Forza Motorsport includes alternate times of day for a small number of environments, but those alternates do not include dark night or weather effects. For Forza 4, the majority of our graphical investment went into our new image-based lighting (IBL) model, as well as new material shaders, which give our cars and tracks such a realistic, detailed look. With IBL, supporting the general look of night is not particularly hard. However, we take frame rate pretty seriously—we believe that having a solid 60 frames per second (FPS) experience with no tearing is very important for a simulation racing game. Delivering night is about more than just getting the general look right. As we found on the original Forza Motorsport, having multiple headlight projections with multiple cast shadows is computationally heavy—even using clever tricks like we did on the less powerful original Xbox platform. This makes delivering a strong night racing experience very difficult at 60 FPS without significant compromise.
Regarding weather conditions, we do simulate different track temperatures. However, we did not get tire data for wet conditions from our tire partners at Pirelli. We do have SAE data about wet conditions, but it’s nowhere near as comprehensive as our Pirelli data. In order to simulate wet correctly, we’d like to get custom real-world test data to input into our system. Of course, physics isn’t the only difficult aspect of delivering wet conditions. Creating believable weather effects also depends upon gobs of particles and lots of shiny, reflective surfaces. There are several tricks for doing this, but it would still have made delivering the new graphical fidelity of FM4 at a solid 60 FPS nearly impossible in our development timeframe.
If we cut down on the number of cars on track, used original Xbox-generation car models, dropped to 30 FPS, or (and this would be the most effective solution) built specific tracks from the ground-up to have less detail and thus extra performance headroom, then night racing and/or weather conditions may have been possible. Some of those trade-offs, we just were not willing to do. Others would require time that we used to develop other features in the game—specifically, the new graphical look of the game in general. Night and weather are features we will continue to evaluate as the franchise continues. We’re waiting for the right time to deliver these features to our fans.
