Vulkan-Docs/doc/specs/vulkan/chapters/fundamentals.txt

// Copyright (c) 2015-2016 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html

[[fundamentals]]
= Fundamentals

This chapter introduces fundamental concepts including the Vulkan
architecture and execution model, API syntax, queues, pipeline
configurations, numeric representation, state and state queries, and the
different types of objects and shaders. It provides a framework for
interpreting more specific descriptions of commands and behavior in the
remainder of the Specification.


[[fundamentals-architecture-model]]
== Architecture Model

Vulkan is designed for, and the API is written for, CPU, GPU, and other
hardware accelerator architectures with the following properties:

* Runtime support for 8, 16, 32 and 64-bit signed and unsigned
  twos-complement integers, all addressable at the granularity of their size
  in bytes.
* Runtime support for 32- and 64-bit floating-point types satisfying the
  range and precision constraints in the
  <<fundamentals-floatingpoint,Floating Point Computation>> section.
* The representation and endianness of these types must: be identical for
  the host and the physical devices.


[NOTE]
.Note
====
Since a variety of data types and structures in Vulkan may: be mapped back
and forth between host and physical device memory, host and device
architectures must: both be able to access such data efficiently in order to
write portable and performant applications.
====

Where the Specification leaves choices open that would affect Application
Binary Interface compatibility on a given platform supporting Vulkan, those
choices are usually made to be compliant to the preferred ABI defined by the
platform vendor. Some choices, such as function calling conventions, may: be
made in platform-specific portions of the +vk_platform.h+ header file.

[NOTE]
.Note
====
For example, the Android ABI is defined by Google, and the Linux ABI is
defined by a combination of gcc defaults, distribution vendor choices, and
external standards such as the Linux Standard Base.
====


[[fundamentals-execmodel]]
== Execution Model

This section outlines the execution model of a Vulkan system.

Vulkan exposes one or more _devices_,
each of which exposes one or more _queues_ which may: process work
asynchronously to one another. The set of queues supported by a device is
partitioned into _families_. Each family supports one or more types of
functionality and may:
contain multiple queues with similar characteristics. Queues within a single
family are considered _compatible_ with one another, and work produced for a
family of queues can: be executed on any queue within that family. This
specification defines four types of functionality that queues may: support:
graphics, compute, transfer, and sparse memory management.

[NOTE]
.Note
====
A single device may: report multiple similar queue
families rather than, or as well as, reporting multiple members of one or
more of those families. This indicates that while members of those families
have similar capabilities, they are _not_ directly compatible with one
another.
====

Device memory is explicitly managed by the application. Each device may:
advertise one or more heaps, representing different areas of memory. Memory
heaps are either device local or host local, but are always visible to the
device. Further detail about memory heaps is exposed via memory types
available on that heap. Examples of memory areas that may: be available on
an implementation include:

  * _device local_ is memory that is physically connected to the device.
  * _device local, host visible_ is device local memory that is visible to
    the host.
  * _host local, host visible_ is memory that is local to the host and
    visible to the device and host.

On other architectures, there may: only be a single heap that can: be used
for any purpose.

A Vulkan application controls a set of devices through the submission of
command buffers which have recorded device commands issued via Vulkan
library calls. The content of command buffers is specific to the underlying
hardware and is opaque to the application. Once constructed, a command
buffer can: be submitted once or many times to a queue for execution.
Multiple command buffers can: be built in parallel by employing multiple
threads within the application.

Command buffers submitted to different queues may: execute in parallel or
even out of order with respect to one another. Command buffers submitted to
a single queue respect the submission order, as described further in
<<fundamentals-queueoperation,Queue Operation>>. Command buffer execution by
the device is also asynchronous to host execution. Once a command buffer is
submitted to a queue, control may: return to the application immediately.
Synchronization between the device and host, and between different queues is
the responsibility of the application.


[[fundamentals-queueoperation]]
=== Queue Operation

Vulkan queues provide an interface to the execution engines of a device.
Commands are recorded into command buffers ahead of execution time.
These command buffers are then submitted to queues for execution. Command
buffers submitted to a single queue are played back in the order they were
submitted, and commands within each buffer are played back in the order they
were recorded.
Work performed by those commands respects the ordering guarantees provided
by explicit and implicit dependencies, as described below. Work submitted
to separate queues may: execute in any relative order unless otherwise
specified. Therefore, the application must: explicitly synchronize work
between queues when needed.

In order to control relative order of execution of work both within a queue
and across multiple queues, Vulkan provides several synchronization
primitives, which include _semaphores_, _events_, _pipeline barriers_, and
_fences_. These are covered in depth in <<synchronization,Synchronization
and Cache Control>>. In broad terms, semaphores are used to synchronize work
across queues or across coarse-grained submissions to a single queue, events
and barriers are used to synchronize work within a command buffer or
sequence of command buffers submitted to a single queue, and fences are used
to synchronize work between the device and the host.

[NOTE]
.Note
====
Implementations have significant freedom to overlap execution of work
submitted to a queue, and this is common due to deep pipelining and
parallelism in Vulkan devices.
====

Work is submitted to queues using queue submission commands that typically
take the form ftext:vkQueue* (e.g. flink:vkQueueSubmit,
flink:vkQueueBindSparse), and usually take a list of semaphores upon which
to wait before work begins and a list of semaphores to signal once work has
completed. Unless otherwise ordered by semaphores, command buffer execution
from multiple queue submissions done using the flink:vkQueueSubmit command
may: overlap (but not be reordered), sparse binding operations done using
the flink:vkQueueBindSparse command from multiple batches may: overlap or be
reordered, and command buffer submissions and sparse binding operations may:
overlap or be reordered against operations of the other type.

Command buffer boundaries, both between primary command buffers of the same
or different batches or submissions as well as between primary and secondary
command buffers, do not introduce any implicit ordering constraints. In
other words, submitting the set of command buffers (which can: include
executing secondary command buffers) between any semaphore or fence
operations plays back the recorded commands as if they had all been recorded
into a single primary command buffer, except that the current state is
<<commandbuffers-statereset,reset>> on each boundary.

Commands recorded in command buffers either perform actions (draw, dispatch,
clear, copy, query/timestamp operations, begin/end subpass operations), set
state (bind pipelines, descriptor sets, and buffers, set dynamic state, push
constants, set render pass/subpass state), or perform synchronization
(set/wait events, pipeline barrier, render pass/subpass dependencies). Some
commands perform more than one of these tasks. State setting commands update
the _current state_ of the command buffer. Some commands that perform
actions (e.g. draw/dispatch) do so based on the current state set
cumulatively since the start of the command buffer. The work involved in
performing action commands is often allowed to overlap or to be reordered,
but doing so mustnot: alter the state to be used by each action command. In
general, action commands are those commands that alter framebuffer
attachments, read/write buffer or image memory, or write to query pools.

Synchronization commands introduce explicit
<<synchronization-execution-and-memory-dependencies,execution and memory
dependencies>> between two sets of action commands, where the second set of
commands depends on the first set of commands. These dependencies enforce
that both the execution of certain
<<synchronization-pipeline-stage-flags,pipeline stages>> in the later set
occur after the execution of certain stages in the source set, and that the
effects of <<synchronization-global-memory-barrier,memory accesses>> performed by
certain pipeline stages occur in order and are visible to each other. When
not enforced by an explicit dependency or otherwise forbidden by the
specification, action commands may: overlap execution or execute out of
order, and may: not see the side effects of each other's memory accesses.

Submitting command buffers and sparse memory operations, signaling fences,
and signaling and waiting on semaphores each provide
<<synchronization-implicit-ordering,Implicit Ordering Guarantees>>.
Signaling a fence or semaphore each guarantees that the previous commands
have completed execution and that memory writes from those commands are
<<synchronization-execution-and-memory-dependencies-available-and-visible,available>>
to future commands. Waiting on a semaphore or submitting command buffers
after a fence has been signaled each guarantees that previous writes that
were available are also
<<synchronization-execution-and-memory-dependencies-available-and-visible,visible>>
to subsequent commands.

[[fundamentals-queueoperation-apiorder]]
Within a subpass of a <<renderpass,render pass instance>>, for a given
(x,y,layer,sample) sample location, the following stages are guaranteed to
execute in _API order_ for each separate primitive that includes that sample
location:

  * depth bounds test
  * stencil test, stencil op and stencil write
  * depth test and depth write
  * occlusion queries
  * blending, logic op and color write

where the API order sorts primitives:

  * First, by the action command that generates them.
  * Second, by the order they are processed by
    <<drawing-primitive-assembly-apiorder,primitive assembly>>.

Within this order, implementations also sort primitives:

  * Third, by an implementation-dependent ordering of new primitives
    generated by tessellation, if a tessellation shader is active.
  * Fourth, by the order new primitives are generated by
    <<geometry-ordering,geometry shading>>, if geometry shading is
    active.
  * Fifth, by an implementation-dependent ordering of primitives generated
    due to the <<primsrast-polygonmode,polygon mode>>.

The device executes command buffers from queues asynchronously from the
host. Control is returned to an application immediately following command
buffer submission to a queue. The application must: synchronize work between
the host and device as needed.

As part of each submission to a queue, a list of semaphores upon which to
wait, and a list of semaphores to signal is provided along with the list of
command buffers to execute. This is covered in more detail in
<<commandbuffers-submission>>.


[[fundamentals-objectmodel-overview]]
== Object Model

The devices, queues, and other entities in Vulkan are represented by
Vulkan objects. At the API level, all objects are referred to by handles.
There are two classes of handles, dispatchable and non-dispatchable.
_Dispatchable_ handle types are a pointer to an opaque type. This pointer
may: be used by layers as part of intercepting API commands, and thus each
API command takes a dispatchable type as its first parameter. Each object of
a dispatchable type must: have a unique handle value during its lifetime.

_Non-dispatchable_ handle types are a 64-bit integer type whose meaning is
implementation-dependent, and may: encode object information directly in the
handle rather than pointing to a software structure. Objects of a
non-dispatchable type may: not have unique handle values within a type or
across types. If handle values are not unique, then destroying one such
handle mustnot: cause identical handles of other types to become invalid,
and mustnot: cause identical handles of the same type to become invalid if
that handle value has been created more times than it has been destroyed.

All objects created or allocated from a sname:VkDevice (i.e. with a
sname:VkDevice as the first parameter) are private to that device, and
mustnot: be used on other devices.


[[fundamentals-objectmodel-lifetime]]
=== Object Lifetime

Objects are created or allocated by ftext:vkCreate* and ftext:vkAllocate*
commands, respectively. Once an object is created or allocated, its
``structure'' is considered to be immutable, though the contents of certain
object types is still free to change. Objects are destroyed or freed by
ftext:vkDestroy* and ftext:vkFree* commands, respectively.

Objects that are allocated (rather than created) take resources from an
existing pool object or memory heap, and when freed return resources to that
pool or heap. While object creation and destruction are generally expected
to be low-frequency occurrences during runtime, allocating and freeing
objects can: occur at high frequency. Pool objects help accommodate improved
performance of the allocations and frees.

It is an application's responsibility to track the lifetime of Vulkan
objects, and not to destroy them while they are still in use.

Application-owned memory is immediately consumed by any Vulkan command it
is passed into. The application can: alter or free this memory as soon as
the commands that consume it have returned.

The following object types are consumed when they are passed into a
Vulkan command and not further accessed by the objects they are used to
create. They can: be destroyed at any time they are not in use by an API
command:

  * sname:VkShaderModule
  * sname:VkPipelineCache
  * sname:VkPipelineLayout

sname:VkDescriptorSetLayout objects may: be accessed by commands that
operate on descriptor sets allocated using that layout, and those descriptor
sets mustnot: be updated with flink:vkUpdateDescriptorSets after the
descriptor set layout has been destroyed. Otherwise, descriptor set layouts
can: be destroyed any time they are not in use by an API command.

The application mustnot: destroy any other type of Vulkan object until
all uses of that object by the device (such as via command buffer execution)
have completed.

The following Vulkan objects can: be destroyed when no command buffers
using the object are executing:

  * sname:VkEvent
  * sname:VkQueryPool
  * sname:VkBuffer
  * sname:VkBufferView
  * sname:VkImage
  * sname:VkImageView
  * sname:VkPipeline
  * sname:VkSampler
  * sname:VkDescriptorPool
  * sname:VkFramebuffer
  * sname:VkRenderPass
  * sname:VkCommandPool
  * sname:VkDeviceMemory
  * sname:VkDescriptorSet

The following Vulkan objects can: be destroyed when work on the queue
that uses the object has been completed:

  * sname:VkFence
  * sname:VkSemaphore
  * sname:VkCommandBuffer
  * sname:VkCommandPool

In general, objects can: be destroyed or freed in any order, even if the
object being freed is involved in the use of another object (e.g. use of a
resource in a view, use of a view in a descriptor set, use of an object in a
command buffer, binding of a memory allocation to a resource), as long as
any object that uses the freed object is not further used in any way except
to be destroyed or to be reset in such a way that it no longer uses the
other object (such as resetting a command buffer). If the object has been
reset, then it can: be used as if it never used the freed object. An
exception to this is when there is a parent/child relationship between
objects. In this case, the application mustnot: destroy a parent object
before its children, except when the parent is explicitly defined to free
its children when it is destroyed (e.g. for pool objects, as defined below).

sname:VkCommandPool objects are parents of sname:VkCommandBuffer objects.
sname:VkDescriptorPool objects are parents of sname:VkDescriptorSet objects.
sname:VkDevice objects are parents of many object types (all that take a
sname:VkDevice as a parameter to their creation).

The following Vulkan objects have specific restrictions for when they
can: be destroyed:

  * sname:VkQueue objects cannot: be explicitly destroyed. Instead, they are
    implicitly destroyed when the sname:VkDevice object they are retrieved
    from is destroyed.
  * Destroying a pool object implicitly frees all objects allocated from
    that pool. Specifically, destroying sname:VkCommandPool frees all
    sname:VkCommandBuffer objects that were allocated from it, and
    destroying sname:VkDescriptorPool frees all sname:VkDescriptorSet
    objects that were allocated from it.
  * sname:VkDevice objects can: be destroyed when all sname:VkQueue objects
    retrieved from them are idle, and all objects created from them have
    been destroyed. This includes the following objects:
  ** sname:VkFence
  ** sname:VkSemaphore
  ** sname:VkEvent
  ** sname:VkQueryPool
  ** sname:VkBuffer
  ** sname:VkBufferView
  ** sname:VkImage
  ** sname:VkImageView
  ** sname:VkShaderModule
  ** sname:VkPipelineCache
  ** sname:VkPipeline
  ** sname:VkPipelineLayout
  ** sname:VkSampler
  ** sname:VkDescriptorSetLayout
  ** sname:VkDescriptorPool
  ** sname:VkFramebuffer
  ** sname:VkRenderPass
  ** sname:VkCommandPool
  ** sname:VkCommandBuffer
  ** sname:VkDeviceMemory
  * sname:VkPhysicalDevice objects cannot: be explicitly destroyed. Instead,
    they are implicitly destroyed when the sname:VkInstance object they are
    retrieved from is destroyed.
  * sname:VkInstance objects can: be destroyed once all sname:VkDevice
    objects created from any of its sname:VkPhysicalDevice objects have been
    destroyed.


[[fundamentals-commandsyntax]]
== Command Syntax and Duration

The Specification describes Vulkan commands as functions or procedures
using C99 syntax. Language bindings for other languages such as C++ and
Javascript may: allow for stricter parameter passing, or object-oriented
interfaces.

With few exceptions, Vulkan uses the standard C types for parameters (int
types from stdint.h, etc). Exceptions to this are using basetype:VkResult
for return values, using basetype:VkBool32 for boolean values,
basetype:VkDeviceSize for sizes and offsets pertaining to device address
space, and basetype:VkFlags for passing bits or sets of bits of predefined
values.

Commands that create Vulkan objects are of the form ftext:vkCreate* and
take stext:Vk*CreateInfo structures with the parameters needed to create the
object. These Vulkan objects are destroyed with commands of the form
ftext:vkDestroy*.

The last in-parameter to each command that creates or destroys a Vulkan
object is pname:pAllocator. The pname:pAllocator parameter can: be set to a
non-`NULL` value such that allocations for the given object are delegated to
an application provided callback; refer to the <<memory-allocation,Memory
Allocation>> chapter for further details.

Commands that allocate Vulkan objects owned by pool objects are of the
form ftext:vkAllocate*, and take stext:Vk*AllocateInfo structures. These
Vulkan objects are freed with commands of the form ftext:vkFree*.
These objects do not take allocators; if host memory is needed, they will
use the allocator that was specified when their parent pool was created.

Information is retrieved from the implementation with commands of the form
ftext:vkGet*.

Commands are recorded into a command buffer by calling API commands of the
form ftext:vkCmd*. Each such command may: have different restrictions on
where it can: be used: in a primary and/or secondary command buffer, inside
and/or outside a render pass, and in one or more of the supported queue
types. These restrictions are documented together with the definition of
each such command.

The _duration_ of a Vulkan command refers to the interval between calling
the command and its return to the caller.


[[fundamentals-threadingbehavior]]
== Threading Behavior

Vulkan is intended to provide scalable performance when used on multiple
host threads. All commands support being called concurrently from multiple
threads, but certain parameters, or components of parameters are defined to
be _externally synchronized_. This means that the caller must: guarantee
that no more than one thread is using such a parameter at a given time.

More precisely, Vulkan commands use simple stores to update software
structures representing Vulkan objects. A parameter declared as
externally synchronized may: have its software
structures updated at any time during the host execution of the command. If
two commands operate on the same object and at least one of the commands
declares the object to be externally synchronized, then the caller must:
guarantee not only that the commands do not execute simultaneously, but also
that the two commands are separated by an appropriate memory barrier (if
needed).

[NOTE]
.Note
====
Memory barriers are particularly relevant on the ARM CPU architecture
which is more weakly ordered than many developers are accustomed to from
x86/x64 programming. Fortunately, most higher-level synchronization
primitives (like the pthread library) perform memory barriers as a part of
mutual exclusion, so mutexing Vulkan objects via these primitives will
have the desired effect.
====

Many object types are _immutable_, meaning the objects cannot: change once
they have been created. These types of objects never need external
synchronization, except that they mustnot: be destroyed while they are in
use on another thread. In certain special cases, mutable object parameters
are internally synchronized such that they do not require external
synchronization. One example of this is the use of a sname:VkPipelineCache
in fname:vkCreateGraphicsPipelines and fname:vkCreateComputePipelines, where
external synchronization around such a heavyweight command would be
impractical. The implementation must: internally synchronize the cache in
this example, and may: be able to do so in the form of a much finer-grained
mutex around the command. Any command parameters that are not labeled as
externally synchronized are either not mutated by the command or are
internally synchronized. Additionally, certain objects related to a
command's parameters (e.g. command pools and descriptor pools) may: be
affected by a command, and must: also be externally synchronized. These
implicit parameters are documented as described below.

Parameters of commands that are externally synchronized are listed below.

include::../hostsynctable/parameters.txt[]

There are also a few instances where a command can: take in a user allocated
list whose contents are externally synchronized parameters. In these cases,
the caller must: guarantee that at most one thread is using a given element
within the list at a given time. These parameters are listed below.

include::../hostsynctable/parameterlists.txt[]

In addition, there are some implicit parameters that need to be externally
synchronized. For example, all pname:commandBuffer parameters that need to
be externally synchronized imply that the pname:commandPool that was passed
in when creating that command buffer also needs to be externally
synchronized. The implicit parameters and their associated object are listed
below.

include::../hostsynctable/implicit.txt[]


[[fundamentals-errors]]
== Errors

Vulkan is a layered API. The lowest layer is the core Vulkan layer, as
defined by this Specification. The application can: use additional layers
above the core for debugging, validation, and other purposes.

One of the core principles of Vulkan is that building and submitting
command buffers should: be highly efficient. Thus error checking and
validation of state in the core layer is minimal, although more rigorous
validation can: be enabled through the use of layers.

The core layer assumes applications are using the API correctly. Except as
documented elsewhere in the Specification, the behavior of the core layer to
an application using the API incorrectly is undefined, and may: include
program termination.
However, implementations must: ensure that incorrect usage by an
application does not affect the integrity of the operating system,
the Vulkan implementation, or other Vulkan client applications
in the system, and does not allow one application to access data
belonging to another application. Applications can: request stronger
robustness guarantees by enabling the pname:robustBufferAccess feature
as described in <<features>>.

Validation of correct API usage is left to validation layers. Applications
should: be developed with validation layers enabled, to help catch and
eliminate errors. Once validated, released applications shouldnot: enable
validation layers by default.


[[fundamentals-validusage]]
=== Valid Usage

Valid usage defines a set of conditions which must: be met in order to achieve
well-defined run-time behavior in an application. These conditions depend only
on Vulkan state, and the parameters or objects whose usage is constrained by
the condition.

Some valid usage conditions have dependencies on run-time limits or feature
availability. It is possible to validate these conditions against Vulkan's
minimum supported values for these limits and features, or some subset of
other known values.

Valid usage conditions do not cover conditions where well-defined behavior
(including returning an error code) exists.

Valid usage conditions should: apply to the command or structure where
complete information about the condition would be known during execution of
an application. This is such that a validation layer or linter can: be
written directly against these statements at the point they are specified.

[NOTE]
.Note
====
This does lead to some non-obvious places for valid usage statements. For
instance, the valid values for a structure might depend on a separate value
in the calling command. In this case, the structure itself will not
reference this valid usage as it is impossible to determine validity from
the structure that it is invalid - instead this valid usage would be
attached to the calling command.

Another example is draw state - the state setters are independent, and can
cause a legitimately invalid state configuration between draw calls; so the
valid usage statements are attached to the place where all state needs to be
valid - at the draw command.
====

Certain usage rules apply to all commands in the API unless explicitly
denoted differently for a command. These rules are as follows.

Any input parameter to a command that is an object handle must: be a valid
object handle, unless otherwise specified. An object handle is valid if:

  * It has been created or allocated by a previous, successful call to the
    API. Such calls are noted in the specification.
  * It has not been deleted or freed by a previous call to the API. Such
    calls are noted in the specification.
  * Any objects used by that object, either as part of creation or
    execution, must: also be valid.

The reserved handle code:VK_NULL_HANDLE can: be passed in place of valid
object handles when _explicitly called out in the specification_. Any
command that creates an object successfully mustnot: return
code:VK_NULL_HANDLE. It is valid to pass code:VK_NULL_HANDLE to any
ftext:vkDestroy* or ftext:vkFree* command, which will silently ignore these
values.

Any parameter that is a pointer must: be a valid pointer. A pointer is valid
if it points at memory containing values of the number and type(s) expected
by the command, and all fundamental types accessed through the pointer (e.g.
as elements of an array or as members of a structure) satisfy the alignment
requirements of the host processor.

Any parameter of an enumerated type must: be a valid enumerant for that
type. A enumerant is valid if:

  * The enumerant is defined as part of the enumerated type.
  * The enumerant is not one of the special values defined for the
    enumerated type, which are suffixed with etext:_BEGIN_RANGE,
    etext:_END_RANGE, etext:_RANGE_SIZE or etext:_MAX_ENUM.

Any parameter that is a flag value must: be a valid combination of bit
flags. A valid combination is either zero or the bitwise OR of valid bit
flags. A bit flag is valid if:

  * The flag is defined as part of the bits type, where the bits type is
    obtained by taking the flag type and replacing the trailing etext:Flags
    with etext:FlagBits. For example, a flag value of type
    ename:VkColorComponentFlags must: contain only values selected from the
    bit flags in elink:VkColorComponentFlagBits.
  * The flag is allowed in the context in which it is being used. For
    example, in some cases, certain bit flags or combinations of bit flags
    are mutually exclusive.

Any parameter that is a structure containing a etext:VkStructureType
ptext:sType member must: have a value of ptext:sType matching the type of
the structure. The correct value is described for each structure type, but
as a general rule, the name of this value is obtained by taking the
structure name, stripping the leading etext:Vk, prefixing each capital
letter with etext:_, converting the entire resulting string to upper case,
and prefixing it with etext:VK_STRUCTURE_TYPE. For example, structures of
type sname:VkImageCreateInfo must: have a ptext:sType value of
ename:VK_STRUCTURE_TYPE_IMAGE_CREATE_INFO.

The values ename:VK_STRUCTURE_TYPE_LOADER_INSTANCE_CREATE_INFO and
ename:VK_STRUCTURE_TYPE_LOADER_DEVICE_CREATE_INFO are reserved for internal
use by the loader, and don't have corresponding Vulkan structures in this
specification.

Any parameter that is a structure containing a basetype:void* ptext:pNext
member must: have a value of ptext:pNext that is either `NULL`, or points to
a valid structure defined by an extension, containing ptext:sType and
ptext:pNext members as described in the <<extensions-interactions,Extension
Interactions>> section. If that extension is supported by the
implementation, then it must: be enabled. Any component of the
implementation (the loader, any enabled layers, and drivers) must: skip
over, without processing (other than reading the pname:sType and pname:pNext
members) any chained structures with pname:sType values not defined by
extensions supported by that component.

Extension structures are not described in the base Vulkan specification,
but either in layered specifications incorporating those extensions,
or in separate vendor-provided documents.

The above rules also apply recursively to members of structures provided as
input to a command, either as a direct argument to the command, or
themselves a member of another structure.

Specifics on valid usage of each command are covered in their individual
sections.


[[fundamentals-returncodes]]
=== Return Codes

While the core Vulkan API is not designed to capture incorrect usage,
some circumstances still require return codes. Commands in Vulkan return
their status via return codes that are in one of two categories:

  * Successful completion codes are returned when a command needs to
    communicate success or status information. All successful completion
    codes are non-negative values.
  * Run time error codes are returned when a command needs to communicate a
    failure that could only be detected at run time. All run time error
    codes are negative values.

All return codes in Vulkan are reported via basetype:VkResult return
values. The possible codes are:

include::../enums/VkResult.txt[]

[[fundamentals-successcodes]]
.Success codes
* ename:VK_SUCCESS
    Command successfully completed
* ename:VK_NOT_READY
    A fence or query has not yet completed
* ename:VK_TIMEOUT
    A wait operation has not completed in the specified time
* ename:VK_EVENT_SET
    An event is signaled
* ename:VK_EVENT_RESET
    An event is unsignaled
* ename:VK_INCOMPLETE
    A return array was too small for the result

[[fundamentals-errorcodes]]
.Error codes
* ename:VK_ERROR_OUT_OF_HOST_MEMORY
    A host memory allocation has failed.
* ename:VK_ERROR_OUT_OF_DEVICE_MEMORY
    A device memory allocation has failed.
* ename:VK_ERROR_INITIALIZATION_FAILED
    Initialization of an object could not be completed for
    implementation-specific reasons.
* ename:VK_ERROR_DEVICE_LOST
    The logical or physical device has been lost. See
    <<devsandqueues-lost-device,Lost Device>>
* ename:VK_ERROR_MEMORY_MAP_FAILED
    Mapping of a memory object has failed.
* ename:VK_ERROR_LAYER_NOT_PRESENT
    A requested layer is not present or could not be loaded.
* ename:VK_ERROR_EXTENSION_NOT_PRESENT
    A requested extension is not supported.
* ename:VK_ERROR_FEATURE_NOT_PRESENT
    A requested feature is not supported.
* ename:VK_ERROR_INCOMPATIBLE_DRIVER
    The requested version of Vulkan is not supported by the driver or
    is otherwise incompatible for implementation-specific reasons.
* ename:VK_ERROR_TOO_MANY_OBJECTS
    Too many objects of the type have already been created.
* ename:VK_ERROR_FORMAT_NOT_SUPPORTED
    A requested format is not supported on this device.

If a command returns a run time error, it will leave any result pointers
unmodified, unless other behavior is explicitly defined in the specification.

Out of memory errors do not damage any currently existing Vulkan objects.
Objects that have already been successfully created can: still be used by
the application.

Performance-critical commands generally do not have return codes. If a run
time error occurs in such commands, the implementation will defer reporting
the error until a specified point. For commands that record into
command buffers (ftext:vkCmd*) run time errors are reported by
fname:vkEndCommandBuffer.

[[fundamentals-numerics]]
== Numeric Representation and Computation

Implementations normally perform computations in floating-point, and must:
meet the range and precision requirements defined under
``Floating-Point Computation'' below.

These requirements only apply to computations performed in Vulkan
operations outside of shader execution, such as texture image
specification and sampling, and per-fragment operations. Range and
precision requirements during shader execution differ and are specified
by the <<spirvenv-precision-operation, Precision and Operation of SPIR-V
Instructions>> section.

In some cases, the representation and/or precision of operations is
implicitly limited by the specified format of vertex or texel
data consumed by Vulkan. Specific floating-point formats are
described later in this section.


[[fundamentals-floatingpoint]]
=== Floating-Point Computation

Most floating-point computation is performed in SPIR-V shader modules. The
properties of computation within shaders are constrained as defined by the
<<spirvenv-precision-operation, Precision and Operation of SPIR-V
Instructions>> section.

Some floating-point computation is performed outside of shaders, such as
viewport and depth range calculations. For these computations, we do not
specify how floating-point numbers are to be represented, or the details of
how operations on them are performed, but only place minimal requirements on
representation and precision as described in the remainder of this section.

ifdef::editing-notes[]
[NOTE]
.editing-note
====
(Jon, Bug 14966) This is a rat's nest of complexity, both in terms of
describing/enumerating places such computation may: take place (other than
``not shader code'') and in how implementations may: do it. We have consciously
deferred the resolution of this issue to post-1.0, and in the meantime, the
following language inherited from the OpenGL Specification is inserted as a
placeholder. Hopefully it can: be tightened up considerably.
====
endif::editing-notes[]

We require simply that numbers' floating-point parts contain enough bits and
that their exponent fields are large enough so that individual results of
floating-point operations are accurate to about 1 part in 10^5^. The
maximum representable magnitude for all floating-point values must: be at
least 2^32^. latexmath:[$x \cdot 0 = 0 \cdot x = 0$] for any non-infinite
and non-NaN latexmath:[$x$]. latexmath:[$1 \cdot x = x \cdot 1 = x$].
latexmath:[$x + 0 = 0 + x = x$]. latexmath:[$0^0 = 1$].

Occasionally, further requirements will be specified. Most
single-precision floating-point formats meet these requirements.

The special values latexmath:[$Inf$] and latexmath:[$-Inf$] encode values
with magnitudes too large to be represented; the special value
latexmath:[$NaN$] encodes ``Not A Number'' values resulting from undefined
arithmetic operations such as latexmath:[$0 / 0$]. Implementations may:
support latexmath:[$Inf$]s and latexmath:[$NaN$]s in their floating-point
computations.

Any representable floating-point value is legal as input to a Vulkan
command that requires floating-point data. The result of providing a value
that is not a floating-point number to such a command is unspecified, but
mustnot: lead to Vulkan interruption or termination. In <<IEEE 754>>
arithmetic, for example, providing a negative zero or a denormalized number
to an Vulkan command must: yield deterministic results, while providing a
latexmath:[$NaN$] or latexmath:[$Inf$] yields unspecified results.


[[fundamentals-fp16]]
=== 16-Bit Floating-Point Numbers

16-bit floating point numbers are defined in the
``16-bit floating point numbers''
section of the Khronos Data Format Specification.

Any representable 16-bit floating-point value is legal as input to a
Vulkan command that accepts 16-bit floating-point data. The result of
providing a value that is not a floating-point number (such as
latexmath:[$Inf$] or latexmath:[$NaN$]) to such a command is
unspecified, but mustnot: lead to Vulkan interruption or termination.
Providing a denormalized number or negative zero to Vulkan must: yield
deterministic results.


[[fundamentals-fp11]]
=== Unsigned 11-Bit Floating-Point Numbers

Unsigned 11-bit floating point numbers are defined in the
``Unsigned 11-bit floating point numbers''
section of the Khronos Data Format Specification.

When a floating-point value is converted to an unsigned 11-bit
floating-point representation, finite values are rounded to the closest
representable finite value.

While less accurate, implementations are allowed to always round in the
direction of zero. This means negative values are converted to zero.
Likewise, finite positive values greater than 65024 (the maximum finite
representable unsigned 11-bit floating-point value) are converted to 65024.
Additionally: negative infinity is converted to zero; positive infinity is
converted to positive infinity; and both positive and negative
latexmath:[$NaN$] are converted to positive latexmath:[$NaN$].

Any representable unsigned 11-bit floating-point value is legal as input
to a Vulkan command that accepts 11-bit floating-point data. The
result of providing a value that is not a floating-point number (such as
latexmath:[$Inf$] or latexmath:[$NaN$]) to such a command is
unspecified, but mustnot: lead to Vulkan interruption or termination.
Providing a denormalized number to Vulkan must: yield deterministic
results.


[[fundamentals-fp10]]
=== Unsigned 10-Bit Floating-Point Numbers

Unsigned 10-bit floating point numbers are defined in the
``Unsigned 10-bit floating point numbers''
section of the Khronos Data Format Specification.

When a floating-point value is converted to an unsigned 10-bit
floating-point representation, finite values are rounded to the closest
representable finite value.

While less accurate, implementations are allowed to always round in the
direction of zero. This means negative values are converted to zero.
Likewise, finite positive values greater than 64512 (the maximum finite
representable unsigned 10-bit floating-point value) are converted to 64512.
Additionally: negative infinity is converted to zero; positive infinity is
converted to positive infinity; and both positive and negative
latexmath:[$NaN$] are converted to positive latexmath:[$NaN$].

Any representable unsigned 10-bit floating-point value is legal as input to
a Vulkan command that accepts 10-bit floating-point data. The result of
providing a value that is not a floating-point number (such as
latexmath:[$Inf$] or latexmath:[$NaN$]) to such a command is unspecified,
but mustnot: lead to Vulkan interruption or termination. Providing a
denormalized number to Vulkan must: yield deterministic results.


[[fundamentals-general]]
=== General Requirements

Some calculations require division. In such cases (including implied
divisions performed by vector normalization), division by zero produces an
unspecified result but mustnot: lead to Vulkan interruption or
termination.


[[fundamentals-fixedconv]]
== Fixed-Point Data Conversions

When generic vertex attributes and pixel color or depth components are
represented as integers, they are often (but not always) considered to be
_normalized_. Normalized integer values are treated specially when
being converted to and from floating-point values, and are usually referred
to as _normalized fixed-point_.

In the remainder of this section, latexmath:[$b$] denotes the bit width of
the fixed-point integer representation. When the integer is one of the types
defined by the API, latexmath:[$b$] is the bit width of that type. When the
integer comes from an <<resources-images,image>> containing color or depth
component texels, latexmath:[$b$] is the number of bits allocated to that
component in its <<features-formats,specified image format>>.

The signed and unsigned fixed-point representations are assumed to be
latexmath:[$b$]-bit binary two's-complement integers and binary unsigned
integers, respectively.


[[fundamentals-fixedfpconv]]
=== Conversion from Normalized Fixed-Point to Floating-Point

Unsigned normalized fixed-point integers represent numbers in the range
latexmath:[$[0,1\]$]. The conversion from an unsigned normalized fixed-point
value latexmath:[$c$] to the corresponding floating-point value
latexmath:[$f$] is defined as

[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\[ f = { c \over { 2^b - 1 } } \]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Signed normalized fixed-point integers represent numbers in the range
latexmath:[$[-1,1\]$]. The conversion from a signed normalized fixed-point
value latexmath:[$c$] to the corresponding floating-point value
latexmath:[$f$] is performed using

[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\[ f = \max( {c \over {2^{b-1} - 1}}, -1.0 ) \]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Only the range latexmath:[$[-2^{b-1}+1,2^{b-1}-1\]$] is used to represent
signed fixed-point values in the range latexmath:[$[-1,1\]$]. For example,
if latexmath:[$b = 8$], then the integer value latexmath:[$-127$]
corresponds to latexmath:[$-1.0$] and the value 127 corresponds to
latexmath:[$1.0$]. Note that while zero is exactly expressible in this
representation, one value (latexmath:[$-128$] in the example) is outside the
representable range, and must: be clamped before use. This equation is used
everywhere that signed normalized fixed-point values are converted to
floating-point.


[[fundamentals-fpfixedconv]]
=== Conversion from Floating-Point to Normalized Fixed-Point

The conversion from a floating-point value latexmath:[$f$] to the
corresponding unsigned normalized fixed-point value latexmath:[$c$] is
defined by first clamping latexmath:[$f$] to the range latexmath:[$[0,1\]$],
then computing

// Equation {glop:fund:convert:eqfloatuint}
[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\[ c = \operatorname{convertFloatToUint} ( f \times ( 2^b - 1 ) , b ) \]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

where latexmath:[$\operatorname{convertFloatToUint}(r,b)$] returns one of
the two unsigned binary integer values with exactly latexmath:[$b$] bits
which are closest to the floating-point value latexmath:[$r$].
Implementations should: round to nearest. If latexmath:[$r$] is equal to an
integer, then that integer value is returned. In particular, if
latexmath:[$f$] is equal to 0.0 or 1.0, then latexmath:[$c$] must: be
assigned 0 or latexmath:[$2^b-1$], respectively.

The conversion from a floating-point value latexmath:[$f$] to the
corresponding signed normalized fixed-point value latexmath:[$c$] is
performed by clamping latexmath:[$f$] to the range latexmath:[$[-1,1\]$],
then computing

// Equation {glop:fund:convert:eqfloatsnorm}
[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\[ c = \operatorname{convertFloatToInt} ( f \times ( 2^{b - 1} - 1 ) , b ) \]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

where latexmath:[$\operatorname{convertFloatToInt}(r,b)$] returns one of the
two signed two's-complement binary integer values with exactly
latexmath:[$b$] bits which are closest to the floating-point value
latexmath:[$r$]. Implementations should: round to nearest. If latexmath:[$r$]
is equal to an integer, then that integer value must: be returned. In particular,
if latexmath:[$f$] is equal to -1.0, 0.0, or 1.0, then latexmath:[$c$]
must: be assigned latexmath:[$-(2^{b-1}-1)$], 0, or latexmath:[$2^{b-1}-1$],
respectively.

This equation is used everywhere that floating-point values are converted to
signed normalized fixed-point.


[[fundamentals-versionnum]]
== API Version Numbers and Semantics

The Vulkan version number is used in several places in the API. In each
such use, the API _major version number_, _minor version number_, and _patch
version number_ are packed into a 32-bit integer as follows:

  * The major version number is a 10-bit integer packed into bits 31-22.
  * The minor version number is a 10-bit integer packed into bits 21-12.
  * The patch version number is a 12-bit integer packed into bits 11-0.

Differences in any of the Vulkan version numbers indicates a change to
the API in some way, with each part of the version number indicating a
different scope of changes.

A difference in patch version numbers indicates that some usually small
aspect of the specification or header has been modified, typically to fix a
bug, and may: have an impact on the behavior of existing functionality.
Differences in this version number shouldnot: affect either _full
compatibility_ or _backwards compatibility_ between two versions, or add
additional interfaces to the API.

A difference in minor version numbers indicates that some amount of new
functionality has been added. This will usually include new interfaces in
the header, and may: also include behavior changes and bug fixes.
Functionality may: be deprecated in a minor revision, but will not be
removed. When a new minor version is introduced, the patch version is reset
to 0, and each minor revision maintains its own set of patch versions.
Differences in this version shouldnot: affect backwards compatibility, but
will affect full compatibility.

A difference in major version numbers indicates a large set of changes to
the API, potentially including new functionality and header interfaces,
behavioral changes, removal of deprecated features, modification or outright
replacement of any feature, and is thus very likely to break any and all
compatibility. Differences in this version will typically require
significant modification to an application in order for it to function.

[[fundamentals-common-objects]]
== Common Object Types

Some types of Vulkan objects are used in many different structures and
command parameters, and are described here. These types include _offsets_,
_extents_, and _rectangles_.


=== Offsets

Offsets are used to describe a pixel location within an image or
framebuffer, as an (x,y) location for two-dimensional images, or an (x,y,z)
location for three-dimensional images. Two- and three-dimensional offsets
are respectively defined by the structures

include::../structs/VkOffset2D.txt[]

include::../validity/structs/VkOffset2D.txt[]

include::../structs/VkOffset3D.txt[]

include::../validity/structs/VkOffset3D.txt[]


=== Extents

Extents are used to describe the size of a rectangular region of pixels within
an image or framebuffer, as (width,height) for two-dimensional images, or as
(width,height,depth) for three-dimensional images. Two- and
three-dimensional extents are respectively defined by the structures

include::../structs/VkExtent2D.txt[]

include::../validity/structs/VkExtent2D.txt[]

include::../structs/VkExtent3D.txt[]

include::../validity/structs/VkExtent3D.txt[]


=== Rectangles

Rectangles are used to describe a specified rectangular region of pixels
within an image or framebuffer. Rectangles include both an offset and an
extent of the same dimensionality, as described above. Two-dimensional
rectangles are defined by the structure

// Comment out until SubresourceRectangle-style structure proposed
// Two- and three-dimensional rectangles are respectively defined by the
// structures

include::../structs/VkRect2D.txt[]

include::../validity/structs/VkRect2D.txt[]

// include::../structs/VkRect3D.txt[]

// include::../validity/structs/VkRect3D.txt[]