A fast video card is supposed to speed up graphics operations, and Photoshop CS4 has support for “Open GL” a programmer’s API for making use of the processing power of the video card’s memory and CPU (GPU).
Therefore, it’s a big surprise to experience a performance downgrade when OpenGL is enabled on my quad-core 3.0 GHz Mac Pro (original model), which has the ATI Radeon X1900XT [specs] with 512MB of video memory. I have dual displays connected, a 30" and a 24". The X1900XT is listed as supported by Adobe.
I switch between color channels frequently (red/green/blue and/or L/a/b). I also click layers on and off frequently. I found that I had to disable OpenGL for a quite substantial improvement in redraw performance eg to make it behave on par with Photoshop CS3.
In short, do not assume that OpenGL is faster; try it with and without for your particular workflow. The list of purported benefits seems marginal, at least for the way I use Photoshop. And Adobe’s technical note only makes me leery of using OpenGL at all. Users with the current model Mac Pro and/or a different video card should not assume my results will be theirs; try it: enable/disable Open GL via Enable OpenGL Drawing, see below.
I also tried with 75% instead of 100% on the Let Photoshop Use memory setting—no difference. Speaking of that setting, I’ve never found that using the Ideal Range is better. Also, Available RAM is a misleading term; my machine has 16GB (16384MB). The terminology should be “Accessible RAM”.
Turn OpenGL OFF for best redraw performance
Of course, disabling OpenGL may result in diminished performance in other areas, though I don’t know what those areas might be yet. The list of benefits seems marginal for my tasks.
I did try Advanced Settings, turning off the Use for Image Drawing checkbox, etc, but redraw performance was still much better with OpenGL disabled entirely, as shown above. On my system, disabling (unchecking) Vertical Sync resulted in the screen not drawing at all.
The default OpenGL settings