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This is NOT about asking how to speed up cycles renders. I am just confused to why a scene has taken so long to render.

Here we go:

I have a camera (WOW) looking up into treetops. Pretty simple. These leaves have subsurface scattering with a black/white texture to apply some change in the colour/subsurface. The trees are a bunch of vertices with a skin modifier, textured. Some simple volumetric clouds. I have a weak volumetrics box to apply a light beam effect surrounding my camera reaching to just above the trees.

For the render settings, I used CPU (I know, kill me. But I started one on GPU and got different results and was just as slow!). Rendered at 4,096 samples at 2,800 x 1,800 (W x H).

I was rendering on a single iMac, which in the past, has done more complex scenes with higher settings within 6 hours easy.

Now for the render time:

99 hours and 52 minutes.

NEARLY 100 HOURS!!! Is it me or is that WAY too long!?!

I would upload the image but it is over 2mb :(

Any reasons into why it is taking so long would be grateful.

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I'm not too sure about the implementation of volumetrics in Cycles, but roughly speaking:

  1. without volumetrics, a ray direction must be evaluated only once every time it hits a surface. So the ray virtually travels freely (no computations) through the empty space, then it hits a surface and (if it hasn't bounced back and forth too many times, depending on your "Bounces" settings), it triggers a single new calculation to decide where the ray should move next. Repeat a few times (up to a dozen) until it finally hits a light or the sky.

  2. with volumetrics, every little interval of space the ray travels, the path tracer has to flip a coin to decide whether it will be scattered or not (until it finally gets scattered enough times to reach the "Volumetric bounces" cap). So along its trip there's a LOT of calculations involved.

If your scene is covered for the most part by volumetric regions, this will require a lot of extra calculation per each single ray (and you are casting 20 BILLION rays in your setup!).

Subsurface scattering is also very computationally intense.

So the reason why your render takes so long is that, even if your geometry and your shaders are "simple" (meaning: it took not too long for you to set them up), they are all over the place and are requiring a ton of calculations per ray (meaning: it takes a lot for the renderer to evaluate them)!

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    $\begingroup$ Absolutely. Very well explained. $\endgroup$ – Rich Sedman Nov 13 '18 at 11:01
  • $\begingroup$ Thanks a heap Nicola! I had a bit of a feeling it was the volumetrics/subsurf but I couldn't find a way to change the samples. That makes a lot of sense though. On mine, the bounces for volumetrics where set to 0, but under geometry, the max steps is at 1024 with a step size of 0.1. Would the max steps do the same thing? If so what would you recommend for them to be set to? Thanks for the quick reply BTW. Very helpful :D $\endgroup$ – Bigfoot Blondy Nov 13 '18 at 11:09
  • $\begingroup$ Tweaking with the max step and step size surely helps, but consider also removing unnecessary volumetrics (a region with very low density is as expensive as a high density region, but could possibly be trimmed off altogether) or even replacing them with transparent image plates if they are not interacting too much with your subject / center of focus*. The kinds of optimisations you can make really are dependent on what your scene looks like! - - *there was a nice example of this in this year Gleb Alexandrov's talk at the Blender Conference, but I think the video is currently unavailable :( $\endgroup$ – Nicola Sap Nov 13 '18 at 11:19
  • $\begingroup$ oh, well then. I think I might have to rethink the large volumetrics!!! The large one I used is subtle and is weak but I kind of need it. I find it frustrating though because I made another scene with an even bigger volumetric box with 40 odd coloured lights passing through it and it was done within 3 hours. Your answer is helpful but it doesn't quite answer my question as the settings you have pointed out are already quite low (besides the max step which seems to be making no difference). BTW, I think I watched that vid. Twas a while ago though. what about subsurface? radius? Scale? $\endgroup$ – Bigfoot Blondy Nov 13 '18 at 11:29
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    $\begingroup$ Thank you! Those settings along with a slight reduction to my subsurf* radius just about quartered the speed. Amazing how such little things make a big difference! All your reply's where very helpful btw. Thanks a heap. $\endgroup$ – Bigfoot Blondy Nov 13 '18 at 11:36

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