In traditional rendering, it is typical to do all the calculations with radiometric units, either as full spectral rendering, or component-wise (XYZ, RGB, etc).
However, as modern rendering adds more physically based models, it's convenient for artists to be able to specify values in the more familiar photometric units, for example, intensity of a light in lumens.
In order to keep the pipeline all in one type of unit, you can do one of the following:
- Convert photometric units to radiometric units using luminous efficacy
- Keep the entire render pipeline in photometric units
- Sébastian Lagarde presents this idea very well in the course notes of his Siggraph 2014 presentation Moving Frostbite to PBR
My questions then are these:
- Are there any disadvantages to rendering exclusively in photometric units?
- As far as I can tell, photometric units are just radiometric units biased to the human eye. Considering that we will view the final image with our eye, I don't see this as a problem.
- Frostbite is a RGB component-wise engine. Would a spectral renderer have any additional disadvantages by rendering exclusively in photometric units?