IRAY: Blonde hair causes render times to quintuple?

Diaspora

I have a scene with one character, she has black hair and it's about 35 seconds a frame, in Iray photoreal.

I change nothing else except now she has the same exact hair but with a blonde material, and suddenly the render time is 5 times the amount.

Any advice on how to get this under control or is this just the inevitable consequence of lighter colored hair in a physically based renderer?

mclaugh

I find that changing hair material rarely changes just the color: it typically changes/adds a lot of additional reources (translucency maps, various gloss maps, normal maps, top coat maps, bump maps, etc.) that may not have been present with, or may be much larger than, the original color. Depending on what and how much was added, I find that it can markedly increase render times.

You could try reverting to the original hair color and manually changing just the base color map and see if that reduces the render time, If it does, reducing the size of the various hair texture resources may be the way forward.

Diaspora

thank you for the lead, I'll report back if that ends up putting a pin in this. 

Diaspora

Damn, that really DID do it, all I did was change the texture feeding the base color, nothing else, and it has identical render times whether it's black or blonde, I suppose you're right that when I change the color via the material preset, it must be changing some other things to up the ante. Thank you

Richard Haseltine

If the renders were in the same session it may be that the GPU memory was exhausted. However, to an extent I would expect colour to have an effect as a lighter colour will reflect more of the incoming energy of a ray and so the ray will (potentially) bounce around for longer, requiring more calculations. The same is true of glossy vs mat.