I have already rendered it twice, once to over 4000samples and it took days, and still splotchy in MLT BiD with max consecutive rejections of 2000. Then redid it again in MLT mode much faster right now
anyway I saw these quotes from Ono - which made me think a little, so I did some testing to figure out the right settings....
Reducing the max number of consecutive rejections reduces noise, especially fireflies, at the cost of introducing some bias into the render.
It is the maximum number of consecutive rejections allowed in the Metropolis-Hastings algorithm before a mutation accept is forced.
------General tracing parameters------
"max_depth" "10000"
"bidirectional" "false"
"metropolis" "true"
"hybrid" "false"
------Metropolis light transport settings------
"large_mutation_prob" "0.4"
"max_change" "0.03"
all rendered to 6 mins 320x240 image size
MNCR 0
Done 6020000.00000 samples (78.38542 samples per pixel)
16646.48344 samples / second (60.07275 micro-seconds / sample)
MNCR 25
Done 11060000.00000 samples (144.01042 samples per pixel)
30426.43564 samples / second (32.86616 micro-seconds / sample)
MNCR 50
Done 10840000.00000 samples (141.14583 samples per pixel)
29920.80770 samples / second (33.42156 micro-seconds / sample)
MNCR 100
Done 10990000.00000 samples (143.09896 samples per pixel)
30385.82236 samples / second (32.91009 micro-seconds / sample)
MNCR 250
Done 10990000.00000 samples (143.09896 samples per pixel)
30314.52187 samples / second (32.98749 micro-seconds / sample)
MNCR 500
Done 10930000.00000 samples (142.31771 samples per pixel)
30181.85997 samples / second (33.13248 micro-seconds / sample)
MNCR 750
Done 10940000.00000 samples (142.44792 samples per pixel)
30254.40539 samples / second (33.05304 micro-seconds / sample)
MNCR 1000
Done 11010000.00000 samples (143.35938 samples per pixel)
30291.60746 samples / second (33.01244 micro-seconds / sample)
Doesn't seem to be any difference in rendering speed like many have believed, but setting it to zero and under 10 definitely slows it way down...?!
You can see with the lower settings, there much less firefly noise spots, but some of shading and reflections on the metal objects are missing.
250 is the lowest I'd go, while 500 seems to the sweet spot, with diminishing returns above that.
and just for kicks I did a render with 10000 setting, no difference in rendering speed, but way splotchy
usually most of those spots do go away with increased rendering time, but only up to a point.
Will 600 as my all round setting