Mac gamers have been anxiously awaiting Doom 3 for the Mac. Hope springs eternal that the Mac version of Doom 3 will perform as well on the Mac as it does on the PC. In our 'Mac versus PC' game test page, we ran Doom 3 at 1600x1200 with Video Quality set to HIGH and the first four Advanced Options all set to YES. The G5/2.5GHz Power Mac with the two fastest graphics cards was handily beaten by 'high end' Windows PCs.

There are already some excellent articles on Doom 3 for the Mac on at other sites. (See links below.) We decided to do an article probing ways to squeeze out a little more performance with only sight loss of visual quality and realism. These are the steps we've found helpful:

STAGE ONE: Run at 1600x1200 with Video Quality set to HIGH; First FOUR Advanced Options set to YES. This is the starting point and the settings we used to compare the Mac to the PC in another article.

STAGE TWO: Turn off SHADOWS in ADVANCED OPTIONS -- a step suggested by the developer.

STAGE THREE: Turn off Anisotropic Filtering either with console command (image_anisotropic 0). This filter only affects distant details and, in my opinion, won't be make a noticeable difference in realism during game play. (In the case of the ATI X800 XT, we used the ATI Displays utility to override the Anisotropic Filtering settings in Doom 3, because it gave us faster times whether set at 8 or OFF.)

STAGE FOUR: Switch from Trilinear Filtering to Bilinear. This is done by replacing:
seta image_filter 'GL_LINEAR_MIPMAP_LINEAR'
with
seta image_filter 'GL_LINEAR_MIPMAP_NEAREST' (in the config file).
This tweak helped the GeForce 6800 Ultra but didn't do anything for the Radeon X800 XT.

STAGE FIVE: This tweak only works on ATI Radeon graphics cards. We used ATIccelerator II to overclock the Radeon X800 XT (core clock set 5% faster to 500MHz and memory clock set 11% faster to 1100MHz). (We don't recommend this. It can cause permanent damage to your graphics card. If you see screen artifacts, then change the settings back to default immediately.)

For the record, we only saw a 5% gain with overclocking when all the tweaks set. But at the 'normal' High Quality setting with Shadows ON and Anisotropy set to 8, the gain from overclocking is 13%.

The graph below shows the gains we achieved with each stage of tweaking. We included the times from various Windows PC configurations as a reference point:

CONCLUSION ON OUR TWEAKS of Doom 3 for the Mac
Even with up to five tweaks, the fastest Macs with the fastest graphics cards could not catch up to the Windows PCs. The Mac is a very sophisticated personal computer. Should we care if it can't play games as well as the 'crude' PC? I say, 'YES!' Why can't the Mac's amazing architecture, rich development tools, and state of the art OS combine to provide top notch OpenGL performance in the newest 3D game? 'Why does Doom 3 run so slow on the Mac compared to the PCs?'

MAC GAME PERFORMANCE BRIEFING FROM THE DOOM 3 DEVELOPERS
Glenda Adams, Director of Development at Aspyr Media, has been involved in Mac game development for over 20 years. I asked her to share a few thoughts on what attempts they had made to optimize Doom 3 on the Mac and what barriers prevented them from getting it to run as fast on the Mac as in comparable Windows PCs. Here's what she wrote:

'Just like the PC version, timedemos should be run twice to get accurate results. The first run the game is caching textures and other data into RAM, so the timedemo will stutter more. Running it immediately a second time and recording that result will give more accurate results.
The performance differences you see between Doom 3 Mac and Windows, especially on high end cards, is due to a lot of factors (in general order from smallest impact to largest):
1. PowerPC architectural differences, including a much higher penalty for float to int conversion on the PPC. This is a penalty on all games ported to the Mac, and can't be easily fixed. It requires re-engineering much of the game's math code to keep data in native formats more often. This isn't 'bad' coding on the PC -- they don't have the performance penalty, and converting results to ints saves memory and can be faster in many algorithms on that platform. It would only be a few percentage points that could be gained on the Mac, so its one of those optimizations that just isn't feasible to do for the speed increase.
2. Compiler differences. gcc, the compiler used on the Mac, currently can't do some of the more complex optimizations that Visual Studio can on the PC. Especially when inlining small functions, the PC has an advantage. Add to this that the PowerPC has a higher overhead for functional calls, and not having as much inlining drops frame rates another few percentage points.
3. More robust and modern OpenGL implementation on OS X. The fact that OpenGL is engineered from the ground up on OS X to be accessible from many applications at once is wonderful for the rest of the world, but does have a performance hit for games. Sharing GL with the rest of the system invokes a small overhead that Windows doesn't have, since Windows can basically assume GL is just in use for one application.
4. OpenGL framework/drivers split on OS X. On Windows, ATI and nVidia are responsible for the OpenGL code all the way from the hardware to the game. On the Mac, Apple handles the top layers of OpenGL and then hands data off to the video card drivers. On Windows this allows the video card manufacturers to do some more direct optimizations that make sure data gets passed to the card as fast as possible. The Mac can't short circuit that process, since there is a fairly well defined boundary between GL and the video card drivers. This is complicated by the more modern GL implementation on OS X as well- Apple can't just put in a bunch of hacks to shove data around the wall and into the cards, just for the game.
5. And the last, but definitely most important factor: Amount of time Apple/ATI/nVidia have had to optimize specifically for Doom 3. On Windows, ATI/NVIDIA spent multiple programmer years tuning their OpenGL implementations for Doom 3, starting back over a year ago while the game was still in development. Apple/ATi/NVIDIA have done an immense amount of work on OS X's GL in the last 3-4 months, but there is no way they could get as much done as the dozens of Windows engineers working on the problem for over a year. 10.3.8 includes a huge number of GL optimizations that make a big difference in Doom 3, and the game wouldn't have been in any shape to ship without these. One of the biggest things ATi & nVidia do on the PC for Doom 3 is have application specific OpenGL optimizations just for the game. They can detect Doom3 is the application using GL, and even which shaders it is downloading -- then they can shift to a mode that is highly optimized just for those cases.
The good news on all of these fronts, especially the last one, is that Doom 3 is such a highly visible benchmarking application, Apple/ATI/NVIDIA/Aspyr are all going to be continuing to work on increasing performance over the coming months/years. Just like what happened with Quake 3, the Mac OS matured, video card drivers got more optimized, and the game was tweaked so that eventually Mac performance is now as good or better than comparable PC hardware (I'd be really interested to see benchmarks with Quake 3 with the original shipping Mac app & version of OS X versus the latest app & current OS on the same hardware). Games drive hardware and the OS, and Doom 3 will likely push Apple to upgrade consumer video cards and continue to spend engineering time in the future to speed up OpenGL.'