The SDK will also contain the skybox Valve used in Lost Coast, because it is apparently a considerable feat to create images supporting HDR data using the current tools such as Photoshop. Adobe has apparently been a bit slow in updating their software in this regard, so Valve ended up hand-authoring the skybox completely, a task with which they don't expect everybody will want to bother. The SDK will also explain how to adjust exposure levels, the brightness range that the auto-exposure feature will cover, the amount of bloom to use, and so on. There are key things to take into account; for example, just as in real life, when looking from a very dark area into a very light area, it is actually more difficult to make out details due to objects being so heavily shadowed.

Though Valve has yet to fully explore the gameplay ramifications of HDR, they are clearly visibly excited to get the tools into the hands of the resourceful mod community to see what kind of ideas will come to fruition. Essentially, they hope to see the Counter-Strike or Garry's Mod of HDR, and they're confident somebody will come up with it.

Source=Shacknews