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IDProjectCategoryView StatusLast Update
0004924The Dark ModPhysicspublic02.07.2022 16:22
Reporterstgatilov Assigned Tostgatilov  
Status resolvedResolutionfixed 
OSWindowsOS Versionx64 
Product VersionTDM 2.06 
Target VersionTDM 2.08Fixed in VersionTDM 2.08 
Summary0004924: Rope physics mad when FPS is low and uncapped
DescriptionWhen uncapped FPS is on (aka com_fixedTic 1), most of physics is modeled with arbitrary deltatime depending on framerate (instead of fixed 16 ms step).
When FPS is low, rope physics becomes crazy if you try to swing on it.

Originally posted here:
Steps To Reproduce1. Load a mission with rope (e.g. PD3: Erasing the Trail).
2. Find the rope (e.g. at coords 5293.07 896.25 108.25 15.2 18.4 0.0).
3. Enable "Uncapped FPS".
4. Make sure FPS gets low (e.g. use antialiasing + high-quality soft shadows).
5. Get yourself onto the rope (e.g. using noclip temporarily).
6. Try to swing on the rope and then jump from it.
7. Look back at the rope.

In my case, I am thrown out from the rope immediately when I try to swing on it. The rope is jumping around like a crazy polyline after that.

The problem happens when:
  com_fixedTic 1, FPS = 30
The problem does not happen when:
  com_fixedTic 0, FPS = 30
  com_fixedTic 1, FPS = 60

The video showing the problem:
TagsNo tags attached.


related to 0004983 closed AIs randomly dying with uncapped and low FPS 
related to 0004665 resolvedstgatilov Mouse Sensitivity Tied to Framerate 
related to 0004698 resolvedstgatilov Timescale command doesn't work with uncapped FPS / com_fixedTic 1 
related to 0005992 resolvedstgatilov Experiment with game tics longer than 17 ms 
child of 0004493 resolvedduzenko Some physics events have changed in 2.05 due to faster fps 




23.12.2018 14:39

developer   ~0011070

Last edited: 23.12.2018 14:41

Isn't com_fixedTic 0 a sufficient and complete fix for this?
If we really want to make it fluid then someone needs to dive into the rope code in search of hardcoded const's



23.12.2018 14:44

administrator   ~0011072

Yes, disabling "Uncapped FPS" removes the problem completely.
So it is not critical for release, hence scheduled for 2.08.

I understand that it is not a simple problem, but I hope someone could fix it in some future =)


31.12.2018 13:26

developer   ~0011195

If someone goes into this issue, maybe it could also be investigated why rope-swing is so clumsy.


06.01.2019 19:29

developer   ~0011250

These crazy rope physics also affect the player when he is attached to it, btw. Try it e.g. on the training mission.

Although I have a decent rig, running the latest Debug build (rev. 7897) resulted in 20 to 30 fps. In the training mission, I just jumped on the hanging rope over the channel and was thrown around like crazy, eventually landing in the water, while awkwardly still being attached to the rope.


07.01.2019 02:57

developer   ~0011251

Last edited: 07.01.2019 04:13

I'm gonna try a little hack to improve 2.07 until a proper solution is derived:


if ( idSessionLocal::com_fixedTic.GetBool() && com_timescale.GetFloat() == 1 && !physicsObj.OnRope() )

Edit: Doesn't work when applied to just game_local...



07.01.2019 04:32

administrator   ~0011252

Nbohr1more, please do not hack everything to make it work.
It might happen soon that nobody will be able to understand how different modes interact with each other.

I understand that debug visualization tools are much easier to send to non-capped mode than support them properly, and they are not for ordinary player. But please do NOT toggle uncapped FPS for ordinary gameplay.


07.01.2019 04:47

developer   ~0011253


I wasn't really happy with this workaround anyway.


01.12.2019 17:25

administrator   ~0011885

One more forum thread about the topic:

Basically, I have a feeling that the problem is caused by instability of ODE solver approach (which is most likely explicit Euler) on large timesteps. The suggested fix is to limit timestep of any game tic by 17 ms.


05.12.2019 08:10

administrator   ~0011889

Done in svn rev 8435.

Now with com_fixedTic = 1, game tics are limited by 17 ms (com_maxTicTimestep).
When com_fixedTic = 0, then all game tics are exactly 16 ms --- as before.
In both modes, at most 10 (com_maxTicsPerFrame) tics are modelled each frame. If frame takes more than 170 ms, then game time perceptively slows down.

Issue History

Date Modified Username Field Change
23.12.2018 12:45 stgatilov New Issue
23.12.2018 12:46 stgatilov Steps to Reproduce Updated
23.12.2018 12:50 stgatilov Product Version TDM 2.07 => TDM 2.06
23.12.2018 12:50 stgatilov Description Updated
23.12.2018 12:50 stgatilov Target Version => TDM 2.08
23.12.2018 12:53 stgatilov Relationship added related to 0004493
23.12.2018 14:39 duzenko Note Added: 0011070
23.12.2018 14:41 duzenko Note Edited: 0011070
23.12.2018 14:44 stgatilov Note Added: 0011072
31.12.2018 13:26 STiFU Note Added: 0011195
06.01.2019 19:29 STiFU Note Added: 0011250
07.01.2019 02:57 nbohr1more Note Added: 0011251
07.01.2019 03:20 nbohr1more Note Edited: 0011251
07.01.2019 03:48 nbohr1more Note Edited: 0011251
07.01.2019 04:12 nbohr1more Note Edited: 0011251
07.01.2019 04:13 nbohr1more Note Edited: 0011251
07.01.2019 04:32 stgatilov Note Added: 0011252
07.01.2019 04:47 nbohr1more Note Added: 0011253
01.02.2019 04:50 stgatilov Relationship added related to 0004983
01.02.2019 05:40 nbohr1more Relationship deleted related to 0004493
01.02.2019 05:41 nbohr1more Relationship added child of 0004493
01.12.2019 17:25 stgatilov Note Added: 0011885
05.12.2019 08:10 stgatilov Note Added: 0011889
05.12.2019 08:43 stgatilov Assigned To => stgatilov
05.12.2019 08:43 stgatilov Status new => resolved
05.12.2019 08:43 stgatilov Resolution open => fixed
05.12.2019 08:43 stgatilov Fixed in Version => TDM 2.08
13.05.2020 04:14 nbohr1more Relationship added related to 0004665
21.07.2020 05:02 nbohr1more Relationship added related to 0004698
02.07.2022 16:22 stgatilov Relationship added related to 0005992