When do I need to use an OpenGL function loader?

Question

I'm a bit confused about when exactly I need to use an OpenGL function loader like GLEW. In general, it seems like you first obtain a window and valid OpenGL context and then attempt to load functions.

• Sometimes these functions are referred to as extensions, sometimes they are called core functions as well. It seems like what's loaded and classified as 'core' and 'extension' is platform dependent. Are the functions that are loaded in addition to some base set?

• Do you need to load functions in the same way on OpenGL ES platforms as well? Taking a quick look at GLEW, I don't see any explicit support for Open GL ES. Other GL function loader libs do explicitly mention support specifically for ES however (like https://github.com/Dav1dde/glad)

Answer 1

OpenGL functions (core or extension) must be loaded at runtime, dynamically, whenever the function in question is not part of the platforms original OpenGL ABI (application binary interface).

• For Windows the ABI covers is OpenGL-1.1
• For Linux the ABI covers OpenGL-1.2 (there's no official OpenGL ABI for other *nixes, but they usually require OpenGL-1.2 as well)
• For MacOS X the OpenGL version available and with it the ABI is defined by the OS version.

This leads to the following rules:

• In Windows you're going to need a function loader for pretty much everything, except single textured, shaderless, fixed function drawing; it may be possible to load further functionality, but this is not a given.
• In Linux you're going to need a function loader for pretty much everything, except basic multitextured with just the basic texenv modes, shaderless, fixed function drawing; it may be possible to load further functionality, but this is not a given.
• In MacOS X you don't need a function loader at all, but the OpenGL features you can use are strictly determined by the OS version, either you have it, or you don't.

The difference between core OpenGL functions and extensions is, that core functions are found in the OpenGL specification, while extensions are functionality that may or may be not available in addition to what the OpenGL version available provides.

Both extensions and newer version core functions are loaded through the same mechanism.