ciphoneopengl-esuser interface
I'm searching for a OpenGL User Interface Library like WxWidgets but not OS dependent.
It would have to provide callbacks for user input since this is OS dependent.
If something like this doesn't yet exist (which I doubt because most games have most of the Basic UI Elements) is there a OpenGL HTML/CSS renderer?
SDL allows you to create an OpenGL context that is accelerated (depending on drivers / hardware support).
I know you tagged as C++, however pygame (python) is a great library for creating 2D games, which also supports an OpenGL context. Pygame is built on SDL.
Clutter is a new OpenGL based GUI library with bindings for Perl, Python, C#, C++, Vala and Ruby. I haven't used it myself. From the website:
Clutter uses OpenGL (and optionally OpenGL ES for use on Mobile and embedded platforms) for rendering but with an API which hides the underlying GL complexity from the developer. The Clutter API is intended to be easy to use, efficient and flexible.
An alternative approach would be to extract features (keypoints) using the scale-invariant feature transform (SIFT) or Speeded Up Robust Features (SURF).
You can find a nice OpenCV code example in Java, C++, and Python on this page: Features2D + Homography to find a known object
Both algorithms are invariant to scaling and rotation. Since they work with features, you can also handle occlusion (as long as enough keypoints are visible).
Image source: tutorial example
The processing takes a few hundred ms for SIFT, SURF is bit faster, but it not suitable for real-time applications. ORB uses FAST which is weaker regarding rotation invariance.
Best Answer
FLTK, GTK+, and QT support OpenGL widgets in a portable way. QT in particular can render all of its widgets in an OpenGL viewport. I know there's been some work on getting GTK to do the same thing, but I haven't heard on the progress of it.
OTK comes up on a quick Google search as an OpenGL-backed widget library.
It's also worth pointing out that Cairo has an OpenGL backend, and there are a number of toolkits that depend on it, including GTK+.