okay,

frankly I never bothered that much about rendertimes as i've got a fat machine (i7 990X, 12 threads @4GHz so yeah, that runs 4 times faster than 4 threads @ 3 GHz), but mainly as my final runs either run in queue at the bacjground or overnight.

So I did some additional investigation to find things out. Lower settings result in lower quality, but what does that mean in real life? I tried settings for Irradiance Caching (IC) and IDL Quality (Q), and I found noticeable quality reduction when either of those fell below 80. Noticeable quality loss means:

 - splotchyness, clearly visible in even, smooth walls and skin

 - clearly visible differences when flipping render results, when it was not clear where to look for beforehand

 - those differences were especially present in AO-sensitive areas, like hair, hat-edges near the head, skirt-edges near the legs, etc. Lower settings seem to arp less light into those areas resulting in darker (self)shadowing.

 - measured differences between render results, RGB values differed 4 or more (that is: a wall with RGB=(196,196,196) gave 200+ in at least one of the RGB channels in at least some spots in the result. Apparently, the eye does notice that.

When both values exceeded 90 the quality loss was gone, not noticeable. Flipping render results revealed no differences even when I kenw where to look, and measured differences were 1 at most: the wall with RGB=196,196,196 deviated to 197 in some of the channels in some spots in the result. The eye does not notice that.

The IC=80 Q=80 setting clocked 25 mins on my machine, including 22 mins for the IDL pre-pass. Raising Q => 90 extended the IDL pre-pass with 2 mins, raising Q is the best way to lose the splotches, which I consider the main quality-losers as these are the most visible. Raising IC => 90 had a far larger impact on rendertime, the IDL pre-pass took another 10 mins more, and another 8 mins when I raised the bar to 95. Higher IC values especially warp more light into the AO-sensitive areas, which I consider the least visible improvement.

Since leaving IC off is mainly a potential saver for very high IC values (say >95) my suggestions for a good result in a decent timeframe are:

 - IDL Quality to 90. Higher values hardly pay off, and the extra rendertime from 80 to 90 is small compared to the extra quality gain. Don't go below 80 for final results.

 - IC on, don't go below 80 for final results, don't go over 90 either because higher values don't pay off. Raising the bar from 80 to 90 might double render time or so, while not having the largest impact on the quality of the result: it mainly brightens some (self)shadowing areas, although raising to 85 does some extra cleaning on the splotches too. Hence IC/85 to IC/90 it is.

On top of all this, I managed to crash firefly in various situations that combined large image, large bucket size, extreme render settings and IC turned off. With IC on and a high value (say 95 or 98) in there, the crashes did not occur. So, for whatever reason, the IC/90 setting recommended above embeds some hidden life-saver as well :)

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though