Just out of interest Brad, relating to HDR images. Is it possible to have a sky luminence in a scene that will traverse the sky at certain intervals, and save each rendering and then merge the renderings to create a HDR image. I am only just getting my head around the HDR methodology, but it would be interesting to create HDR images from renders with different lighting parameters, as opposed to photographs taken at different times of day of the same scene.
What is your take on this possibility ?.
The idea behind HDR is to gather as much lighting information as possible from a scene and store to that information as a 32-bit digital image. This is done by acquiring at least (3) LDR images of differing exposure and merging those into an HDR image via special software.
The dynamic range of the light in a scene is essentially the difference between its darkest and brightest areas. The dynamic range of light in a room with the sun spilling in through a window would be much higher than the same room lit by only incandescent light bulbs. The higher the dynamic range, the more exposures you'll need to effectively cover that range.
To capture the light information you start by taking a snapshot that properly exposes the darkest area of the scene (generally the shadows). Then you adjust the exposure up by 1 or 2 stops and take another snapshot. Continue taking shots, while increasing the exposure by the same increment each time, until the brightest area of the scene is properly exposed. Now you have a collection of images that represents the dynamic range of your scene. When merging these into an HDR image most software will ask you for the increment of exposure used and some will just guess.
Basically, the idea is to capture the images as quickly as possible rather than throughout a day. This is most important with outside scenes because cars, birds, trees, clouds, etc. all move which makes merging the images a big job. It becomes less automatic and much more hands-on to align and edit out portions of the images.
So, to answer your question Michael, I haven't tried generating an HDRI from multiple renderings. I think you could certainly generate a usable file that way with some trial and error. I've seen many posts in various forums on the subject of "faking" HDR. I think the trick would be setting up the scene once and then adjusting the luminance brightness to follow a typical capturing sequence. As mentioned above, the exposure difference between images needs to be consistent for merging software to reliably merge the images into a single HDRI.
I'd love to see what you come up with while experimenting.