OpenGL and multiple threads

Hello, since I haven’t been able to find decent documentation on this subject I need to ask these questions here:

In OpenGL each thread needs its own Rendering Context?

I can do reads/writes to different DisplayDevices ( ATI extension - make_current_read) but this works only with the same Rendering Context?

When drawing object A in thead 1 and object B in thread 2, will a readpixels from thread 2 display only object B?

Has anybody used multithreading with OpenGL succesfully, and what did you gain by it?

Thanks for any insight into this matter.

Originally posted by def:
In OpenGL each thread needs its own Rendering Context? […]

Each thread can have at most one current context. A context can be current for only one thread at a time.
So, being “current” does not imply that the context must be different. However it implies that one unique context must be exclusively accessed by one thread at a time.
Recall that OpenGL is by nature a client/server architecture. So having multiple thread issuing commands concurrently in the same context would lead to a breakup of the GL commands protocol and inconsistent status in the state machine.

Efficient use of multiprogramming, at a safe level, is for example to carry out independent calculations in parallel.
Obviously multi-threading really pays off on SMP systems.

If I can remember, when having a decent hardware, GL commands usually are inexpensive to be executed
(they are buffered by the driver). So generally they should not be considered a major performance hit. What needs the most care in coding is state change (to be minimized), and data transfer.

I suggest that you reconsider your code and eventually revert to single threading, that in the case you presented would seem to be the natural choice.