As we have seen, shadow maps are essentially textures that are projected onto a surface. Those gamers who have played games over the past, say, 6 years will known that general textures have become sharper and more detailed in 3D games; this has been possible because the textures themselves have become bigger, i.e. their resolution (number of pixels) has increased. The same is true for shadow maps, in that the greater the resolution used, the better looking the shadow will be. Neverwinter Nights 2 from Obsidian uses shadow maps and allows one to use a range of resolutions (the difference is more apparent during real-time motion):
With low resolution depth buffers, the tree shadows look rounded and fuzzy.
With more pixels in them, high res depth buffers help to retain the correct shape of the trees in their shadows.
Low res shadows in the distance look blocky and in motion, cause aliasing problems.
High res with filtering = realistic and smooth shadows!
Since the low res maps make the shadows look blocky, it obviously makes sense to use the highest res maps that you can. Unfortunately, larger depth buffers need more power from the graphics processor to render as they contain more pixels. There is also a limit to how large textures can actually be; really big ones can be very taxing! But even massive textures can't contain enough detail to provide realistic shadows on complex surfaces; at certain angles, a single big map will cause problems too.
Alan Wake uses cascaded shadow maps to ensure that the environment is as realistic as possible - both close-up and in the far distance.
3DMark06 uses extremely high resolution cascaded shadow maps to ensure that the surfaces are lit as accurately as possible.
So to get around this, Alan Wake (and 3DMark06) use cascaded shadow maps. The scene is split up into several sections, along the axis that travels away from the view point. A shadow map is then generated for each part, rather than one for the whole frame (hence the term "cascade - arrange in series"). Although costly in terms of performance, the end results are very good, especially when the surfaces and shadows are at sharp angles to the camera.
For a more detailed look at shadowing technology, please read the following presentation by NVIDIA - download the pdf file here.
