Vulkan-Docs/doc/specs/vulkan/appendices/VK_KHR_swapchain.txt

// Copyright (c) 2014-2017 Khronos Group. This work is licensed under a
// Creative Commons Attribution 4.0 International License; see
// http://creativecommons.org/licenses/by/4.0/

include::meta/VK_KHR_swapchain.txt[]

*Last Modified Date*::
    2016-05-04
*IP Status*::
    No known IP claims.
*Contributors*::
  - Patrick Doane, Blizzard
  - Ian Elliott, LunarG
  - Jesse Hall, Google
  - Mathias Heyer, NVIDIA
  - James Jones, NVIDIA
  - David Mao, AMD
  - Norbert Nopper, Freescale
  - Alon Or-bach, Samsung
  - Daniel Rakos, AMD
  - Graham Sellers, AMD
  - Jeff Vigil, Qualcomm
  - Chia-I Wu, LunarG
  - Jason Ekstrand, Intel
  - Matthaeus G. Chajdas, AMD
  - Ray Smith, ARM

The +VK_KHR_swapchain+ extension is the device-level companion to the
+VK_KHR_surface+ extension.
It introduces slink:VkSwapchainKHR objects, which provide the ability to
present rendering results to a surface.

=== New Object Types

  * slink:VkSwapchainKHR

=== New Enum Constants

  * Extending elink:VkStructureType:
  ** ename:VK_STRUCTURE_TYPE_SWAPCHAIN_CREATE_INFO_KHR
  ** ename:VK_STRUCTURE_TYPE_PRESENT_INFO_KHR
  * Extending elink:VkImageLayout:
  ** ename:VK_IMAGE_LAYOUT_PRESENT_SRC_KHR
  * Extending elink:VkResult:
  ** ename:VK_SUBOPTIMAL_KHR
  ** ename:VK_ERROR_OUT_OF_DATE_KHR

=== New Enums

None

=== New Structures

  * slink:VkSwapchainCreateInfoKHR
  * slink:VkPresentInfoKHR

=== New Functions

  * flink:vkCreateSwapchainKHR
  * flink:vkDestroySwapchainKHR
  * flink:vkGetSwapchainImagesKHR
  * flink:vkAcquireNextImageKHR
  * flink:vkQueuePresentKHR

=== Issues

1) Does this extension allow the application to specify the memory backing
of the presentable images?

*RESOLVED*: No.
Unlike standard images, the implementation will allocate the memory backing
of the presentable image.

2) What operations are allowed on presentable images?

*RESOLVED*: This is determined by the image usage flags specified when
creating the presentable image's swapchain.

3) Does this extension support MSAA presentable images?

*RESOLVED*: No.
Presentable images are always single-sampled.
Multi-sampled rendering must use regular images.
To present the rendering results the application must manually resolve the
multi- sampled image to a single-sampled presentable image prior to
presentation.

4) Does this extension support stereo/multi-view presentable images?

*RESOLVED*: Yes.
The number of views associated with a presentable image is determined by the
pname:imageArrayLayers specified when creating a swapchain.
All presentable images in a given swapchain use the same array size.

5) Are the layers of stereo presentable images half-sized?

*RESOLVED*: No.
The image extents always match those requested by the application.

6) Do the "`present`" and "`acquire next image`" commands operate on a
queue? If not, do they need to include explicit semaphore objects to
interlock them with queue operations?

*RESOLVED*: The present command operates on a queue.
The image ownership operation it represents happens in order with other
operations on the queue, so no explicit semaphore object is required to
synchronize its actions.

Applications may want to acquire the next image in separate threads from
those in which they manage their queue, or in multiple threads.
To make such usage easier, the acquire next image command takes a semaphore
to signal as a method of explicit synchronization.
The application must later queue a wait for this semaphore before queuing
execution of any commands using the image.

7) Does flink:vkAcquireNextImageKHR block if no images are available?

*RESOLVED*: The command takes a timeout parameter.
Special values for the timeout are 0, which makes the call a non-blocking
operation, and code:UINT64_MAX, which blocks indefinitely.
Values in between will block for up to the specified time.
The call will return when an image becomes available or an error occurs.
It may, but is not required to, return before the specified timeout expires
if the swapchain becomes out of date.

8) Can multiple presents be queued using one flink:vkQueuePresentKHR call?

*RESOLVED*: Yes.
slink:VkPresentInfoKHR contains a list of swapchains and corresponding image
indices that will be presented.
When supported, all presentations queued with a single
flink:vkQueuePresentKHR call will be applied atomically as one operation.
The same swapchain must not appear in the list more than once.
Later extensions may provide applications stronger guarantees of atomicity
for such present operations, and/or allow them to query whether atomic
presentation of a particular group of swapchains is possible.

9) How do the presentation and acquire next image functions notify the
application the targeted surface has changed?

*RESOLVED*: Two new result codes are introduced for this purpose:

  * ename:VK_SUBOPTIMAL_KHR - Presentation will still succeed, subject to
    the window resize behavior, but the swapchain is no longer configured
    optimally for the surface it targets.
    Applications should query updated surface information and recreate their
    swapchain at the next convenient opportunity.
  * ename:VK_ERROR_OUT_OF_DATE_KHR - Failure.
    The swapchain is no longer compatible with the surface it targets.
    The application must query updated surface information and recreate the
    swapchain before presentation will succeed.

These can be returned by both flink:vkAcquireNextImageKHR and
flink:vkQueuePresentKHR.

10) Does the flink:vkAcquireNextImageKHR command return a semaphore to the
application via an output parameter, or accept a semaphore to signal from
the application as an object handle parameter?

*RESOLVED*: Accept a semaphore to signal as an object handle.
This avoids the need to specify whether the application must destroy the
semaphore or whether it is owned by the swapchain, and if the latter, what
its lifetime is and whether it can be re-used for other operations once it
is received from flink:vkAcquireNextImageKHR.

11) What types of swapchain queuing behavior should be exposed? Options
include swap interval specification, mailbox/most recent vs.
FIFO queue management, targeting specific vertical blank intervals or
absolute times for a given present operation, and probably others.
For some of these, whether they are specified at swapchain creation time or
as per-present parameters needs to be decided as well.

*RESOLVED*: The base swapchain extension will expose 3 possible behaviors
(of which, FIFO will always be supported):

  - Immediate present: Does not wait for vertical blanking period to update
    the current image, likely resulting in visible tearing.
    No internal queue is used.
    Present requests are applied immediately.
  - Mailbox queue: Waits for the next vertical blanking period to update the
    current image.
    No tearing should be observed.
    An internal single-entry queue is used to hold pending presentation
    requests.
    If the queue is full when a new presentation request is received, the
    new request replaces the existing entry, and any images associated with
    the prior entry become available for re-use by the application.
  - FIFO queue: Waits for the next vertical blanking period to update the
    current image.
    No tearing should be observed.
    An internal queue containing [eq]#ptext:numSwapchainImages - 1# entries
    is used to hold pending presentation requests.
    New requests are appended to the end of the queue, and one request is
    removed from the beginning of the queue and processed during each
    vertical blanking period in which the queue is non-empty

Not all surfaces will support all of these modes, so the modes supported
will be returned using a surface info query.
All surfaces must support the FIFO queue mode.
Applications must choose one of these modes up front when creating a
swapchain.
Switching modes can be accomplished by recreating the swapchain.

12) Can ename:VK_PRESENT_MODE_MAILBOX_KHR provide non-blocking guarantees
for flink:vkAcquireNextImageKHR? If so, what is the proper criteria?

*RESOLVED*: Yes.
The difficulty is not immediately obvious here.
Naively, if at least 3 images are requested, mailbox mode should always have
an image available for the application if the application does not own any
images when the call to flink:vkAcquireNextImageKHR was made.
However, some presentation engines may have more than one "`current`" image,
and would still need to block in some cases.
The right requirement appears to be that if the application allocates the
surface's minimum number of images + 1 then it is guaranteed non-blocking
behavior when it does not currently own any images.

13) Is there a way to create and initialize a new swapchain for a surface
that has generated a ename:VK_SUBOPTIMAL_KHR return code while still using
the old swapchain?

*RESOLVED*: Not as part of this specification.
This could be useful to allow the application to create an "`optimal`"
replacement swapchain and rebuild all its command buffers using it in a
background thread at a low priority while continuing to use the
"`suboptimal`" swapchain in the main thread.
It could probably use the same "`atomic replace`" semantics proposed for
recreating direct-to-device swapchains without incurring a mode switch.
However, after discussion, it was determined some platforms probably could
not support concurrent swapchains for the same surface though, so this will
be left out of the base KHR extensions.
A future extension could add this for platforms where it is supported.

14) Should there be a special value for
slink:VkSurfaceCapabilitiesKHR::pname:maxImageCount to indicate there are no
practical limits on the number of images in a swapchain?

*RESOLVED*: Yes.
There where often be cases where there is no practical limit to the number
of images in a swapchain other than the amount of available resources (I.e.,
memory) in the system.
Trying to derive a hard limit from things like memory size is prone to
failure.
It is better in such cases to leave it to applications to figure such soft
limits out via trial/failure iterations.

15) Should there be a special value for
slink:VkSurfaceCapabilitiesKHR::pname:currentExtent to indicate the size of
the platform surface is undefined?

*RESOLVED*: Yes.
On some platforms (Wayland, for example), the surface size is defined by the
images presented to it rather than the other way around.

16) Should there be a special value for
slink:VkSurfaceCapabilitiesKHR::pname:maxImageExtent to indicate there is no
practical limit on the surface size?

*RESOLVED*: No.
It seems unlikely such a system would exist.
0 could be used to indicate the platform places no limits on the extents
beyond those imposed by Vulkan for normal images, but this query could just
as easily return those same limits, so a special "`unlimited`" value does
not seem useful for this field.

17) How should surface rotation and mirroring be exposed to applications?
How do they specify rotation and mirroring transforms applied prior to
presentation?

*RESOLVED*: Applications can query both the supported and current transforms
of a surface.
Both are specified relative to the device's "`natural`" display rotation and
direction.
The supported transforms indicates which orientations the presentation
engine accepts images in.
For example, a presentation engine that does not support transforming
surfaces as part of presentation, and which is presenting to a surface that
is displayed with a 90-degree rotation, would return only one supported
transform bit: ename:VK_SURFACE_TRANSFORM_ROT90_BIT_KHR.
Applications must transform their rendering by the transform they specify
when creating the swapchain in pname:preTransform field.

18) Can surfaces ever not support etext:VK_MIRROR_NONE? Can they support
vertical and horizontal mirroring simultaneously? Relatedly, should
etext:VK_MIRROR_NONE[_BIT] be zero, or bit one, and should applications be
allowed to specify multiple pre and current mirror transform bits, or
exactly one?

*RESOLVED*: Since some platforms may not support presenting with a transform
other than the native window's current transform, and prerotation/mirroring
are specified relative to the device's natural rotation and direction,
rather than relative to the surface's current rotation and direction, it is
necessary to express lack of support for no mirroring.
To allow this, the etext:MIRROR_NONE enum must occupy a bit in the flags.
Since etext:MIRROR_NONE must be a bit in the bitmask rather than a bitmask
with no values set, allowing more than one bit to be set in the bitmask
would make it possible to describe undefined transforms such as
etext:VK_MIRROR_NONE_BIT | etext:VK_MIRROR_HORIZONTAL_BIT, or a transform
that includes both "`no mirroring`" and "`horizontal mirroring
simultaneously.
Therefore, it is desirable to allow specifying all supported mirroring
transforms using only one bit.
The question then becomes, should there be a
etext:VK_MIRROR_HORIZONTAL_AND_VERTICAL_BIT to represent a simultaneous
horizontal and vertical mirror transform? However, such a transform is
equivalent to a 180 degree rotation, so presentation engines and
applications that wish to support or use such a transform can express it
through rotation instead.
Therefore, 3 exclusive bits are sufficient to express all needed mirroring
transforms.

19) Should support for sRGB be required?

*RESOLVED*: In the advent of UHD and HDR display devices, proper color space
information is vital to the display pipeline represented by the swapchain.
The app can discover the supported format/color-space pairs and select a
pair most suited to its rendering needs.
Currently only the sRGB color space is supported, future extensions may
provide support for more color spaces.
See issues 23 and 24.

20) Is there a mechanism to modify or replace an existing swapchain with one
targeting the same surface?

*RESOLVED*: Yes.
This is described above in the text.

21) Should there be a way to set prerotation and mirroring using native APIs
when presenting using a Vulkan swapchain?

*RESOLVED*: Yes.
The transforms that can be expressed in this extension are a subset of those
possible on native platforms.
If a platform exposes a method to specify the transform of presented images
for a given surface using native methods and exposes more transforms or
other properties for surfaces than Vulkan supports, it might be impossible,
difficult, or inconvenient to set some of those properties using Vulkan KHR
extensions and some using the native interfaces.
To avoid overwriting properties set using native commands when presenting
using a Vulkan swapchain, the application can set the pretransform to
"`inherit`", in which case the current native properties will be used, or if
none are available, a platform-specific default will be used.
Platforms that do not specify a reasonable default or do not provide native
mechanisms to specify such transforms should not include the inherit bits in
the pname:supportedTransforms bitmask they return in
slink:VkSurfaceCapabilitiesKHR.

22) Should the content of presentable images be clipped by objects obscuring
their target surface?

*RESOLVED*: Applications can choose which behavior they prefer.
Allowing the content to be clipped could enable more optimal presentation
methods on some platforms, but some applications might rely on the content
of presentable images to perform techniques such as partial updates or
motion blurs.

23) What is the purpose of specifying a slink:VkColorSpaceKHR along with
slink:VkFormat when creating a swapchain?

*RESOLVED*: While Vulkan itself is color space agnostic (e.g. even the
meaning of R, G, B and A can be freely defined by the rendering
application), the swapchain eventually will have to present the images on a
display device with specific color reproduction characteristics.
If any color space transformations are necessary before an image can be
displayed, the color space of the presented image must be known to the
swapchain.
A swapchain will only support a restricted set of color format and -space
pairs.
This set can be discovered via flink:vkGetPhysicalDeviceSurfaceFormatsKHR.
As it can be expected that most display devices support the sRGB color
space, at least one format/color-space pair has to be exposed, where the
color space is ename:VK_COLOR_SPACE_SRGB_NONLINEAR.

24) How are sRGB formats and the sRGB color space related?

*RESOLVED*: While Vulkan exposes a number of SRGB texture formats, using
such formats does not guarantee working in a specific color space.
It merely means that the hardware can directly support applying the
non-linear transfer functions defined by the sRGB standard color space when
reading from or writing to images of that these formats.
Still, it is unlikely that a swapchain will expose a etext:*_SRGB format
along with any color space other than ename:VK_COLOR_SPACE_SRGB_NONLINEAR.

On the other hand, non-etext:*_SRGB formats will be very likely exposed in
pair with a SRGB color space.
This means, the hardware will not apply any transfer function when reading
from or writing to such images, yet they will still be presented on a device
with sRGB display characteristics.
In this case the application is responsible for applying the transfer
function, for instance by using shader math.

25) How are the lifetime of surfaces and swapchains targeting them related?

*RESOLVED*: A surface must outlive any swapchains targeting it.
A slink:VkSurfaceKHR owns the binding of the native window to the Vulkan
driver.

26) How can the client control the way the alpha channel of swap chain
images is treated by the presentation engine during compositing?

*RESOLVED*: We should add new enum values to allow the client to negotiate
with the presentation engine on how to treat image alpha values during the
compositing process.
Since not all platforms can practically control this through the Vulkan
driver, a value of ename:VK_COMPOSITE_ALPHA_INHERIT_BIT_KHR is provided like
for surface transforms.

27) Is flink:vkCreateSwapchainKHR the right function to return
ename:VK_ERROR_NATIVE_WINDOW_IN_USE_KHR, or should the various
platform-specific slink:VkSurfaceKHR factory functions catch this error
earlier?

*RESOLVED*: For most platforms, the slink:VkSurfaceKHR structure is a simple
container holding the data that identifies a native window or other object
representing a surface on a particular platform.
For the surface factory functions to return this error, they would likely
need to register a reference on the native objects with the native display
server some how, and ensure no other such references exist.
Surfaces were not intended to be that heavy-weight.

Swapchains are intended to be the objects that directly manipulate native
windows and communicate with the native presentation mechanisms.
Swapchains will already need to communicate with the native display server
to negotiate allocation and/or presentation of presentable images for a
native surface.
Therefore, it makes more sense for swapchain creation to be the point at
which native object exclusivity is enforced.
Platforms may choose to enforce further restrictions on the number of
slink:VkSurfaceKHR objects that may be created for the same native window if
such a requirement makes sense on a particular platform, but a global
requirement is only sensible at the swapchain level.

=== Examples

[NOTE]
.Note
====
The example code for the +VK_KHR_surface+ and +VK_KHR_swapchain+ extensions
was removed from the appendix after revision 1.0.29.
This WSI example code was ported to the cube demo that is shipped with the
official Khronos SDK, and is being kept up-to-date in that location (see:
https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers/blob/master/demos/cube.c).
====

=== Version History

 * Revision 1, 2015-05-20 (James Jones)
   - Initial draft, based on LunarG KHR spec, other KHR specs, patches
     attached to bugs.

 * Revision 2, 2015-05-22 (Ian Elliott)
   - Made many agreed-upon changes from 2015-05-21 KHR TSG meeting.
     This includes using only a queue for presentation, and having an
     explicit function to acquire the next image.
   - Fixed typos and other minor mistakes.

 * Revision 3, 2015-05-26 (Ian Elliott)
   - Improved the Description section.
   - Added or resolved issues that were found in improving the Description.
     For example, pSurfaceDescription is used consistently, instead of
     sometimes using pSurface.

 * Revision 4, 2015-05-27 (James Jones)
   - Fixed some grammatical errors and typos
   - Filled in the description of imageUseFlags when creating a swapchain.
   - Added a description of swapInterval.
   - Replaced the paragraph describing the order of operations on a queue
     for image ownership and presentation.

 * Revision 5, 2015-05-27 (James Jones)
   - Imported relevant issues from the (abandoned)
     vk_wsi_persistent_swapchain_images extension.
   - Added issues 6 and 7, regarding behavior of the acquire next image and
     present commands with respect to queues.
   - Updated spec language and examples to align with proposed resolutions
     to issues 6 and 7.

 * Revision 6, 2015-05-27 (James Jones)
   - Added issue 8, regarding atomic presentation of multiple swapchains
   - Updated spec language and examples to align with proposed resolution to
     issue 8.

 * Revision 7, 2015-05-27 (James Jones)
   - Fixed compilation errors in example code, and made related spec fixes.

 * Revision 8, 2015-05-27 (James Jones)
   - Added issue 9, and the related VK_SUBOPTIMAL_KHR result code.
   - Renamed VK_OUT_OF_DATE_KHR to VK_ERROR_OUT_OF_DATE_KHR.

 * Revision 9, 2015-05-27 (James Jones)
   - Added inline proposed resolutions (marked with [JRJ]) to some XXX
     questions/issues.
     These should be moved to the issues section in a subsequent update if
     the proposals are adopted.

 * Revision 10, 2015-05-28 (James Jones)
   - Converted vkAcquireNextImageKHR back to a non-queue operation that uses
     a VkSemaphore object for explicit synchronization.
   - Added issue 10 to determine whether vkAcquireNextImageKHR generates or
     returns semaphores, or whether it operates on a semaphore provided by
     the application.

 * Revision 11, 2015-05-28 (James Jones)
   - Marked issues 6, 7, and 8 resolved.
   - Renamed VkSurfaceCapabilityPropertiesKHR to VkSurfacePropertiesKHR to
     better convey the mutable nature of the info it contains.

 * Revision 12, 2015-05-28 (James Jones)
   - Added issue 11 with a proposed resolution, and the related issue 12.
   - Updated various sections of the spec to match the proposed resolution
     to issue 11.

 * Revision 13, 2015-06-01 (James Jones)
   - Moved some structures to VK_EXT_KHR_swap_chain to resolve the spec's
     issues 1 and 2.

 * Revision 14, 2015-06-01 (James Jones)
   - Added code for example 4 demonstrating how an application might make
     use of the two different present and acquire next image KHR result
     codes.
   - Added issue 13.

 * Revision 15, 2015-06-01 (James Jones)
   - Added issues 14 - 16 and related spec language.
   - Fixed some spelling errors.
   - Added language describing the meaningful return values for
     vkAcquireNextImageKHR and vkQueuePresentKHR.

 * Revision 16, 2015-06-02 (James Jones)
   - Added issues 17 and 18, as well as related spec language.
   - Removed some erroneous text added by mistake in the last update.

 * Revision 17, 2015-06-15 (Ian Elliott)
   - Changed special value from "-1" to "0" so that the data types can be
     unsigned.

 * Revision 18, 2015-06-15 (Ian Elliott)
   - Clarified the values of VkSurfacePropertiesKHR::minImageCount and the
     timeout parameter of the vkAcquireNextImageKHR function.

 * Revision 19, 2015-06-17 (James Jones)
   - Misc.
     cleanup.
     Removed resolved inline issues and fixed typos.
   - Fixed clarification of VkSurfacePropertiesKHR::minImageCount made in
     version 18.
   - Added a brief "Image Ownership" definition to the list of terms used in
     the spec.

 * Revision 20, 2015-06-17 (James Jones)
   - Updated enum-extending values using new convention.

 * Revision 21, 2015-06-17 (James Jones)
   - Added language describing how to use
     VK_IMAGE_LAYOUT_PRESENT_SOURCE_KHR.
   - Cleaned up an XXX comment regarding the description of which queues
     vkQueuePresentKHR can be used on.

 * Revision 22, 2015-06-17 (James Jones)
   - Rebased on Vulkan API version 126.

 * Revision 23, 2015-06-18 (James Jones)
   - Updated language for issue 12 to read as a proposed resolution.
   - Marked issues 11, 12, 13, 16, and 17 resolved.
   - Temporarily added links to the relevant bugs under the remaining
     unresolved issues.
   - Added issues 19 and 20 as well as proposed resolutions.

 * Revision 24, 2015-06-19 (Ian Elliott)
   - Changed special value for VkSurfacePropertiesKHR::currentExtent back to
     "-1" from "0".
     This value will never need to be unsigned, and "0" is actually a legal
     value.

 * Revision 25, 2015-06-23 (Ian Elliott)
   - Examples now show use of function pointers for extension functions.
   - Eliminated extraneous whitespace.

 * Revision 26, 2015-06-25 (Ian Elliott)
   - Resolved Issues 9 & 10 per KHR TSG meeting.

 * Revision 27, 2015-06-25 (James Jones)
   - Added oldSwapchain member to VkSwapchainCreateInfoKHR.

 * Revision 28, 2015-06-25 (James Jones)
   - Added the "inherit" bits to the rotatation and mirroring flags and the
     associated issue 21.

 * Revision 29, 2015-06-25 (James Jones)
   - Added the "clipped" flag to VkSwapchainCreateInfoKHR, and the
     associated issue 22.
   - Specified that presenting an image does not modify it.

 * Revision 30, 2015-06-25 (James Jones)
   - Added language to the spec that clarifies the behavior of
     vkCreateSwapchainKHR() when the oldSwapchain field of
     VkSwapchainCreateInfoKHR is not NULL.

 * Revision 31, 2015-06-26 (Ian Elliott)
   - Example of new VkSwapchainCreateInfoKHR members, "oldSwapchain" and
     "clipped".
   - Example of using VkSurfacePropertiesKHR::{min|max}ImageCount to set
     VkSwapchainCreateInfoKHR::minImageCount.
   - Rename vkGetSurfaceInfoKHR()'s 4th param to "pDataSize", for
     consistency with other functions.
   - Add macro with C-string name of extension (just to header file).

 * Revision 32, 2015-06-26 (James Jones)
   - Minor adjustments to the language describing the behavior of
     "oldSwapchain"
   - Fixed the version date on my previous two updates.

 * Revision 33, 2015-06-26 (Jesse Hall)
   - Add usage flags to VkSwapchainCreateInfoKHR

 * Revision 34, 2015-06-26 (Ian Elliott)
   - Rename vkQueuePresentKHR()'s 2nd param to "pPresentInfo", for
     consistency with other functions.

 * Revision 35, 2015-06-26 (Jason Ekstrand)
   - Merged the VkRotationFlagBitsKHR and VkMirrorFlagBitsKHR enums into a
     single VkSurfaceTransformFlagBitsKHR enum.

 * Revision 36, 2015-06-26 (Jason Ekstrand)
   - Added a VkSurfaceTransformKHR enum that is not a bitmask.
     Each value in VkSurfaceTransformKHR corresponds directly to one of the
     bits in VkSurfaceTransformFlagBitsKHR so transforming from one to the
     other is easy.
     Having a separate enum means that currentTransform and preTransform are
     now unambiguous by definition.

 * Revision 37, 2015-06-29 (Ian Elliott)
   - Corrected one of the signatures of vkAcquireNextImageKHR, which had the
     last two parameters switched from what it is elsewhere in the
     specification and header files.

 * Revision 38, 2015-06-30 (Ian Elliott)
   - Corrected a typo in description of the vkGetSwapchainInfoKHR()
     function.
   - Corrected a typo in header file comment for VkPresentInfoKHR::sType.

 * Revision 39, 2015-07-07 (Daniel Rakos)
   - Added error section describing when each error is expected to be
     reported.
   - Replaced bool32_t with VkBool32.

 * Revision 40, 2015-07-10 (Ian Elliott)
   - Updated to work with version 138 of the "vulkan.h" header.
     This includes declaring the VkSwapchainKHR type using the new
     VK_DEFINE_NONDISP_HANDLE macro, and no longer extending VkObjectType
     (which was eliminated).

 * Revision 41 2015-07-09 (Mathias Heyer)
   - Added color space language.

 * Revision 42, 2015-07-10 (Daniel Rakos)
   - Updated query mechanism to reflect the convention changes done in the
     core spec.
   - Removed "queue" from the name of
     VK_STRUCTURE_TYPE_QUEUE_PRESENT_INFO_KHR to be consistent with the
     established naming convention.
   - Removed reference to the no longer existing VkObjectType enum.

 * Revision 43, 2015-07-17 (Daniel Rakos)
   - Added support for concurrent sharing of swapchain images across queue
     families.
   - Updated sample code based on recent changes

 * Revision 44, 2015-07-27 (Ian Elliott)
   - Noted that support for VK_PRESENT_MODE_FIFO_KHR is required.
     That is ICDs may optionally support IMMEDIATE and MAILBOX, but must
     support FIFO.

 * Revision 45, 2015-08-07 (Ian Elliott)
   - Corrected a typo in spec file (type and variable name had wrong case
     for the imageColorSpace member of the VkSwapchainCreateInfoKHR struct).
   - Corrected a typo in header file (last parameter in
     PFN_vkGetSurfacePropertiesKHR was missing "KHR" at the end of type:
     VkSurfacePropertiesKHR).

 * Revision 46, 2015-08-20 (Ian Elliott)
   - Renamed this extension and all of its enumerations, types, functions,
     etc.
     This makes it compliant with the proposed standard for Vulkan
     extensions.
   - Switched from "revision" to "version", including use of the
     VK_MAKE_VERSION macro in the header file.
   - Made improvements to several descriptions.
   - Changed the status of several issues from PROPOSED to RESOLVED, leaving
     no unresolved issues.
   - Resolved several TODOs, did miscellaneous cleanup, etc.

 * Revision 47, 2015-08-20 (Ian Elliott--porting a 2015-07-29 change from
   James Jones)
   - Moved the surface transform enums to VK_WSI_swapchain so they could be
     re-used by VK_WSI_display.

 * Revision 48, 2015-09-01 (James Jones)
   - Various minor cleanups.

 * Revision 49, 2015-09-01 (James Jones)
   - Restore single-field revision number.

 * Revision 50, 2015-09-01 (James Jones)
   - Update Example #4 to include code that illustrates how to use the
     oldSwapchain field.

 * Revision 51, 2015-09-01 (James Jones)
   - Fix example code compilation errors.

 * Revision 52, 2015-09-08 (Matthaeus G. Chajdas)
   - Corrected a typo.

 * Revision 53, 2015-09-10 (Alon Or-bach)
   - Removed underscore from SWAP_CHAIN left in
     VK_STRUCTURE_TYPE_SWAPCHAIN_CREATE_INFO_KHR.

 * Revision 54, 2015-09-11 (Jesse Hall)
   - Described the execution and memory coherence requirements for image
     transitions to and from VK_IMAGE_LAYOUT_PRESENT_SOURCE_KHR.

 * Revision 55, 2015-09-11 (Ray Smith)
   - Added errors for destroying and binding memory to presentable images

 * Revision 56, 2015-09-18 (James Jones)
   - Added fence argument to vkAcquireNextImageKHR
   - Added example of how to meter a CPU thread based on presentation rate.

 * Revision 57, 2015-09-26 (Jesse Hall)
   - Replace VkSurfaceDescriptionKHR with VkSurfaceKHR.
   - Added issue 25 with agreed resolution.

 * Revision 58, 2015-09-28 (Jesse Hall)
   - Renamed from VK_EXT_KHR_device_swapchain to VK_EXT_KHR_swapchain.

 * Revision 59, 2015-09-29 (Ian Elliott)
   - Changed vkDestroySwapchainKHR() to return void.

 * Revision 60, 2015-10-01 (Jeff Vigil)
   - Added error result VK_ERROR_SURFACE_LOST_KHR.

 * Revision 61, 2015-10-05 (Jason Ekstrand)
   - Added the VkCompositeAlpha enum and corresponding structure fields.

 * Revision 62, 2015-10-12 (Daniel Rakos)
   - Added VK_PRESENT_MODE_FIFO_RELAXED_KHR.

 * Revision 63, 2015-10-15 (Daniel Rakos)
   - Moved surface capability queries to VK_EXT_KHR_surface.

 * Revision 64, 2015-10-26 (Ian Elliott)
   - Renamed from VK_EXT_KHR_swapchain to VK_KHR_swapchain.

 * Revision 65, 2015-10-28 (Ian Elliott)
   - Added optional pResult member to VkPresentInfoKHR, so that
     per-swapchain results can be obtained from vkQueuePresentKHR().

 * Revision 66, 2015-11-03 (Daniel Rakos)
   - Added allocation callbacks to create and destroy functions.
   - Updated resource transition language.
   - Updated sample code.

 * Revision 67, 2015-11-10 (Jesse Hall)
   - Add reserved flags bitmask to VkSwapchainCreateInfoKHR.
   - Modify naming and member ordering to match API style conventions, and
     so the VkSwapchainCreateInfoKHR image property members mirror
     corresponding VkImageCreateInfo members but with an 'image' prefix.
   - Make VkPresentInfoKHR::pResults non-const; it is an output array
     parameter.
   - Make pPresentInfo parameter to vkQueuePresentKHR const.

 * Revision 68, 2016-04-05 (Ian Elliott)
   - Moved the "validity" include for vkAcquireNextImage to be in its proper
     place, after the prototype and list of parameters.
   - Clarified language about presentable images, including how they are
     acquired, when applications can and cannot use them, etc.
     As part of this, removed language about "ownership" of presentable
     images, and replaced it with more-consistent language about presentable
     images being "acquired" by the application.

 * 2016-08-23 (Ian Elliott)
   - Update the example code, to use the final API command names, to not
     have so many characters per line, and to split out a new example to
     show how to obtain function pointers.
     This code is more similar to the LunarG "cube" demo program.

 * 2016-08-25 (Ian Elliott)
   - A note was added at the beginning of the example code, stating that it
     will be removed from future versions of the appendix.