0006262: Rounding Error in Calculation of Projectile Velocities

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Date Submitted

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0006262

Dwarf Fortress

Throwing/Shooting

public

2012-12-12 05:18

2014-08-03 13:29

Reporter

Pirate Bob

Assigned To

Toady One

Priority

normal

Severity

trivial

Reproducibility

always

Status

resolved

Resolution

fixed

Platform

Computer

Windows/Linux

OS Version

all

Product Version

0.34.11

Target Version

Fixed in Version

0.40.05

Summary

0006262: Rounding Error in Calculation of Projectile Velocities

Description

Projectile (bolts, arrows, blowdarts) velocities appear to be calculated using the formula:
velocity=[SHOOT_FORCE]/(([SOLID_DENSITY]*[SIZE])/1,000,000),
and then capped to [SHOOT_MAXVEL]. All numbers are treated as integers throughout, and therefore floored to the nearest integer after each multiplication/division. [SHOOT_FORCE], [SIZE] and [SHOOT_MAXVEL] are defined by the launcher (bow, crossbow, etc) and [SOLID_DENSITY] by the ammo material. See this thread for details and derivation: http://www.bay12forums.com/smf/index.php?topic=116151.msg3716892;topicseen#msg3716892 [^]

As a result:
1) Any bolts/arrows with [SOLID_DENSITY] between 6666 and 13333 (all metals except adamantine) will have velocity equal to [SHOOT_FORCE].
2) Bolts lighter than [SOLID_DENSITY:6666] (wood, bone, adamantine) will have velocity equal to [SHOOT_MAXVEL].
3) Since both [SHOOT_FORCE] and [SHOOT_MAXVEL] are 1000, all bolts will have velocity 1000.
4) Armor penetration (in most cases) and damage depend on bolt momentum, so bolts made of denser materials will be more effective.

Steps To Reproduce

Download the script chkproj.lua from http://www.bay12forums.com/smf/index.php?topic=116151.msg3716892;topicseen#msg371689 [^] and use it to check projectile velocities, which are given by the variable unk22. By changing [SHOOT_FORCE], [SHOOT_MAXVAL] and bolt [SOLID_DENSITY] one can quickly verify the rounding behavior.

This has also been verified by measuring deflection of bolts off armored targets. All metal bolts were found to have exactly the same rate of deflection as a function of momentum when momentum was calculated using the velocity formula described, and mass=([SOLID_DENSITY]*[SIZE])/1,000,000. See http://www.bay12forums.com/smf/index.php?topic=116151.msg3733282#msg3733282. [^]

Additional Information

This could easily be fixed by changing the velocity calculation to:
velocity=1,000,000*SHOOT_FORCE/[SOLID_DENSITY]/[SIZE]
If this is not possible, then the original calculation could be used with all numbers converted to floating point within the calculation, and then the final velocity rounded to an integer.

The effects of this bug cannot be detected without modding/hacking in the current version because all bolts have such high momenta that small differences have no noticeable impact. I have therefore marked it as trivial. However, if bolt momenta were reduced to give armor a chance of stopping projectiles (see [url=http://www.bay12games.com/dwarves/mantisbt/view.php?id=5516 [^]]5516[/url]), then this bug would become significant.

It is also possible, but I think very unlikely, that this is "intended behavior". This bug renders the [SHOOT_FORCE] parameter almost useless, as all projectiles are fired with the same velocity. If Toady wanted all projectiles fired with the same velocity, he could have just set a fixed projectile velocity rather than calculating it from [SHOOT_FORCE].

Tags

No tags attached.

Attached Files

Relationships

related to	0005516	resolved	Toady One	Absurdly Powerful Bolts/Arrows
related to	0006364	resolved	Toady One	Weight fraction not considered when calculating weapon velocities
related to	0006870	resolved	Toady One	Bolts, crossbows and other ranged weapons have excessive SHOOT_FORCE values, leading to ranged combat issues

Notes
(0023774) Pirate Bob (reporter) 2012-12-12 05:58	This bug should be marked as related to 0005516, and vice versa, but I can't figure out how to do that (maybe admin privileges are required?). Also, sorry about the nasty looking links - I thought maybe BBcode would work, but apparently not...

(0023944) Quietust (reporter) 2013-04-13 15:34	For reference, the "chkproj.lua" mentioned above is a script for use with DFHack and thus isn't likely to be useful to Toady in tracking down the problem (since he can debug the game directly).

(0023947) ScegfOd (reporter) 2013-04-19 05:00	this bug reminded me of a C++ tutorial i was looking at once upon a time... does Toady program in C++? type casting might fix this bug easily http://www.cprogramming.com/tutorial/c/lesson11.html [^]

(0024164) UristDaVinci (reporter) 2013-10-09 12:39	I used windbg on the relevant code, and this "bug" is caused by only the integer weight of the bolt being used in the integer division SHOOT_FORCE/WEIGHT. It appears that this code was imported from earlier versions of Dwarf Fortress (such as "40d"), where weight numbers were all larger. This code wasn't updated when fractional weights were introduced. It's almost not a bug, instead just a non-updated feature like siege weapons.

Issue History
Date Modified	Username	Field	Change
2012-12-12 05:18	Pirate Bob	New Issue
2012-12-12 05:51	Pirate Bob	Issue Monitored: Pirate Bob
2012-12-12 05:58	Pirate Bob	Note Added: 0023774
2012-12-12 07:18	PetWolverine	Issue Monitored: PetWolverine
2012-12-12 09:48	Knight Otu	Relationship added	related to 0005516
2013-04-13 13:07	Pirate Bob	Tag Attached: Probable Quick Fix
2013-04-13 15:34	Quietust	Note Added: 0023944
2013-04-19 05:00	ScegfOd	Note Added: 0023947
2013-08-02 04:28	Knight Otu	Relationship added	related to 0006364
2013-10-09 12:39	UristDaVinci	Note Added: 0024164
2013-10-28 11:22	JimiD	Issue Monitored: JimiD
2014-01-17 10:33	Kirig Stonebeard	Issue Monitored: Kirig Stonebeard
2014-07-14 11:28	Footkerchief	Relationship added	has duplicate 0006870
2014-07-24 09:30	Footkerchief	Tag Detached: Probable Quick Fix
2014-07-24 14:35	Footkerchief	Relationship replaced	related to 0006870
2014-07-24 15:05	Toady One	Status	new => resolved
2014-07-24 15:05	Toady One	Fixed in Version	=> Next Version
2014-07-24 15:05	Toady One	Resolution	open => fixed
2014-07-24 15:05	Toady One	Assigned To	=> Toady One
2014-08-03 13:29	PetWolverine	Issue End Monitor: PetWolverine