reduction (GDR)''' in melee combat. Normally, each attack that hits you is reduced by a random amount up to your AC. GDR is a form of insurance against poor AC rolls; if you have sufficient AC, you are guaranteed to reduce the damage taken by a percentage of the enemy's maximum attack damage equal to your GDR. As an enemy's damage output is about as variable as your AC, a high GDR will allow you to completely negate an enemy's attacks much more often than normal, and those attacks that do get through will always be significantly diminished.
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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 GDR Affects==
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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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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==
 
==Calculating GDR==
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In most cases, your GDR% = (14*(Body Armour Base AC-2)^(1/2))%. No other type of armour provides any benefits to 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 Armours===
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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%
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| 10 ([[Plate armour]]) || 28.5%
 
|-
 
|-
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| [[Ring mail]], [[steam dragon armour]] || 5 || 24%
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| 16 (+2 [[Robe]], aux) || 32%
 
|-
 
|-
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| [[Scale mail]], [[mottled dragon armour]]  || 6 || 28%
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| 32 (+10 [[Plate armour]], aux) || 38.1%
 
|-
 
|-
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| [[Swamp dragon armour]] || 7 || 31%
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| 40 (+14 [[Crystal plate armour|Crystal plate]], aux) ||40.2%
 
|-
 
|-
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| [[Chain mail]], [[fire dragon armour]] || 8 || 34%
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| 50 || 42.5%
 
|-
 
|-
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| [[Ice dragon armour]] || 9 || 37%
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| 100 || 50.6%
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|-
 
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| [[Plate armour]], [[pearl dragon armour]], [[storm dragon armour]] || 10 || 39%
 
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|-
 
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| [[Gold dragon armour]] || 12 || 44%
 
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|-
 
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| [[Crystal plate armour]] || 14 || 48%
 
 
|-
 
|-
 
|}
 
|}
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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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There are also 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%). Finally, [[gargoyle]] characters receive an innate boost to GDR. This boost is inversely proportional to the GDR provided by your body armour.
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==Damage Reduction==
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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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==What GDR Affects==
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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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GDR only works against one thing: physical damage dealt by monsters attacking 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 elemental melee damage, such as an [[ice beast]]'s cold damage. However, most such elemental attacks need to deal physical damage to you before their added effects trigger, so high GDR will make them happen less often.
 
  
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==Guaranteed Damage Reduction Cap==
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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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GDR is '''not''' a form of damage reduction separate from AC. If you have excellent GDR and terrible AC, it will do you no good. Regardless of your GDR%, you are only ''guaranteed'' an amount of damage reduction equal to 1/2 of your AC. If an enemy's damage roll exceeds this guaranteed minimum, the game tests the attack against your AC. If the damage reduction from AC winds up being greater than your guaranteed damage reduction, that is used instead.
 
  
 
==Strategy==
 
==Strategy==
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GDR's usefulness depends heavily on how often you find yourself in melee combat, how high your AC is, and how much you're willing to sacrifice your [[ev]]asion defense (in general, the worse an armour is for your evasion, the higher its GDR will be). Unless you intend to wade into close combat whenever possible, ''and'' you have no intention of training [[Dodging]], you'll likely be better off avoiding super-heavy armour which maximizes GDR and focusing on optimizing your AC, EV, spells, and resistances to keep yourself alive. Characters that actively avoid melee, whether it be through fast movement speed or ranged attacks, will barely notice the effect of their GDR.
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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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==Example==
 
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When dealing with melee combat, 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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[[File:GDR_Example.png]]
 
  
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This chart illustrates an important aspect of GDR: GDR helps most when facing numerous small attacks. When facing 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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For most characters, GDR will slightly reduce the maximum physical damage enemies can deal.
  
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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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==History==
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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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