registry/vulkan/chapters/VK_GOOGLE_display_timing/queries.txt

// Copyright (c) 2016-2018 Google LLC.
//
// SPDX-License-Identifier: CC-BY-4.0

== Display Timing Queries

Traditional game and real-time-animation applications frequently use
ename:VK_PRESENT_MODE_FIFO_KHR so that presentable images are updated during
the vertical blanking period of a given refresh cycle (RC) of the
presentation engine's display.
This avoids the visual anomaly known as tearing.

However, synchronizing the presentation of images with the RC does not
prevent all forms of visual anomalies.
Stuttering occurs when the geometry for each presentable image is not
accurately positioned for when that image will be displayed.
The geometry may appear to move too little some RCs, and too much for
others.
Sometimes the animation appears to freeze, when the same image is used for
more than one RC.

In order to minimize stuttering, an application needs to correctly position
their geometry for when the presentable image will be displayed to the user.
To accomplish this, applications need various timing information about the
presentation engine's display.
They need to know when presentable images were actually presented, and when
they could have been presented.
Applications also need to tell the presentation engine to display an image
no sooner than a given time.
This can allow the application's animation to look smooth to the user, with
no stuttering.
The `VK_GOOGLE_display_timing` extension allows an application to satisfy
these needs.

The presentation engine's display typically refreshes the pixels that are
displayed to the user on a periodic basis.
The period may be fixed or variable.
In many cases, the presentation engine is associated with fixed refresh rate
(FRR) display technology, with a fixed refresh rate (RR, e.g. 60Hz).
In some cases, the presentation engine is associated with variable refresh
rate (VRR) display technology, where each refresh cycle (RC) can vary in
length.
This extension treats VRR displays as if they are FRR.

[open,refpage='vkGetRefreshCycleDurationGOOGLE',desc='Obtain the RC duration of the PE\'s display',type='protos']
--
To query the duration of a refresh cycle (RC) for the presentation engine's
display, call:

include::{generated}/api/protos/vkGetRefreshCycleDurationGOOGLE.txt[]

  * pname:device is the device associated with pname:swapchain.
  * pname:swapchain is the swapchain to obtain the refresh duration for.
  * pname:pDisplayTimingProperties is a pointer to a
    sname:VkRefreshCycleDurationGOOGLE structure.

include::{generated}/validity/protos/vkGetRefreshCycleDurationGOOGLE.txt[]
--

[open,refpage='VkRefreshCycleDurationGOOGLE',desc='Structure containing the RC duration of a display',type='structs']
--
The sname:VkRefreshCycleDurationGOOGLE structure is defined as:

include::{generated}/api/structs/VkRefreshCycleDurationGOOGLE.txt[]

  * pname:refreshDuration is the number of nanoseconds from the start of one
    refresh cycle to the next.

include::{generated}/validity/structs/VkRefreshCycleDurationGOOGLE.txt[]
--

[NOTE]
.Note
====
The rate at which an application renders and presents new images is known as
the image present rate (IPR, aka frame rate).
The inverse of IPR, or the duration between each image present, is the image
present duration (IPD).
In order to provide a smooth, stutter-free animation, an application will
want its IPD to be a multiple of pname:refreshDuration.
For example, if a display has a 60Hz refresh rate, pname:refreshDuration
will be a value in nanoseconds that is approximately equal to 16.67ms.
In such a case, an application will want an IPD of 16.67ms (1X multiplier of
pname:refreshDuration), or 33.33ms (2X multiplier of pname:refreshDuration),
or 50.0ms (3X multiplier of pname:refreshDuration), etc.

In order to determine a target IPD for a display (i.e. a multiple of
pname:refreshDuration), an application needs to determine when its images
are actually displayed.
Let's say that an application has an initial target IPD of 16.67ms (1X
multiplier of pname:refreshDuration).
It will therefore position the geometry of a new image 16.67ms later than
the previous image.
Let's say that this application is running on slower hardware, so that it
actually takes 20ms to render each new image.
This will create visual anomalies, because the images will not be displayed
to the user every 16.67ms, nor every 20ms.
In this case, it is better for the application to adjust its target IPD to
33.33ms (i.e. a 2X multiplier of pname:refreshDuration), and tell the
presentation engine to not present images any sooner than every 33.33ms.
This will allow the geometry to be correctly positioned for each presentable
image.

Adjustments to an application's IPD may be needed because different views of
an application's geometry can take different amounts of time to render.
For example, looking at the sky may take less time to render than looking at
multiple, complex items in a room.
In general, it is good to not frequently change IPD, as that can cause
visual anomalies.
Adjustments to a larger IPD because of late images should happen quickly,
but adjustments to a smaller IPD should only happen if the
pname:actualPresentTime and pname:earliestPresentTime members of the
slink:VkPastPresentationTimingGOOGLE structure are consistently different,
and if pname:presentMargin is consistently large, over multiple images.
====

[open,refpage='vkGetPastPresentationTimingGOOGLE',desc='Obtain timing of a previously-presented image',type='protos']
--
The implementation will maintain a limited amount of history of timing
information about previous presents.
Because of the asynchronous nature of the presentation engine, the timing
information for a given flink:vkQueuePresentKHR command will become
available some time later.
These time values can be asynchronously queried, and will be returned if
available.
All time values are in nanoseconds, relative to a monotonically-increasing
clock (e.g. `CLOCK_MONOTONIC` (see clock_gettime(2)) on Android and Linux).

To asynchronously query the presentation engine, for newly-available timing
information about one or more previous presents to a given swapchain, call:

include::{generated}/api/protos/vkGetPastPresentationTimingGOOGLE.txt[]

  * pname:device is the device associated with pname:swapchain.
  * pname:swapchain is the swapchain to obtain presentation timing
    information duration for.
  * pname:pPresentationTimingCount is a pointer to an integer related to the
    number of sname:VkPastPresentationTimingGOOGLE structures to query, as
    described below.
  * pname:pPresentationTimings is either `NULL` or a pointer to an array of
    sname:VkPastPresentationTimingGOOGLE structures.

If pname:pPresentationTimings is `NULL`, then the number of newly-available
timing records for the given pname:swapchain is returned in
pname:pPresentationTimingCount.
Otherwise, pname:pPresentationTimingCount must: point to a variable set by
the user to the number of elements in the pname:pPresentationTimings array,
and on return the variable is overwritten with the number of structures
actually written to pname:pPresentationTimings.
If the value of pname:pPresentationTimingCount is less than the number of
newly-available timing records, at most pname:pPresentationTimingCount
structures will be written, and ename:VK_INCOMPLETE will be returned instead
of ename:VK_SUCCESS, to indicate that not all the available timing records
were returned.

include::{generated}/validity/protos/vkGetPastPresentationTimingGOOGLE.txt[]
--

[open,refpage='VkPastPresentationTimingGOOGLE',desc='Structure containing timing information about a previously-presented image',type='structs']
--
The sname:VkPastPresentationTimingGOOGLE structure is defined as:

include::{generated}/api/structs/VkPastPresentationTimingGOOGLE.txt[]

  * pname:presentID is an application-provided value that was given to a
    previous fname:vkQueuePresentKHR command via
    slink:VkPresentTimeGOOGLE::pname:presentID (see below).
    It can: be used to uniquely identify a previous present with the
    flink:vkQueuePresentKHR command.
  * pname:desiredPresentTime is an application-provided value that was given
    to a previous flink:vkQueuePresentKHR command via
    slink:VkPresentTimeGOOGLE::pname:desiredPresentTime.
    If non-zero, it was used by the application to indicate that an image
    not be presented any sooner than pname:desiredPresentTime.
  * pname:actualPresentTime is the time when the image of the
    pname:swapchain was actually displayed.
  * pname:earliestPresentTime is the time when the image of the
    pname:swapchain could have been displayed.
    This may: differ from pname:actualPresentTime if the application
    requested that the image be presented no sooner than
    slink:VkPresentTimeGOOGLE::pname:desiredPresentTime.
  * pname:presentMargin is an indication of how early the
    fname:vkQueuePresentKHR command was processed compared to how soon it
    needed to be processed, and still be presented at
    pname:earliestPresentTime.

The results for a given pname:swapchain and pname:presentID are only
returned once from fname:vkGetPastPresentationTimingGOOGLE.

The application can: use the sname:VkPastPresentationTimingGOOGLE values to
occasionally adjust its timing.
For example, if pname:actualPresentTime is later than expected (e.g. one
pname:refreshDuration late), the application may increase its target IPD to
a higher multiple of pname:refreshDuration (e.g. decrease its frame rate
from 60Hz to 30Hz).
If pname:actualPresentTime and pname:earliestPresentTime are consistently
different, and if pname:presentMargin is consistently large enough, the
application may decrease its target IPD to a smaller multiple of
pname:refreshDuration (e.g. increase its frame rate from 30Hz to 60Hz).
If pname:actualPresentTime and pname:earliestPresentTime are same, and if
pname:presentMargin is consistently high, the application may delay the
start of its input-render-present loop in order to decrease the latency
between user input and the corresponding present (always leaving some margin
in case a new image takes longer to render than the previous image).
An application that desires its target IPD to always be the same as
pname:refreshDuration, can also adjust features until
pname:actualPresentTime is never late and pname:presentMargin is
satisfactory.
--

The full `apiext:VK_GOOGLE_display_timing` extension semantics are described
for swapchains created with ename:VK_PRESENT_MODE_FIFO_KHR.
For example, non-zero values of
sname:VkPresentTimeGOOGLE::pname:desiredPresentTime must: be honored, and
fname:vkGetPastPresentationTimingGOOGLE should: return a
sname:VkPastPresentationTimingGOOGLE structure with valid values for all
images presented with fname:vkQueuePresentKHR.
The semantics for other present modes are as follows:

  * ename:VK_PRESENT_MODE_IMMEDIATE_KHR.
    The presentation engine may: ignore non-zero values of
    sname:VkPresentTimeGOOGLE::pname:desiredPresentTime in favor of
    presenting immediately.
    The value of
    sname:VkPastPresentationTimingGOOGLE::pname:earliestPresentTime must: be
    the same as
    sname:VkPastPresentationTimingGOOGLE::pname:actualPresentTime, which
    should: be when the presentation engine displayed the image.
  * ename:VK_PRESENT_MODE_MAILBOX_KHR.
    The intention of using this present mode with this extension is to
    handle cases where an image is presented late, and the next image is
    presented soon enough to replace it at the next vertical blanking
    period.
    For images that are displayed to the user, the value of
    sname:VkPastPresentationTimingGOOGLE::pname:actualPresentTime must: be
    when the image was displayed.
    For images that are not displayed to the user,
    fname:vkGetPastPresentationTimingGOOGLE may: not return a
    sname:VkPastPresentationTimingGOOGLE structure, or it may: return a
    sname:VkPastPresentationTimingGOOGLE structure with the value of zero
    for both sname:VkPastPresentationTimingGOOGLE::pname:actualPresentTime
    and sname:VkPastPresentationTimingGOOGLE::pname:earliestPresentTime.
    It is possible that an application can: submit images with
    sname:VkPresentTimeGOOGLE::pname:desiredPresentTime values such that new
    images may: not be displayed.
    For example, if sname:VkPresentTimeGOOGLE::pname:desiredPresentTime is
    far enough in the future that an image is not presented before
    fname:vkQueuePresentKHR is called to present another image, the first
    image will not be displayed to the user.
    If the application continues to do that, the presentation may: not
    display new images.
  * ename:VK_PRESENT_MODE_FIFO_RELAXED_KHR.
    For images that are presented in time to be displayed at the next
    vertical blanking period, the semantics are identical as for
    ename:VK_PRESENT_MODE_FIFO_RELAXED_KHR.
    For images that are presented late, and are displayed after the start of
    the vertical blanking period (i.e. with tearing), the values of
    sname:VkPastPresentationTimingGOOGLE may: be treated as if the image was
    displayed at the start of the vertical blanking period, or may: be
    treated the same as for ename:VK_PRESENT_MODE_IMMEDIATE_KHR.