[MC-89043] Slime blocks moved by pistons often fail to bounce up the player

The bug

When a piston moves a slime block players are not always launched.

How to reproduce

Place a upwards facing sticky piston
Place a slime block on top
Place a redstone block on the side of the slime block
Update the piston
Jump on the slime block (which now moves up and down)
→ ❌ You will see that you just rarely get bounced up. For entities other than the player it works fine.

Example

This video shows it: https://www.youtube.com/watch?v=qgFt5L5f9jY

Additional information

It works fine for other entities, so it probably has to do with the client/server communication or similar.

Potential Fix

I noticed there is a flag called "velocityChanged" in MCP for entities. It is for example used when an entity is hit and needs to get knock back.

When the flag is set, the EntityTracker will send an extra packet containing the entities velocity.
Setting this flag when an entity is bounced by a slime block pretty much fixes this issue. (There is still an related one that if you fall on a moving slime block you don't get bounced which might have similar effects.)

Video with a comparison flag set vs flag not set: https://www.youtube.com/watch?v=VKHBJxtSlc0&t=2m42s

Linked issues

is duplicated by 14

MC-89124 Walking/sprinting near a slime block pushed in any orientation by a piston doesn't bounce players Resolved

MC-89382 pushed upwards slime blocks dont bounce players Resolved

MC-89537 Slime blocks don't push players up while moving Resolved

MC-90519 Players are not propelled by extending slime blocks Resolved

MC-93174 Slime Block Resolved

9 more links

relates to 3

MC-88833 Pistons don't move/bounce entities Resolved

MC-90063 Slime block propulsion Resolved

MC-44401 Falling on a Slime Block while holding the down the jump key will cause the player to NOT be launched high while still neglecting fall damage. Resolved

Attachments

2015-09-25 17-57-28.flv

My Movie.mp4

Comments 32

Dlawso the Really Lucky Rabbit 2015-09-19T01:09:38Z

IMO, most likely caused by the fix of (and possibly a regression of) MC-88833.

Panda4994 2015-09-19T01:13:01Z

Yeah, it definitely is caused by the (awesome!) recent changes to how pistons move entities.
I'm not sure where exactly this one is coming from yet, but I'm sure it can be resolved without backing off the previous fixes 🙂

tim Biesenbeek 2015-09-21T21:21:53Z

Still confirmd in 15w38b & 15w39b

SunCat 2015-09-24T13:24:10Z

Confirmed in 15w39c (MC-89382)

tim Biesenbeek 2015-09-25T15:58:13Z

15w39c Confirmd.

22 more comments

tim Biesenbeek 2015-10-19T14:00:17Z

Confirmed for 15w42a - still not working
still not working correctly, works sometimes though
never got it working in the middle of grouped slime blocks

Daniel Burnett 2015-11-09T15:52:35Z

Confirmed in 15w45a. Even if it's wired to only push when you're on top of it (such as with a command block proximity detector), you only get pushed up about 25% of the time when walking over it. If you stand still on it, it works fine, but if you move, then it seems to depend on either your speed or the exact distance from the block before when it triggers.

[Mod] Neko 2015-12-12T23:22:40Z

Confirmed for 15w50a

Triceror 2016-01-26T15:10:52Z

it might be a no brainer, but also confirmed in 1.8.9

tim Biesenbeek 2016-02-22T23:15:36Z

If you are moving - it 5/7 times fails
If server tps is lower then 20 - it fails.

every version is imporving the reliability

confrimd for 16w06 - 1.9pre1 - 1.9pre2

tim Biesenbeek 2016-02-25T10:49:42Z

Singleplayer is fine -multplayer its like 85 / 100 times now working
1.9pre3

tim Biesenbeek 2016-02-27T12:08:34Z

moving seems to be more bugged then pre3
standing still 100/100

confirmed 1.9pre4

tim Biesenbeek 2016-02-29T19:36:47Z

https://twitter.com/SeargeDP/status/704383283061202945
"No code changes between 1.9pre4 and 1.9"- SeargeDP
so, still confirmd 1.9 :/

James 2016-03-30T19:20:57Z

Slime blocks still only occasionally launch the player in 1.9.2. Maybe about a third of all times for me.

Also, whether it works or not seems to somehow be tied to the game's performance. I tried it on another, better computer and slime blocks definitely launched me more often, maybe about 80% of the time. Anyway, that may be why people get different probabilities.

tim Biesenbeek 2016-03-31T07:43:29Z

@Jelmer
I got fiberglass internet, and a super beafy server; and in multiplayer is more stable then single player on my laptop (when on lan)
so it's all relative
(Desktop is other way around)

Koala_eiO 2016-04-10T21:19:51Z

I came here to report this and found this already existing report.
I never had issues before, but since 1.9 came out all my jump pads broke.

James 2016-08-04T17:05:37Z

To add to my previous comment, I made two very interesting observations that may be helpful.

1) Whether the player is launched or not seems to not only be related to the performance, but to the frame rate itself. Now that sounds ridiculous and it probably shouldn't work like that, but I tested it several times by limiting the frame rate. At 10 FPS and probably lower successful launching rarely happens, above maybe 30 or 40 FPS it seems to succeed almost every time. All in all however it seems more reliable than it was in 1.9.

2) Other entities (tested with items and creepers) seem to be correctly launched every single time, even at a frame rate of 10 FPS.

RimaNari 2016-08-04T17:20:26Z

@unknown Does lowering the FPS in SP give any constraints to the TPS of the internal server as well? I assume it does, so your observations would be in agreement with that. If not though, that would certainly be strange.

James 2016-08-04T17:41:20Z

Apparently it does, but I'm pretty sure it shouldn't.

Alugia 2017-08-08T19:11:49Z

Confirmed for 1.12.1

Oval 2018-09-17T00:31:29Z

Confirmed for 1.13.1.

Meh-Vah d'Jukaar 2019-04-17T10:56:10Z

This seems to have gotten way worse since the latest 1.14 snapshots. Now if a player is moving on them while the piston is extending, he almost never gets launched up.

tim Biesenbeek 2019-04-17T10:58:49Z

Since 1.13 I never got up while moving
No worse or better experience for me

Marc G 2019-04-27T14:40:08Z

Got a lot worse in the official 1.14 release.

Jeremy P 2019-05-21T22:26:34Z

This is still a problem in 1.14.2 Pre-Release 2

Nathan B 2019-07-23T03:29:13Z

This is still a huge problem in 1.14.4. Please fix this bug, it's so annoying for us redstoners

James Fitzpatrick 2019-08-08T14:00:13Z

Significantly worse on 1.14.4 to the point of making slime block bounce mechanisms entirely unusable. Bounces almost never work on the first attempt now, and I've had periods of upwards of a minute of constant triggering where the block will never cause a bounce.

James Fitzpatrick 2019-11-22T14:04:24Z

The cause of the bug seems to be moving whilst the slime block moves. If I stand perfectly still the bounce will work almost every time, but walking or falling as it moves causes it to fail to have any effect.

Mark Sway 2019-12-15T23:01:40Z

Confirmed in 1.15.

Seems to be related to movement of the player, as mentioned above, but occurs sporadically. I've been able to successfully chain upwards of 20 bounces while sprinting in a row, yet at other times I can't even manage a single one.

Panda4994 2020-02-14T15:53:29Z

This should be noticeably better in 20w07a, however it can still fail if the player falls onto the moving slime block because of MC-123217.

Alexander Mitchell 2020-03-22T08:44:56Z

This is also a problem in Bedrock edition...

Spare_Sock 2024-11-28T05:02:28Z

Issue seems to have reoccurred as of 1.20.3 but only when the player is in a boat.

It doesn't effect other mobs in boats
happens when moving or at a standstill as long as player is in it.
Did not see any problems with launching myself on slimeblocks only when in a boat.

There was another post on this specifically but it was linked to this one as a duplicate so I'm commenting here.

Hope this is clear let me know if more info would be helpful