The reference pages for OpenGL 4.5 include complete descriptions of all OpenGL commands and functions contained in the Core Profile of OpenGL 4.5, and the OpenGL Shading Language 4.40 Specification.
The legacy reference pages for OpenGL 3.3 include complete descriptions of all commands and functions contained in the Core Profile of OpenGL 3.3, and the OpenGL Shading Language 3.30 Specifications.
The legacy OpenGL 2.1 reference pages include descriptions of all OpenGL commands contained in the OpenGL 2.1 Specification, including commands that have been deprecated and removed in more modern versions of OpenGL (but are still included in compatibility profiles of OpenGL). Also included are descriptions of GLU and GLX entry points.
Several quick reference guides are available in PDF format from the links below. The quick reference guides are fold-out annotated lists of the functions included in OpenGL, along with their parameters and other useful information.
The OpenGL Registry, hosted outside of the SDK, contains the formal Specifications for OpenGL and the OpenGL Shading Language, as well as for companion APIs like GLX and GLU. Specifications are the authoritative documents about how OpenGL is intended to work. They are also the most difficult to read, being written primarily for OpenGL implementers, not developers using OpenGL; so you may find the reference pages combined with tutorials to be a better choice.
The Registry also contains specifications for extensions describing optional features of OpenGL, and header files defining interfaces for extensions. Extension specifications are written as supplements to the core OpenGL Specification. Most new features are first introduced as extensions and only later become part of a core OpenGL version, so using extensions is important to staying current with the evolution of the API.
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