// Copyright (c) 2015-2016 The Khronos Group Inc. // Copyright notice at https://www.khronos.org/registry/speccopyright.html [[introduction]] = Introduction This chapter is Informative except for the sections on Terminology and Normative References. This document, referred to as the ``{apiname} Specification'' or just the ``Specification'' hereafter, describes the {apiname} graphics system: what it is, how it acts, and what is required to implement it. We assume that the reader has at least a rudimentary understanding of computer graphics. This means familiarity with the essentials of computer graphics algorithms and terminology as well as with modern GPUs (Graphic Processing Units). The canonical version of the Specification is available in the official {apiname} Registry, located at URL http://www.khronos.org/registry/vulkan/ [[introduction-whatis]] == What is the {apiname} Graphics System? {apiname} is an API (Application Programming Interface) for graphics and compute hardware. The API consists of many commands that allow a programmer to specify shader programs, compute kernels, objects, and operations involved in producing high-quality graphical images, specifically color images of three-dimensional objects. [[introduction-programmer]] === The Programmer's View of {apiname} To the programmer, {apiname} is a set of commands that allow the specification of _shader programs_ or _shaders_, _kernels_, data used by kernels or shaders, and state controlling aspects of {apiname} outside the scope of shaders. Typically, the data represents geometry in two or three dimensions and texture images, while the shaders and kernels control the processing of the data, rasterization of the geometry, and the lighting and shading of _fragments_ generated by rasterization, resulting in the rendering of geometry into the framebuffer. A typical {apiname} program begins with platform-specific calls to open a window or otherwise prepare a display device onto which the program will draw. Then, calls are made to open _queues_ to which _command buffers_ are submitted. The command buffers contain lists of commands which will be executed by the underlying hardware. The application can: also allocate device memory, associate _resources_ with memory and refer to these resources from within command buffers. Drawing commands cause application-defined shader programs to be invoked, which can: then consume the data in the resources and use them to produce graphical images. To display the resulting images, further platform-specific commands are made to transfer the resulting image to a display device or window. [[introduction-implementor]] === The Implementor's View of {apiname} To the implementor, {apiname} is a set of commands that allow the construction and submission of command buffers to a device. Modern devices accelerate virtually all {apiname} operations, storing data and framebuffer images in high-speed memory and executing shaders in dedicated GPU processing resources. The implementor's task is to provide a software library on the host which implements the {apiname} API, while mapping the work for each {apiname} command to the graphics hardware as appropriate for the capabilities of the device. [[introduction-ourview]] === Our View of {apiname} We view {apiname} as a pipeline having some programmable stages and some state-driven fixed-function stages that are invoked by a set of specific drawing operations. We expect this model to result in a specification that satisfies the needs of both programmers and implementors. It does not, however, necessarily provide a model for implementation. An implementation must: produce results conforming to those produced by the specified methods, but may: carry out particular computations in ways that are more efficient than the one specified. [[introduction-bugs]] == Filing Bug Reports Issues with and bug reports on the {apiname} Specification and the API Registry can: be filed in the Khronos Vulkan Github repository, located at URL http://github.com/KhronosGroup/Vulkan-Docs Please tag issues with appropriate labels, such as ``Specification'', ``Ref Pages'' or ``Registry'', to help us triage and assign them appropriately. Unfortunately, Github does not currently let users who do not have write access to the repository set Github labels on issues. In the meantime, they can be added to the title line of the issue set in brackets, e.g. ''[Specification]''. [[introduction-terminology]] == Terminology The key words *must*, *must not*, *required*, *shall*, *shall not*, *should*, *should not*, *recommend*, *may*, and *optional* in this document are to be interpreted as described in RFC 2119: http://www.ietf.org/rfc/rfc2119.txt *must*:: This word, or the terms *required* or *shall*, mean that the definition is an absolute requirement of the specification. *must not*:: This phrase, or the phrase *shall not*, means that the definition is an absolute prohibition of the specification. *should*:: This word, or the adjective *recommended*, means that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course. *should not*:: This phrase, or the phrase *not recommended*, means that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label. *may*:: This word, or the adjective *optional*, means that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item. An implementation which does not include a particular option must be prepared to interoperate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which does include a particular option must be prepared to interoperate with another implementation which does not include the option (except, of course, for the feature the option provides). The additional terms *can* and *cannot* are to be interpreted as follows: *can*:: This word means that the particular behavior described is a valid choice for an application, and is never used to refer to implementation behavior. *cannot*:: This word means that the particular behavior described is not achievable by an application. For example, an entry point does not exist, or shader code is not capable of expressing an operation. [NOTE] .Note ================== There is an important distinction between *cannot* and *must not*, as used in this Specification. *Cannot* means something the application literally is unable to express or accomplish through the API, while *must not* means something that the application is capable of expressing through the API, but that the consequences of doing so are undefined and potentially unrecoverable for the implementation. ================== ifdef::editing-notes[] [NOTE] .editing-note ==== TODO (Jon) - We might need to augment the RFC 2119 definition of *must not* to include some of the previous note, since at present it is defined solely in terms of implementation behavior. See Gitlab issue #9. ==== endif::editing-notes[] [[introduction-normative]] == Normative References Normative references are references to external documents or resources to which implementers of {apiname} must: comply. [[IEEE 754]]:: _IEEE Standard for Floating-Point Arithmetic_, IEEE Std 754-2008, http://dx.doi.org/10.1109/IEEESTD.2008.4610935, August, 2008. [[Khronos Data Format Specification]]:: A. Garrard, _Khronos Data Format Specification, version 1.1_, https://www.khronos.org/registry/dataformat/specs/1.1/dataformat.1.1.html, February 16, 2015. [[Khronos SPIR-V Extended Instructions for GLSL Specification]]:: J. Kessenich, _SPIR-V Extended Instructions for GLSL, Version 1.00_, https://www.khronos.org/registry/spir-v/, February 10, 2016. [[Khronos SPIR-V Specification]]:: J. Kessenich and B. Ouriel, _The Khronos SPIR-V Specification, Version 1.00_, https://www.khronos.org/registry/spir-v/, February 10, 2016.