Yes and no. As long as you merely adjust things in the viewport, OpenGL will still be used, even if MP is on. however, as soon as you do final renders or RAM previews, OpenGL will be disabled and all effects fall back into software-only mode. It's a limitation in how OpenGL uses an "exclusive" model for allocating resources. Only one program instance can control those. So to cut a long story short: Don't let anythings stop you from using MP and OpenGL at the same time, just be aware of the limitations. Only disable eitehr feature, if you experience problems like faulty renders or crashes.
Thanks for that. The reason for my question is that AE CS3 is hanging-up on RAM preview. I went through all the Adobe documents I could find. I turned off "Render Multiple Frames Simultaneously", I took the OpenGL Plug-in out of the Plug-ins folder. I edited the Text prefs file, changing "MaxNumberOf Processes" to "6" (it was originally at "0" - which puzzles me). I forced a rebuild of the prefs file ...and all possible permutations of the above.
When AE hangs while attempting RAM preview, its icon in the Dock (couldn't find any reference to this feature in the manual) shows a progress bar that reads 639 of 720 - the latter number, I suppose, referring to the total number of frames in my 30 sec @ 24 fps comp. I have 16GB of RAM installed. My size and quality settings are both at 50% (could this be inappropriate).
Trawling the Internet tells me that this is far from an unusual problem. See, for example, this thread from Creative Cow last year, a series of complaints with no solution forthcoming:
I can probably get by without RAM preview, but having spent the money on an 8Core machine it would be great to have it work.
2 x 2.26 GHz Quad-Core Intel Xeon
16 GB 1066 MHz DDR3
Any recommendations for optimum settings?
Any further ideas, given these specifics, very much welcomed.
What footage types and what effects are you using? Most failures to use MP depend on outdated effects or "unsafe" media formats. Could also simply be a file I/O bottleneck, that bombs out the processes due to them being unable to load the source media (imagine the data rates of 8 processes trying to access the same HD footage...) or write the results. The pref option you mention is actually an experimental feature in CS3 and it being set to zero simply means "use all cores on all CPUs" changing it to other values may improve matters, but that's all on your own head . I think over at mograph.net there is a longer thread on configuring MP and the conclusion was to use 5 cores on a system with 16GB RAM. That was of course for CS4, which provides better control over the process...
Thanks again. The footage is a 393.9 MB 16 bit TIFF equirectangular panorama. The effect is Trapcode Horizon. I've given up on RAM preview, but am still repeatedly having to force quit and restart AE when I scrub in the Timeline.