We've added the latest entry in the DiRT series to our test suite at the suggestion of AMD, who has been working with Codemasters for years on optimizations for Eyefinity and DirectX 11. Although Showdown is based on the same game engine as its predecessors, it adds an advanced lighting path that uses DirectCompute to allow fully dynamic lighting. In addition, the game has an optional global illumination feature that approximates the diffusion of light off of surfaces in the scene. We enabled the new lighting path, but global illumination is a little too intensive for at least some of these cards.
This is a fantastic game, by the way. My pulse was pounding at the end of each 90-second test run.
Uh oh. Just a look at the raw frame time plots bodes poorly for the GeForces, all of 'em.
The frequent spikes to ~40 milliseconds on the GeForce GPUs take their toll on the FPS average, but their impact is more readily apparent in the 99th percentile frame times, which are nearly double those of the Radeons.
Frame latencies are higher across the board on the GeForces versus the Radeons, but you can see how the "tail" spikes upward on the GeForce cards somewhere in the last 3-8% of frames. Those frames, we know from the plots, are distributed throughout each test run, not confined to a single spot on the track.
The results from our 50-ms threshold offer a bit of a corrective for us. Although the Radeons are clearly faster overall, even the older GeForces don't waste much time on really long-latency frames. That jibes with my seat-of-the-pants impression that Showdown was quite playable on all of the cards, even if it wasn't as smooth on the GeForces as on the Radeons.