mirror of
https://gitlab.freedesktop.org/monado/monado.git
synced 2024-12-28 18:46:18 +00:00
49 lines
2.5 KiB
Markdown
49 lines
2.5 KiB
Markdown
# Frame Pacing/Timing {#frame-pacing}
|
|
|
|
<!--
|
|
Copyright 2021, Collabora, Ltd. and the Monado contributors
|
|
SPDX-License-Identifier: BSL-1.0
|
|
-->
|
|
|
|
A "brief" overview of the various time-points that a frame goes through, from
|
|
when the application gets go ahead to render the frame to when pixels are turned
|
|
into photons. This is only a single frame, where all of the timings are achieved and
|
|
the application is single threaded. The HMD also only turns on the display
|
|
during the vblank period, meaning the pixel to photon transformation is delayed
|
|
from scanout starting to the vblank period (like for the Index).
|
|
|
|
* `xrWaitFrame` returns to the application, referred to as **wake_up**.
|
|
* The app does a logic step to move the simulation to the next predicted
|
|
display time.
|
|
* `xrBeginFrame` is called by the application, referred to as **begin**.
|
|
* The app renders the current views using the GPU.
|
|
* `xrEndFrame` is called by the application submitting the views.
|
|
* The compositor wakes up to do its distorting rendering and any warping,
|
|
checking if the applications rendering has finished. When the compositor
|
|
submits the work to the GPU, referred to as **submit**.
|
|
* The compositor queues its swapbuffer to the display engine.
|
|
* Scanout starts, the kernel checked if the compositors rendering was completed
|
|
in time. We refer to this time as **present**, this seems to be common.
|
|
* During the vblank period the display lights up turning the pixels into
|
|
photons. We refer to this time as **display**, same as in OpenXR.
|
|
|
|
The names for timepoints are chosen to align with the naming in
|
|
[`VK_GOOGLE_display_timing`][], reading that extension can provide further
|
|
information.
|
|
|
|
## Main compositor perspective
|
|
|
|
* @ref xrt_comp_wait_frame - It is within this function that the frame timing is
|
|
predicted, see @ref u_pa_predict and @ref u_pc_predict. The compositor will
|
|
then wait to **wake_up** time and then return from this function.
|
|
* @ref xrt_comp_begin_frame - The app or IPC server calls this function when it
|
|
is done with CPU work and ready to do GPU work.
|
|
* @ref xrt_comp_discard_frame - The frame is discarded.
|
|
* @ref xrt_comp_layer_begin - Called during transfers of layers.
|
|
* @ref xrt_comp_layer_projection - This and other layer functions are
|
|
called to list the layers the compositor should render.
|
|
* @ref xrt_comp_layer_commit - The compositor starts to render the frame,
|
|
trying to finish at the **present** time.
|
|
|
|
[`VK_GOOGLE_display_timing`]: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VK_GOOGLE_display_timing.html
|