3D and Technical Artist Shane Marks posts a look at a technique to using a diffuse convolved environment map for lighting scenes rather than using a typical standard HDR image. Shane shows how you can get better and faster Final Gather renders by processing the HDR images by applying a diffuse convolution filter to them.
HDRI maps are inherently problematic as you could have pixels that are extremely bright and extremely dark next to each other
This can be done by using some freely available tools that can be used to modify the HDR into something “level” that final gather will work better and faster with, and in reality it is a simple blur processing, not unlike a gaussian blur.
A Problem with HDR’s
The problem lies in the HDR maps where you could have one extremely bright pixel right next to a really dark one. Final gather is almost like a reverse ray trace, where the camera sends out rays to the objects in the scene to estimate global illumination, sampling directions in the hemisphere over that final gather point.
Knowing how final gather works, if a ray encounters a bright pixel on the environment map while another hits a dark pixel you will inevitably get a splotchy looking render. At this point you may feel the need to up the final gather settings, but trying to clean up the render in this way will greatly increase the time it takes to render.
Creating a Better Image for Final Gather
Using Picturenaut, you can process the HDR image to even out the intensities of the pixels in the map. This is done with a “smart blur” that will do a diffuse convolution on the image. A diffuse convolution filter will preserve the edges of the HDR image and better approximates the lighting contribution within the HDR.
In the end, your HDR file will be lower resolution, and provide you with better rendering results with less samples.
Having a “pre-blurred” HDR prevents you from having to use higher samples to get the correct illumination solution, as it almost is “pre-Sampled” as far as final gathering is concerned.
Check out the post for Better HDRI Maps for Final Gather (General Technique) over at Shane Mark’s site here.