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Since physics effects donít translate very well to screenshots, I decided to make a little video instead Ė just a few scenes from the game to show you what these little details bring to the table. Then I found a much, much better done video on Youtube that kicks the crap out of what I put together, so you can watch that instead. There are spoilers, so just skip ahead to the 1:15 mark if you want to avoid those. Otherwise, enjoy!
Advanced cloth simulations are responsible for many of the PhysX effects in the game. Banners and flags flutter and sway, shredding when a Batarang or gun fire hits. Caution tape and cobwebs tear as Batman moves through them. Sheets of paper are strewn about the floors throughout Arkham interiors. They furl and roll around in reaction to characters kicking them. During combat, Batmanís swift movements cause them to fly into the air and float back down again. Without PhysX, they only exist as static piles and textures stuck onto surfaces. On the outside, fallen leaves act in much the same way, adding to the appearance of your interactions with the game world.
Volumetric smoke, steam, and fog generated by PhysX are much like the effect introduced with Shader Model 3.0/DirectX 10 and seen in games such as STALKER: Clear Sky. The whole point is to be rid of the two-dimensional sprites that form crude imitations and donít react at all to their surroundings, clipping through objects or the boundaries of the level itself. NVIDIA PhysX treats smoke as an actual object that takes up space and is contained in the game world. It moves around somewhat like a liquid, if the droplets were the size of softballs. This is called smoothed particle hydrodynamics and allows smoke to directly interact with not only the environment, but characters, as well.
Most games employ only basic physics on a few things like boxes or barrels, maybe bodies and other various items. Objects are given properties that allow them to be subjected to forces like gravity and explosions. Beyond that, the game also needs to be able to know if two objects are touching so that it might make corrections to ensure they donít break the laws of physics by occupying the same space. This is known as collision detection, a quite complicated and resource-draining operation, and is the extent of most software-based physics.
Having a lot of so-called rigid bodies in a scene will put a strain on the CPU because of all the calculations that need to be done to make sure they behave properly. This can be solved with the hardware acceleration provided by NVIDIA GPUs. Some scenes in Batman: AA involve hundreds or even thousands of dynamic objects on screen at once, something that previously could not even be conceived! It will bring your framerate down quite a bit even now, but chips get more powerful and algorithms are improved, so itís exciting to see what can be done with this technology in future games.