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https://github.com/genxium/DelayNoMore
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50 lines
3.4 KiB
Markdown
50 lines
3.4 KiB
Markdown
# What to be concerned for internet syncing
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1. Server received too late (solution: force confirmation)
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2. Client received too late (solution: prediction and frame chasing, big impact on user experience because the graphics will be inconsistent if mismatches occur too often)
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# Potential avalanche from local lag
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Under the current "input delay" algorithm, the lag of a single player would cause all the other players to receive outdated commands, e.g. when at a certain moment
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- player#1: renderFrameId = 100, significantly lagged due to local CPU overheated
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- player#2: renderFrameId = 240
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- player#3: renderFrameId = 239
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- player#4: renderFrameId = 242
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players #2, #3 #4 would receive "outdated(in their subjective feelings) but all-confirmed commands" from then on, thus forced to rollback and chase many frames - the lag due to "large range of frame-chasing" would then further deteriorate the situation - like an avalanche.
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**BE CAUTIOUS, THIS SITUATION HAPPENS QUITE OFTEN FOR REAL DEVICES** where different operating systems and temporary CPU overheat cause different lags for different player in a same battle! If not properly handled, slow tickers would be `inputing in the "history" of other players`, resulting in too frequent prediction mismatch and thus inconsistent graphics for other players!
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In a "no-server & p2p" setup, I couldn't think of a proper way to cope with such edge case. Solely on the frontend we could only mitigate the impact to players #2, #3, #4, e.g. a potential lag due to "large range of frame-chasing" is proactively avoided in `<proj-root>/frontend/assets/scripts/Map.js, function update(dt)`.
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To be fair, **a "p2p" setup can reduce round-trip to single-trip**, but w/o a point of authority in such case player#1 needs a way to recognize the slowness (e.g. check the received peer inputs) and ticks faster for a while to catch up; in contrast in a "server as authority" setup, the server could force confirming an inputFrame without player#1's upsync, and notify player#1 to apply a "roomDownsyncFrame" as well as drop all its outdated local inputFrames.
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# Start up frames
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renderFrameId | generatedInputFrameId | toApplyInputFrameId
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-------------------|----------------------------|----------------------
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0, 1, 2, 3 | 0, _EMP_, _EMP_, _EMP_ | 0
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4, 5, 6, 7 | 1, _EMP_, _EMP_, _EMP_ | 0
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8, 9, 10, 11 | 2, _EMP_, _EMP_, _EMP_ | 1
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12, 13, 14, 15 | 3, _EMP_, _EMP_, _EMP_ | 2
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It should be reasonable to assume that inputFrameId=0 is always of all-empty content, because human has no chance of clicking at the very first render frame.
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# Alignment of the current setup
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The following setup is chosen deliberately for some "%4" number coincidence.
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- NstDelayFrames = 2
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- InputDelayFrames = 4
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- InputScaleFrames = 2
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If "InputDelayFrames" is changed, the impact would be as follows, kindly note that "372%4 == 0".
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### pR.InputDelayFrames = 4
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renderFrameId | toApplyInputFrameId
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--------------------------|----------------------------------------------------
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368, 369, 370, 371 | 91
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372, 373, 374, 375 | 92
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### pR.InputDelayFrames = 5
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renderFrameId | toApplyInputFrameId
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--------------------------|----------------------------------------------------
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..., ..., ..., 368 | 90
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369, 370, 371, 372 | 91
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373, 374, 375, ... | 92
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