OpenGL Extensions和Version

Talk About

• OpenGL Extensions有哪些分类？
• Conformant OpenGL implementation是什么?
• OpenGL Extensions和OpenGL Versions有什么关系？
• OpenGL Extensions和conformant OpenGL implementation有什么关系？

OpenGL Extensions Category

严格来说，OpenGL Extensions可以分为三类，但也可以分为两类。

如果按照Khronos的Extensions Name Convention可以分为以下三类：

• vendor-specific (“NV”, “ATI”, etc)
• multivendor (EXT)
• Khronos-approved (“ARB”, “OES”, “OML”)

具体说， Khronos的Extension命名规则是api_category_name, 对于OpenGL API来说api就是GL, category就是上面的三类，对于第三类Khronos-approved Extensions, category还会细分

• “ARB”或”KHR”专门指OpenGL Extensions
• “OES”专门指OpenGL ES Extensions
• “OML”专门指OpenML Extensions

如果按照Khronos的OpenGL Registry Extension Specifications可以分为以下两类:

• OpenGL ARB Extensions Specifications
• OpenGL Vendor and EXT Extension Specifications

Khronos-approved Extensions

这里重点理一下Khronos-approved Extensions(或者**”GL_ARB_“和“GL_KHR_“**), 因为Khronos-approved Extensions与下面两个问题密切相关：

• OpenGL Extensions和OpenGL Versions有什么关系？
• OpenGL Extensions和conformant OpenGL implementation有什么关系？

先看一下OpenGL 4.6 Core Profile Specification

K.3 ARB and Khronos Extensions

OpenGL extensions that have been approved by the Khronos OpenGL Architectural Review Board Working Group(ARB), or jointly approved by the ARB and the Khronos OpenGL ES Working Group(KHR), are summarized in this section. ARB and KHR extensions are NOT required to be supported by a CONFORMANT OpenGL implementation, but are expected to be widely available; they define functionality that is likely to move into the REQUIRED feature set in a future revision of the specification.

所有被ARB批准的，或者被ARB和KHR联合批准的OpenGL Extensions被罗列在本章节。这些Extensions不是一个Conformant OpenGL实现所必须支持的，但它们应该被大多数OpenGL实现都支持；它们所定义的功能很有可能在未来的OpenGL Specification里加到必须支持的特性集中。

K.3.1 Naming Conventions

为了区分ARB和KHR extensions和core OpenGL extensions(required core features), 以及vendor-specific extensions, 有以下extension命名规范：

• 每个Khronos-approved extension都有一个唯一的名字字符串”GL_ARB_name”或”GL_KHR_name”, 如果某个OpenGL实现支持某个extension,那么这个extension的名字字符串就要出现在调用glGetStringi(EXTENSIONS);的返回值里。
• 所有在这个extension里定义的函数，函数名都以FunctionARB或FunctionKHR命名。
• 所有在这个extension里定义的枚举，枚举名都以NAME_ARB或NAME_KHR命名。

K.3.2 Promoting Extensions to Core Features

Khronos-approved, multivendor, 以及vendor-specific extensions都可以被_晋升_(promoted)加入到OpenGL核心特性集(required core features). 一旦被加入，相应的extension specification将被合入到core specification. 在这个extension里定义的函数和枚举也将_删除_它们的ARB, KHR, EXT, 或者vendor后缀。而且支持这些被_晋升_的extension的OpenGL实现应该继续支持带后缀的函数名和枚举名。

What Is The Relationship Between OpenGL Extensions And OpenGL Versions?

下面这张图展示各种OpenGL extensions集合的关系。只有那些Required Core Features(Extensions)才会被纳入到各个OpenGL Core Profile Specification， 也即OpenGL versions是通过不断加入新的Required Core Features来升级的。

OpenGL (ES) and GLSL (ES) Version

Khronos发布OpenGL(ES)及GLSL(ES)的时间

Standard	Birth
OpenGL	1992
OpenGL ES	2003
GLSL	2004
GLSL ES	2007

OpenGL Version

GL Version	Release	GLSL Version	Preprocessor
1.0	1992
1.1	1997
1.2	1998
1.3	2001
1.4	2002
1.5	2003
2.0	2004	1.10	#version 110
2.1	2006	1.20	#version 120
3.0	2008	1.30	#version 130
3.1	2009	1.40	#version 140
3.2	2009	1.50	#version 150
3.3	2010	3.30	#version 330
4.0	2010	4.00	#version 400
4.1	2010	4.10	#version 410
4.2	2011	4.20	#version 420
4.3	2012	4.30	#version 430
4.4	2013	4.40	#version 440
4.5	2014	4.50	#version 450
4.6	2017	4.60	#version 460

OpenGL (Shading Language) ES Version

OpenGL ES	Release	GLSL ES (ESSL)	Directive	GLSL based
1.0	2003	N/A	N/A	N/A
1.1	2004	N/A	N/A	N/A
2.0	2007	1.00	#version 100	1.20
3.0	2012	3.00	#version 300 es	3.30
3.1	2014	3.10	#version 310 es	3.30
3.2	2015	3.20	#version 320 es	3.30

PIPE_CAP_* vs Extensions in Mesa

/**
 * Use pipe_screen::get_param() to query PIPE_CAP_ values to determine
 * which GL extensions are supported.
 * Quite a few extensions are always supported because they are standard
 * features or can be built on top of other gallium features.
 * Some fine tuning may still be needed.
 */
void st_init_extensions(struct pipe_screen *screen,
                        struct gl_constants *consts,
                        struct gl_extensions *extensions,
                        struct st_config_options *options,
                        gl_api api)

/**
 * Examine enabled GL extensions to determine GL version.
 */
static GLuint
compute_version(const struct gl_extensions *extensions,
                const struct gl_constants *consts, gl_api api)

FAQ

1. 为什么GLSL ES 1.00后面是3.00版本？

在GLSL ES 3.00 Spec里有这样一段说明：

This version of the language is based on version 3.30 of the desktop GLSL. However it includes a number of features that are in version 4.20 but not 3.30. The previous version of GLSL ES was version 1.00 so this version could be called version 2.00.

RESOLUTION: Follow the desktop GLSL convention so that the language version matches the API version. Hence this version will be called 3.00

GLSL ES 3.10 & 3.20里同样有这段说明，所以，3.00主要是为了对应OpenGL ES API的版本号，3.10, 3.20同样是。