Specular causing slowdown

[re:FORM]

I have a displaced mesh that I want to have strong highlight reflections (reflection glossiness set to 0.9). To do this I have an omni light with multiplier of 1, and diffuse unticked so it only affects the specular of the surface.
When this light is switched on, I get the nice specular highlights, but it severely slows down the render speed. Is this because vray has to use alot more samples to correctly anti-alias the reflections? Is there a more sensible way to achieve this effect? (I want it to look like the super-bright and sharp reflections you get on water from the sun)

I've tried using a self illuminated plane, but I couldn't get a sharp enough reflection from it.

Any ideas?

[re:FORM]

Ah, ticking 'clamp output' and 'sub-pixel mapping' in the colour mapping tab speeds up the render, but you loose the sharpness of reflection.

Would increasing the brightness of the omni providing the specular highlights bring back the nice sharp highlights, but also slow the render time down again (thus negating the improvement after ticking the boxes as above)?

[Morbid Angel]

i think the slowdown will happen no matter what. This is because every desplaced edge is now picking up a highlite, and that highlite needs lots of aa samples to get proper look.

[re:FORM]

Thanks morbid.

Actually, this isn't a displacement issue, the same slowdown occurs if I only use a bump map.

Interesting...

It appears to be directly related to using glossy reflections to produce a highlight. Is there any other way to get glossy reflections without this slowdown?

Here's the material I'm trying to create.... a wet-shiny fat material.


The render closeup shows the option with both 'sub-pixel mapping' and 'clamp output' off, but with the light causing the speculars at 0.1 mult.

The background render shows how it looks with those colour mapping options switched on.... you can see how I loose the nice speculars.

I hope I'm just going about creating this shader the wrong way ?

Any suggestions?

[Morbid Angel]

you may want to try using vray blend mat and comp a standart mat with black color and only specular...that might be faster.

[RErender]

If you just want the speculars and don't need reflections you can just turn off "trace reflections" in the vraymat, then it will behave a lot like the speculars on a max standard mat.

[Morbid Angel]

yes, i thought he already done that no?

[RErender]

mmm, maybe

[re:FORM]

Well its funny you should say that

I've been trying that approach today, but I just couldn't get the result I was looking for. The main problem was that the more I increased the coat material, the duller the base material became. I assume this was becuase the reflection (coat) material had the fresnel box ticked. Would it work if I left the coat material as a pure mirror (black diffuse) material, and then controlled the amount with a falloff map ? I thought we were only supposed to use vray-API maps in the vrayBlend material?

I'm going to try unlocking the highlight glossiness from the reflection glossiness, and then set refl. gloss. to 1 and highlight.glos to 0.9.... given that I don't actually need raytraced reflections of anything, I've also got 'trace reflections' off.

Thanks for you suggestions!

[Morbid Angel]

well often i dont use vray fresnel, instead i use falloff map. More control.

[re:FORM]

If you just want the speculars and don't need reflections you can just turn off "trace reflections" in the vraymat, then it will behave a lot like the speculars on a max standard mat.

Yep this is what I have, but the speculars still cause a big slowdown.

The region render takes 82s with no sub-pixel colour mapping, but only 27s with sub-pixel colourmapping on. This is a huge difference in render speed.

Looking at the vray log, the major difference with the sub-pixel mapping applied is that the "number of raycasts" goes from 2,362,310 to 713,588

What would be the best way to optimise this? Either I live with the dull reflections and get a nice fast render, or I find a way to reduce the sampling applied to the speculars.

[re:FORM]

well often i dont use vray fresnel, instead i use falloff map. More control.

True, but this wouldn't solve the problem that when the reflection isnt at 100%(white) the base material gets dulled by the colour of the coat material.

Is there a way to use the belnd map so that only the reflection is placed over the base material?

[Morbid Angel]

yes, the color of the second layer (one with reflection) has to be black, so it will lay over only reflection.

[re:FORM]

Looking at ways to adjust the sampling on the speculars, I noticed that the number of reflection glossiness samples makes no difference to the render speed. I assume this is because I am using Qmc sampling with 1/100 samples?

Rendertime seems to be directly proportional to the brightness of the light providing the speculars.

[re:FORM]

yes, the color of the second layer (one with reflection) has to be black, so it will lay over only reflection.

But, and I am probably wrong here , if the reflection material has fresnel or falloff, you end up with black diffuse affecting the base material. You can only do this with a solid white in the reflection slot.