Hair shader rendering extremly long

[frodopytlicek]

Hi,
I have big issue with render time with hair shader in closeups. Its sooo long and I have no idea how to optimize it. In the shader is nothing special and boucens are on default. Only problem is that there are 78 lights Any suggestions for optimization?

[^Lele^]

Even with compensation, white/bright hair will be slow to converge.
If it's a closeup, it's more screen taken, and it'll be longer still.
The suggestion is to not have the 78 lights all influence the hair, but rather reduce the lights' influences to the minimum necessary amount.

Without a scene to try out, though, it's hard to tell if there could be other ways to speed it up.

[frodopytlicek]

Problem is the look is approved and I cannot change it. In draft renders it was fine but now on final rendertime could be up to 8h per frame

[^Lele^]

If you're look-locked, you're look-locked.

If the hair is big enough on screen (say, a bit more than a couple or three pixels per strand) maybe denoising a higher noise-level render could be doable.
You could surely shave some time by rendering the image with fixed sampling (i.e. for the bucket renderer, untick `max subdivs` and only use the min subdivs control.) at a level you're comfortable with (say, 10 subdivs.) and that produces a good enough (if you can, multi-frame) denoised result.

Needless to say, if the detail is severely sub-pixel then you're out of luck with denoising.
But you could still shave some important time by sampling fixed to a noise level you can use directly.
Using fixed sampling will also allow you to budget the job better on the farm, as you won't risk super-long convergence areas, at the cost of uneven noise levels across the image.

[frodopytlicek]

Using fixed sampling helped more than I think thans. About the denoiser, yeah its good but... there is a mig bug in denoiser when rendering into layers (characterds separaded from background, etc...) unfortunately I dont have ticket number, but it should be somewhere in your system. Only thing I have is request number #72466.

[Joelaff]

You could at least try lowering the adaptive lights count in the render settings. Sometimes as low as 3 or 4 makes very little difference to the rendered result, but can sometimes be faster. Worth a try.

I believe upping the Light Cache samples will help improve the selection of which lights are used in Adaptive Lights. So a combination of some more samples there, but lower Adaptive Lights might help.

[frodopytlicek]

Thanks, I finaly get it to work by turning off some lights which were not so important for hair look and render the hair separately.