OpenGL Context

1 OpenGL Context

• 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.

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• 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. Most OpenGL objects are sharable, including Sync Objects and GLSL Objects. Container Objects are not sharable, nor are Query Objects.

• Objects can be separated into two different categories: regular objects and container objects.

regular objects	container objects
Query Objects	Framebuffer Objects
Renderbuffer Objects	Program Pipeline Objects
Sampler Objects	Transform Feedback Objects
Texture Objects	Vertex Array Objects

• Query Objects are OpenGL Objects that are used for asynchronous queries of certain kinds of information.

• 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.