LucasArts engineer Marco Salvi has posted an in-depth explanation of his method for HDR rendering, prompted by a SIGGRAPH 2006 lecture series by Valve employees. Salvi first touches on Valve's own HDR process (added to the Source engine in 2005), which he points out runs "with MSAA on relatively old hardware [and] executes tone mapping and MSAA resolve in the proper correct order with no extra performance cost, something that a lot of modern games can't still get right today."

"Through image segmentation techniques," Valve's code "’simply’ tries to determine if the previous frame has been under or over exposed and a new exposure value is adjusted to compensate for problems with the previous frame(s)." Salvi notes that this method doesn't allow for reliably determining average logarithmic luminance, which in his experience has led to "an overall flat and over or under saturated feeling." So he introduces his own plan: "get rid of the exposure search through previous frames feedback and compute it the proper way!"

Salvi's method computes per-pixel logarithmic luminance and outputs that to the alpha channel; he includes a code example of the relevant math, and delves into further specific details.

As a bonus, Valve's Gary McTaggart and Chris Green, both presenters of the lectures that spurred Salvi's post, drop by in the blog's comment section to explain their reasoning behind the route they took for the Source engine's HDR implementation.