Understanding Denoising Feature with CoronaHairMtl

[julienbe]

Hi
I have used Corona on many Advertising projects and this is a very very nice (and robust ) render engine.
Since the new 1.7 version I try to understand how the new Corona Hair shader works with noise.

With CoronaHairMtl, the denoising feature doesn't seem to change the aspect of areas where hair and fur is present, while the filtering is much more strong with standard CoronaMtl.
Here you can see some of my tests - 8 passes / 50 sec





How can we acheive this soft look using denoising on hair and fur ?

[maru]

It seems that if you zoom in on the hair close enough, they get "blurred" by denoising as you would expect (when you can clearly see the individual hairs). But they get noisier once you move the camera farther away.
My guess is that this is by design to make sure the hair does not get too splotchy, but I think this could be improved, or at least there could be some control over how much denoising should be applied to hair. 

[maru]

I want to know where to find this information and to publish it.

Huh? Wat information do you mean?

[julienbe]

Indeed, some control over the "fuzziness" or softness of the fur would be great - by a more global denoising filter maybe ?
When you work on some "pixar-like" animated characters, sometimes you need some extra soft effect rather than pure realistic hair rendering where you can see every strand tip.

[julienbe]

If I want something that start to look like soft animal fur, i must go for 40 passes. Same results for you guys ?

[Ondra]

Long story short:denoiser is not magic - it is just blurring filter that knows where to blur and where to leave the image alone. Blurring hair is practically impossible without destroying the fine details, so the denoiser applies just minimal blurring there.

[romullus]

If I want something that start to look like soft animal fur, i must go for 40 passes. Same results for you guys ?

IMO 40 passes is not enough for hair to get good AA. I would look for 100+ passes for soft hairs. Of course you can lower GIvsAA, so you don't have to spend too many samples on GI.

[julienbe]

Thank you for your answer Ondra !
I understand that the Denoising feature can't solve the noise on fine details with hair and fur.

Adding a CoronaHairMtl shader to Corona Render was an excellent improvement in 1.7 and a "must have" for VFX artists.
Maybe we can imagine that, in a near future, this shader will be optimized and react more efficiently to noise.

Furthermore, the Denoising feature could blur a little more that specific shader since we can already reduce the amount of filtering.

JULIEN