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An OpenGL context represents many things. A context stores all of the state associated with this instance of OpenGL. It represents the (potentially visible) default framebuffer that rendering commands will draw to when not drawing to a framebuffer object. Think of a context as an object that holds all of OpenGL; when a context is destroyed, OpenGL is destroyed.
Contexts are localized within a particular process of execution (an application, more or less) on an operating system. A process can create multiple OpenGL contexts. Each context can represent a separate viewable surface, like a window in an application.
Each context has its own set of OpenGL Objects, which are independent of those from other contexts. A context's objects can be shared with other contexts. Any OpenGL object types which are not containers are sharable, as well as Sync Objects and GLSL Objects (excluding program pipeline objects). All container objects are not shared between contexts.
Any object sharing must be made explicitly, either as the context is created or before a newly created context creates any objects. However, contexts do not have to share objects; they can remain completely separate from one another.
In order for any OpenGL commands to work, a context must be current; all OpenGL commands affect the state of whichever context is current. The current context is a thread-local variable, so a single process can have several threads, each of which has its own current context. However, a single context cannot be current in multiple threads at the same time.
Context information queries
There is a lot of information about an OpenGL context can be queried.
OpenGL version number
To get the OpenGL major and minor version numbers, you can call these functions:
glGetIntegerv(GL_MAJOR_VERSION, *); glGetIntegerv(GL_MINOR_VERSION, *);
These are available on OpenGL version 3.0 and above contexts. If those are not available, you can use this instead:
The string returned starts with "<major version>.<minor version>". Following the minor version, there can be another '.', then a vendor-specific build number. The string may have more content, which is completely vendor-specific (thus not a part of the OpenGL standard).
For example, the returned string may be like "2.0.6914 WinXP SSE/SSE2/SSE3/3DNow!". 2.0 is the actual version number of GL supported. 6914 is a driver build number. WinXP is the OS. SSE/SSE2/SSE3/3DNow! are CPU features that the driver can use.
Sometimes glGetString(GL_VERSION) also returns also the bus type used, such as AGP or PCI or PCIEx.
The OpenGL implementations have a vendor string associated with it that is used for identifying the maker of the implementation.
It could be "ATI Technologies", "NVIDIA Corporation", "INTEL" and so on. Note that there's no guarantee that the string for a specific vendor will remain the same in future implementations. On Windows, if it says "Microsoft" then you are using the Windows software renderer or the Windows Direct3D wrapper. You probably haven't installed the graphics drivers yet in that case.
This names the renderer used by the implementation.
This string is often the name of the GPU. In the case of Mesa3d, it would be i.e "Gallium 0.4 on NVA8". It might even say "Direct3D" if the Windows Direct3D wrapper is being used.
Extensions can be queried one by one. To do this, first use glGetIntegerv(GL_NUM_EXTENSIONS, *) to get the number of extensions supported by the implementation. Then iterate over each one with this:
Where k is less than the GL_NUM_EXTENSIONS value.
Legacy extension list
|Warning: This section describes legacy OpenGL APIs that have been removed from core OpenGL 3.1 and above (they are only deprecated in OpenGL 3.0). It is recommended that you not use this functionality in your programs.|
A string containing a space-separated list of extension names can be queried as follows:
Shading language version
The primary version of GLSL supported by the implementation can be queried:
The version string has almost the same format as the context version string. The difference is that the minor version always has two digits.
Supported GLSL versions
|Core in version||4.6|
|Core since version||4.3|
Support for specific versions of GLSL can be queried via an indexed get. The number of supported GLSL versions is queried with glGetIntegerv(GL_NUM_SHADING_LANGUAGE_VERSIONS, *). Each individual version can be queried from an indexed string:
Where k is on the range 0 to GL_NUM_SHADING_LANGUAGE_VERSIONS - 1.
These strings use the formatting of GLSL's #version declaration. This has the following forms:
The profile is only present for versions of GLSL that have this distinction.
Note that GLSL only gained the #version declaration in version 1.10; if an implementation exposes support for GLSL 1.00 (through ARB_shading_language_100), then it will return an empty string ("").
Versions of the GLSL for OpenGL ES can also be supported. If an implementation returns "100", this does not refer to GLSL 1.00. It instead refers to support for OpenGL ES's 2.0's GLSL ES version 1.00. The profile can also be "es", which represents a version of OpenGL ES's shading language. So version "300 es" represents GLSL ES 3.00. GLSL ES 1.00 does not use the "es" profile name.