How to select the GPU for openGL rendering?

OOfff. This is a subtle and complicated issue. Are you on windows or OS X?

If you create a window with Jitter on one monitor, the GPU that the monitor is connected two will 'own' the content. Ie: its textures and memory will be resident on that GPU*

If you MOVE the window (at least on OS X), it is my understanding that OS X's drivers will move the GPU resources to the GPU that owns that monitor. You may have to re-create your GL world. You can do this by having a named matrix in your patch, and set your jit.gl.renderer to drawto namedmatrix, drawto windowdestination, where namedmatrix is your jit.gl.matrix namedmatrix 4 char 320 240 (this forces software rendering), and then immediately switch it back to your named window, which goes back to hardware land. This may kick it on to the second GPU.

If you have two monitors hooked up to one GPU, nothing happens, it works, as youve stated.

If you have two monitors hooked up to two different GPUs, the memory/textures etc are moved, and set up on the second GPU, as far as I am aware (using the drawto trick)*

Now if you span the window across two monitors, depending on the GPU configuration, some things can happen:

If you have two monitors on one GPU, you are fine, content plays just as fast (just as fast as it should at least being resized, as you noticed)

If you have two monitors on two GPUs, the content IS NOT AUTOMATICALLY MOVED AND REBUILT AT FULL SPEED ACROSS THE TWO. Since the content is NEEDED ON BOTH GPUS, the GPU where the window was initially created 'OWNS' the content, and READBACK happens. This readback makes things SLOW. it downloads data to the main CPU memory and then re-uploads it to the second GPU.**

Its not clear what exactly is going on with that dual head on the second GPU from your description.

Is nothing GL related on the 'main' GPU anymore when you move the window to the projectors?

something like:

gpu1 : (DVI output 1) -> monitor 1
gpu1 : (DVI output 2) -> monitor 2

gpu2: -> (DVI output 1) -> dual head -> monitor 1 & monitor2

If everything is on gpu 2 like above, try the drawto command which rebuilds the gl client list, iirc, and may move the context over.

If you arent on OS X, man, have fun, because GL on windows with Jitter seems to be really weird and not quite 'the same'.

Good luck!

* this is how I understand it, I may be wrong, but this seems to follow what ive seen.

** You can do something about it if you need to render on both GPUs (just for your info), but it requires care and a deep understanding of GL.

Make N OpenGL contexts for each GPU that you want (say one monitor per GPU). Make them resident on each GPU by creating the window on that GPU or moving it.

Upload your the textures to each context. Ie:

If you want texturea on all three monitors (assuming 3 gpus), upload the original bitmap 3 times to 3 different textures youve built on each context.

If you want to have your scene 'spanned', offset each camera on each context so they line up next to one another, and use ortho projection so your scene looks as though it is one whole scene.

Since you will eventually be using a TripleHead, its not an issue for you, but MultiGPU (non SLI'd) is NOT easy, and someone smarter than me will probably point out issues in what ive said, which makes it all the more complicated to manage and deal with.