UPDATE!

Apple has got the message and has provided a graphics update for OS X Snow Leopard which will go some way towards closing the gap between the two platforms.

“When we launched Steam on Mac OS X back in May, there was a lot of buzz about performance, particularly relative to Windows running on the same machine,” says a statement from Steam bosses.

“While we met our goal of making sure all of our customers had an acceptable gaming experience at launch, we have spent a large chunk of effort in the intervening months working with Apple and their GPU vendors to close the performance gap with Windows.”

Steam reckons a combination of changes to its own code and the latest graphics update available from Apple today removes a variety of software bottlenecks, resulting in significant graphics performance enhancements for Mac gamers.

In addition to low-level implementation changes which have improved performance across the board, Apple has also removed some implementation inefficiencies which allow Steam games to improve visual quality, most notably in the area of GPU occlusion queries.

Since the latest update to 10.6.4, Steam has been able to properly implement ‘occlusion query’ which is a GPU-based mechanism which allows OpenGL to draw convincing lighting effects, particularly when a light source is obscured by in-game objects.

“A given light source may be partly or wholly occluded by other geometry in the scene and we use the occlusion query to determine how occluded it is. The percentage of a given light source’s screen area which is actually visible is used to scale the intensity of an additive glow sprite which is drawn over the frame without any z-buffering,” – Steam developer said.

“We have been able to measure performance improvements with the latest software update, but we are anticipating even more speedups if Apple implements the uniform_buffer_object extension and GLSL 1.3 in a future update. With these additional features, we will be able to sidestep this particular CPU bottleneck, allowing us to win back a bunch of CPU time and, ultimately, performance.”

The gaming community is reporting dramatic performance improvements on iMac (Late 2009 and Mid 2010), Mac mini (Early 2009 and Mid 2010), Mac Pro (Early 2009), MacBook (Early 2009 and Mid 2010) and MacBook Pro (15-inch, Mid 2010) and MacBook Pro (17-inch, Mid 2010) models. Older systems are generally already operating at the limits of the hardware, and Valve Software says it’s unlikely that any significant performance improvements can be achieved in the future.

In conclusion, Steam employees said, “We’re very excited about the performance improvements that Apple and the GPU vendors have been able to deliver this summer and we are working with them to further improve performance.”