I have a couple of questions concerning the ambient file.
Suppose I have to calculate images for different rooms of a building, one perspective at a time of each room, with some room being more ‘open’ looking onto other rooms, would you suggest to have just one ambient file (of course the geometry and the parameters would stay unchanged) and simply add on it?
What happens if one calculation doesn’t go through for some reason, let’s say because I am forced to stop it and the same calculation is then started again? I see that it doesn’t start from zero, but simply adds to what was calculated since then (at least this is my impression). Would the final calculation be reliable?
Regarding your first question, if -ab is 2 or greater and there is significant light-sharing between rooms, you are usually better off sharing the same ambient file. The only possible down side to doing this is the potential for light leaks at T-junctions, where walls have not been given any thickness, and memory overhead if your ambient file gets very large, but both are rare occurrences.
Rerunning a calculation with the same ambient file produces a better result the second time. We often do what we call and “overture calculation” for just this purpose, where we render the same view at much lower resolution and discard the result before the final rendering pass. If you use the rad program, it will do this for you automatically if you specify an AMBFILE.