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Active: 665 users

Defusion by terrain – interesting Mine behaviour

Forum Index > Brood War Strategy
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1 2 Next All
Freakling
Profile Joined October 2012
Germany1529 Posts
Last Edited: 2021-02-27 11:15:58
February 21 2021 17:00 GMT
#1
This clip was brought up on the NHFFA discord, together with the question: "Why was there no damage here?"


The answer, it turns out, is rather interesting, courtesy of tscmoo:

tscmoo on SSCAIT discord wrote:
spider mines explode through the "castspell" iscript opcode, this leads to this check https://github.com/OpenBW/openbw/blob/master/bwgame.h#L14942 and then this spider mine specific check https://github.com/OpenBW/openbw/blob/master/bwgame.h#L14610 which, because of this line https://github.com/OpenBW/openbw/blob/master/bwgame.h#L5656, checks if the spider mine itself is on walkable terrain. if the spider mine is not on walkable terrain, then it does not deal any damage

bw pathfinding being what it is, put the spider mine on unwalkable terrain the moment it got in range
also slightly worth noting that this walkability test is buildtile/megatile resolution, so effectively it checks if more than 12 minitiles within that are are walkable
in that regard, the bw pathfinding might not be doing anything wrong here, it's just a resolution issue with the walkability test.


To add some more background to that:
  • Each terrain tile is made up of 4×4 minitiles, which are either walkable or unwalkable.
  • At game-start the game classifies each tile with at least 4 unwalkable minitiles as unwalkable, all other tiles as walkable.
  • Whether a ground unit can occupy a location depends solely on the minitile walkability property.
  • The tile the mine from the clip detonates on has 8/16 unwalkable minitiles and is thus classified as an unwalkable tile, which makes the Mine dud out.


Conclusion: Near unwalkable terrain, such as cliff edges, in particularlyin tight chokes, or doodads, there is a certain chance that Mine damage will be completely mitigated.

EDIT: Here's the whole situation recreated in ScmDraft with overlays showing the relevant walkabily properties:
+ Show Spoiler [Show animation] +
[image loading]


EDIT2: Thanks to StaticNine for another video demonstrating the effect. Note that the video title does not explain the conditions and cause of the effect correctly:
+ Show Spoiler [show video] +
kogeT
Profile Joined September 2013
Poland2037 Posts
February 21 2021 17:36 GMT
#2
Just another argument for PvT imba. :D
https://www.twitch.tv/kogetbw
Highgamer
Profile Joined October 2015
1407 Posts
February 21 2021 19:56 GMT
#3
And they complain about goon AI...
EMPaThy789
Profile Joined July 2009
New Zealand878 Posts
February 22 2021 02:54 GMT
#4
Time to lose all my dragoons trying to abuse this in a game.
FlaShFTW
Profile Blog Joined February 2010
United States10154 Posts
February 22 2021 03:22 GMT
#5
I love how this one clip is getting so much traction. My goon is a hero now, and his tale will be echoed throughout Auir.
Writer#1 KT and FlaSh Fanboy || Woo Jung Ho Never Forget || Teamliquid Political Decision Desk
TL+ Member
ggsimida
Profile Joined August 2015
1148 Posts
Last Edited: 2021-02-22 06:01:36
February 22 2021 06:01 GMT
#6
what an intriguing 20+ yr old game

thx for sharing
Freakling
Profile Joined October 2012
Germany1529 Posts
February 22 2021 10:13 GMT
#7
On February 22 2021 12:22 FlaShFTW wrote:
I love how this one clip is getting so much traction. My goon is a hero now, and his tale will be echoed throughout Auir.

Wait till Bisu finds out – and perfects his cliff-hugging technique
MeSaber
Profile Joined December 2009
Sweden1235 Posts
February 22 2021 13:35 GMT
#8
Cool and all, my question then is how do i know where the unwalkable terrain is? Is it randomized on game start or in map editor?

No way of knowing?
-.-
Freakling
Profile Joined October 2012
Germany1529 Posts
February 22 2021 14:26 GMT
#9
If you want to know for sure, the easiest way it to open the map in ScmDraft:
[image loading]
Highgamer
Profile Joined October 2015
1407 Posts
February 22 2021 14:58 GMT
#10
Jeez, no wonder stuff gets stuck on ramps sometimes...
Freakling
Profile Joined October 2012
Germany1529 Posts
February 22 2021 15:04 GMT
#11
The red is unwalkable tiles. As I explained where stuff can go is determined on the minitile level, which is the grey overlay.
As far as this topic is concerned: Grey prohibits movement, red makes Mines dud.
Highgamer
Profile Joined October 2015
1407 Posts
Last Edited: 2021-02-22 19:12:19
February 22 2021 19:10 GMT
#12
Just one quick question: What exactly do you mean by "grey prohibits movement"?

From your explanation above - 'ground units can occupy walkable mini-tiles' - I had the impression that units can walk/move on walkable mini-tiles, despite the whole tile being classified as unwalkable. Maybe only by patch-finding accident, but isn't that what the mine in the video did, no?

Or did you mean: "Normally units won't step on walkable mini-tiles if the whole tile is unwalkable"?
Are units being pushed off of a walkable mini-tile (in an unwalkable whole-tile) should they accidentally move onto it?

Thx in advance.
Freakling
Profile Joined October 2012
Germany1529 Posts
Last Edited: 2021-02-22 20:28:46
February 22 2021 20:25 GMT
#13
Units can occupy any walkable space they can fit in. But if the tile is flagged unwalkable any mine explosions on it will be duds. The mine can move onto the tile because it is partly walkable (has walkable minitiles), but doesn't deal damage because the whole tile is flagged as unwalkable.

Or, to be very technical about it:
  • A ground unit's collision box may not overlap with any unwalkable minitiles
  • If a ground unit overlaps with unwalkable terrain it will be force-moved to the "nearest" (according to the game's idea of nearest…) walkable area (this just as a side note, it is not relevant for the effect discussed in this thread).
  • The centre of the collision box (rounded up coordinates) is the actual unit position, which is what is checked to determine the terrain flag checks for the unit.


Maybe it's not quite clear what I mean with grey, because the grey overlay actually looks dark red under the red overlay… Can you see that there are two overlays visible on that image, the darker one with 4× the resolution of the other? Maybe I should make a gif…
Highgamer
Profile Joined October 2015
1407 Posts
Last Edited: 2021-02-23 09:57:55
February 23 2021 09:52 GMT
#14
Thx Freakling, it's all cleared up now that you mentioned that the grey overlay is where it's deep red.

I think you're fine if you maybe just use the terms "deep red/light red" instead of "grey/red" in your above post.

Other than that, precise in-depth BW-knowledge as always.
Freakling
Profile Joined October 2012
Germany1529 Posts
February 23 2021 10:10 GMT
#15
I added an animation to the OP that shows the terrain analysis step by step.
MeSaber
Profile Joined December 2009
Sweden1235 Posts
February 23 2021 18:40 GMT
#16
Nice!
-.-
Dangermousecatdog
Profile Joined December 2010
United Kingdom7084 Posts
Last Edited: 2021-02-23 20:30:27
February 23 2021 20:13 GMT
#17
So there are mini-tiles that are either walkable or unwalkable, but a tile as a whole is either flagged as walkable or unwalkable? Are there any other circumstance where a whole tile being flagged walkable or unwalkable matters? Other than building placment presumably. Scarab duds too due to this perhaps?

I guess with this info the best way to make a spider mine dud is make it hit a unit on the sides of a ramp or throw your unit to a corner and hope for the best.
Freakling
Profile Joined October 2012
Germany1529 Posts
Last Edited: 2021-02-25 20:13:14
February 24 2021 10:15 GMT
#18
On February 24 2021 05:13 Dangermousecatdog wrote:
So there are mini-tiles that are either walkable or unwalkable, but a tile as a whole is either flagged as walkable or unwalkable? Are there any other circumstance where a whole tile being flagged walkable or unwalkable matters? Other than building placment presumably.
Pathfinding regions are generated and connected based on the walkability and terrain level tile flags (both generated from the respective minitile flags at game-start), so indirectly they have a huge bearing on long-range pathfinding and a plethora of associated behaviours.

Buildability is only determined by the buildability flag, a game-generated "partially walkable" flag (which is set as soon as one minitile of a tile in unwlkable) and a creep flag for Zerg (there are some special cases regarding partially unwalkable, but buildable tiles).
Creep spread is only determined by the buildability flag though (and has some unintended buggy interaction with a deprecated "flipped doodad sprite" flag).

Scarab duds too due to this perhaps?
Read the quote from the OP, follow the references! The relevant flag check is specifically hardcoded for Mines only.
Scarabs dud due to timeout, when they cannot reach their target due to other units blocking the way, or when the Reaver owner aborts the attack (gives the Reaver a stop order).

I guess with this info the best way to make a spider mine dud is make it hit a unit on the sides of a ramp or throw your unit to a corner and hope for the best.
Something like this. In practice it is situationally useful to know and somewhat luck based, I guess.
Freakling
Profile Joined October 2012
Germany1529 Posts
Last Edited: 2021-02-25 20:10:40
February 24 2021 10:20 GMT
#19
double post, delete please
Dangermousecatdog
Profile Joined December 2010
United Kingdom7084 Posts
Last Edited: 2021-02-25 20:51:16
February 25 2021 20:50 GMT
#20
Thanks Freakling, for the informative answers.
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