Difference between revisions of "Core And Compatibility in Contexts"

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OpenGL version 3.0 introduced the idea of [[Deprecation|deprecation]]; some sections of the specification could be marked as deprecated. This means that they can be removed from the core specification in later versions, which happened in version 3.1.
#REDIRECT [[OpenGL_Context#Context_types]]
However, since many implementations support the deprecated and removed features anyway, some implementations want to be able to provide a way for users of higher GL versions to gain access to the old APIs. Several techniques were tried, and it has settled down into a division between '''Core''' and '''Compatibility'''
== OpenGL 3.1 and ARB_compatibility ==
A new extension, ARB_compatibility, was introduced when OpenGL 3.1 was revealed. The presence of this extension is a signal to the user that deprecated or removed features are still available through the original entrypoints and enumerations. The behavior of such implementations is defined with a separate, much larger, [[OpenGL Specification]]. Thus, there was a backwards-compatible specification and a non-backwards compatible specification.
== OpenGL 3.2 and Profiles ==
When OpenGL 3.0 was introduced, a new context creation mechanism was introduced as well. Since later versions could remove functionality, it was important that the user be able to ask for a specific version of OpenGL, one that they had written their code against.
However, the ARB_compatibility extension posed a problem: the implementation decided whether to define it or not. A user could not ask to ''not'' have ARB_compatibility; it was forced on them. Thus, another change was made to context creation.
The 3.0 form of context creation allows the user to ask for a profile. Initially, only one profile was available: core. In 3.2, a new profile was added: compatibility. An implementation is only ''required'' to define core, so compatibility is not guaranteed to be available. However, it's a safe bet that it is.
The 3.2 wording also changed, allowing 3.0 and higher contexts to be given when not directly asked, so long as they support ARB_compatibility or are the compatibility profile of that version of OpenGL. So if you ask for 2.1, the implementation is free to give you 3.2 compatibility; it was not previously free to do this.
Because of the change in wording, the way to detect the changed functionality is to check the extension string. If WGL_ARB_create_context_profile is present, then it uses the 3.2 wording. Otherwise it uses the 3.0/3.1 wording.

Latest revision as of 05:18, 26 September 2017