# Stereo problems on a Mac Pro with 2 Cinema displays **URL:** \ **Category:** Uncategorized\ **Created:** [October 27, 2009, 12:37am UTC](https://psychtoolbox.discourse.group/t/stereo-problems-on-a-mac-pro-with-2-cinema-displays/610 "2009-10-27T00:37:57Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Allen\_Ingling](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Allen\_Ingling](https://psychtoolbox.discourse.group/u/Allen_Ingling)\ **Post date:** [October 27, 2009, 12:37am UTC](https://psychtoolbox.discourse.group/t/stereo-problems-on-a-mac-pro-with-2-cinema-displays/610/1 "2009-10-27T00:37:57Z")
I am trying to run the StereoDemo in Stereomode 10 for multi displays. I am using Matlab R2007a on a new Mac Pro 2.66GHz Quad-Core Intel Xeon. My video cards are two NVIDIA GeForce GT 120. Currently, the two cinema displays are connected to a single card. The cinema displays are the old model (23 inch). While the StereoDemo works fine on my Macbook pro with a second display connected, I get the following error when I try to run it on the Mac Pro. PTB-ERROR[ContextCreation failed: invalid share context]:The specified display may not support double buffering and/or stereo output. There could be insufficient video memory PTB-ERROR[Low-Level setup of window failed]:The specified display may not support double buffering and/or stereo output. There could be insufficient video memory ---
**Author:** ![Allen\_Ingling](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Allen\_Ingling](https://psychtoolbox.discourse.group/u/Allen_Ingling)\ **Post date:** [October 28, 2009, 8:04am UTC](https://psychtoolbox.discourse.group/t/stereo-problems-on-a-mac-pro-with-2-cinema-displays/610/2 "2009-10-28T08:04:25Z")
Hi Mario, Thanks for your advice - I am very relieved. Yes it was the dual video card issue. I had one display in each card - D'oh! StereoMode 10 works fine with just two monitors connected to the same video card. I haven't tried a third in the second card yet -- Mac's cables are too short for my current setup. Cheers, Phil > --- In [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com), "Mario" \ wrote: > \> > \> > \> > \> I assume you have more than 2 displays connected? In that case the demo is > \> probably trying to display on the wrong displays. It is hard-coded to use > \> screens 0 and 1, assuming a dual-display setup. > \> > \> If i have to guess, your desktop monitor (with the dock and menu-bar etc.) is > \> connected to > \> one GPU and shows up as screenid 0, whereas the two displays for stereo work are > \> connected to the 2nd GPU and correspond to screens 1 and 2. In that case or > \> similar cases, the hardcoded screenids 0 and 1 would try to open the left-eye > \> window on your first GPU and the right-eye window on the 2nd GPU, which is a > \> no-go. > \> > \> Apparently, "the worlds most advanced operating system", driving "up to eight > \> displays in the fastest Mac graphics ever" > \> (\<[http://www.apple.com/macpro/features/graphics.html](http://www.apple.com/macpro/features/graphics.html)\>) (quote of the iPhone > \> company), can't even share resources across different graphics cards, so > \> whenever your 2 stereo display monitors are not attached to the same graphics > \> card, it will bomb out with the "invalid share context" error message. > \> > \> I really wonder why they advertise 8 display support if they still can't get > \> simple dual-display operation right even in Snow Leopard -- not speaking of > \> Leopard which was an utter disaster on NVidia cards and it took them over 14 > \> months to fix that for Snow Leopard. > \> > \> Test which screenids the command Screen('Screens') returns? Probably 0, 1 and 2? > \> > \> Try if PerceptualVBLSyncTest([1,2], 10); works. It would try stereomode 10 on > \> screens 1 and 2. > \> > \> Then you'll need to change the hardcoded values in the demos to make them work. > \> Unfortunately it gets difficult to automatically select the 2 out of n displays > \> on a n \> 2 display setup, so the demos need tweaking. > \> > \> I hope that is the problem, because otherwise things get ugly. Let us know if > \> this is the problem. > \> -mario > \> > \> > \> --- In [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com), "philipmgrove" \ wrote: > \> \> > \> \> I am trying to run the StereoDemo in Stereomode 10 for multi displays. I am using Matlab R2007a on a new Mac Pro 2.66GHz Quad-Core Intel Xeon. My video cards are two NVIDIA GeForce GT 120. Currently, the two cinema displays are connected to a single card. The cinema displays are the old model (23 inch). > \> \> > \> \> While the StereoDemo works fine on my Macbook pro with a second display connected, I get the following error when I try to run it on the Mac Pro. > \> \> > \> \> PTB-ERROR[ContextCreation failed: invalid share context]:The specified display may not support double buffering and/or stereo output. There could be insufficient video memory > \> \> > \> \> > \> \> PTB-ERROR[Low-Level setup of window failed]:The specified display may not support double buffering and/or stereo output. There could be insufficient video memory > \> \> > \>