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Kent Daleng
392fc27de1
* wiki testing * wiki updates * use .md with anchors, revert sidebar * bump wiki action * add some more anchors, fix some language * change links to be more descriptive by themselves
23 lines
1.7 KiB
Markdown
23 lines
1.7 KiB
Markdown
On a TTY, only one frame can be submitted to an output at a time, and the compositor must wait until the output repaints (indicated by a VBlank) to be able to submit the next frame.
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In niri we keep track of this via the `RedrawState` enum that you can find in an `OutputState`.
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Here's a diagram of state transitions for the `RedrawState` state machine:
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<picture>
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<source media="(prefers-color-scheme: dark)" srcset="./img/RedrawState-dark.drawio.png">
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<img alt="RedrawState state transition diagram" src="./img/RedrawState-light.drawio.png">
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</picture>
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`Idle` is the default state, when the output does not need to be repainted.
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Any operation that may cause the screen to update calls `queue_redraw()`, which moves the output to a `Queued` state.
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Then, at the end of an event loop dispatch, niri calls `redraw()` for every `Queued` output.
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If the redraw causes damage (i.e. something on the output changed), we move into the `WaitingForVBlank` state, since we cannot redraw until we receive a VBlank event.
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However, if there's no damage, we do not return to `Idle` right away.
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Instead, we set a timer to fire roughly at when the next VBlank would occur, and transition to a `WaitingForEstimatedVBlank` state.
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This is necessary in order to throttle frame callbacks sent to applications to at most once per output refresh cycle.
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Without this throttling, applications can start continuously redrawing without damage (for instance, if the application window is partially off-screen, and it is only the off-screen part that changes), and eating a lot of CPU in the process.
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Then, either the estimated VBlank timer completes, and we go back to `Idle`, or maybe we call `queue_redraw()` once more and try to redraw again.
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