Retrieve the Stencil Buffer?

[PickleWorld]

Is there an easy way to copy the contents of the stencil buffer to main memory?

You can copy the depth or frame buffer with glCopyTexImage2D(... GL_RGBA/GL_DEPTH_COMPONENT, ....) but the spec says nothing about stencil?

Is it possible? Or do I have to clear the framebuffer, set the stencil, draw a black quad and then copy the framebuffer or something every time instead?

[skynet]

glReadPixels() with GL_STENCIL_INDEX or GL_DEPTH_STENCIL as 'format' parameter and an appropriate type (GL_UNSIGNED_INT or UNSIGNED_INT_24_8) copies the stencil values to main memory.

Be aware that you cannot use glReadPixels() on multisampled FBOs.
Paragraph 4.3. of the OpenGL specs tells you everything you need to know ;-)