glfw: GLFW is a free, Open Source, multi-platform library for opening a window, creating an OpenGL context and managing input. It is easy to integrate into existing applications and does not lay claim to the main loop. GLFW is written in C and has native support for Windows, Mac OS X and many Unix-like systems using the X Window System, such as Linux and FreeBSD. GLFW is licensed under the zlib/libpng license.
glew: The OpenGL Extension Wrangler Library (GLEW) is a cross-platform open-source C/C++ extension loading library. GLEW provides efficient run-time mechanisms for determining which OpenGL extensions are supported on the target platform.
glut: The [Open]GL Utility Toolkit (GLUT) is a utility library for OpenGL applications to put the platform-dependent details (e.g. creating windows and responding to mouse events and key presses) under one hood, in an easy and portable way.
freeglut: freeglut is an open-source alternative to the OpenGL Utility Toolkit (GLUT) library.
摘自[stackoverflow]
glew：就是你要使用对老opengl的扩展，你要使用一些新的接口就要用这个！
glfw：类似于统一opengl的窗口，窗口管理与glut类似
glut：opengl的工具包，隐藏平台之间的差别与细节，提供提供统一的方法来处理事件，消息等
freeglut：替代品glut
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