ggx question

[jackieteh]

Hi, someone told me that BRDF GGX suitable for metal only, is that true?

[Dave_Wortley]

Nope, good for plastics, rubbers especially and it's my default for all materials these days.

[Macker]

Yeah I use it for almost everything too.

[Dave_Wortley]

Worth saying that I adjust the GTR Tail Falloff on every material, you have more control but you now have two parameters to adjust for glossiness.

[jenya.andersson]

Even for a skin spec, tho this is in renderman RIS its the same GGX BDRF model

https://youtu.be/DKlbaU_uWpI?t=1m1s

[Richard7666]

Worth saying that I adjust the GTR Tail Falloff on every material, you have more control but you now have two parameters to adjust for glossiness.

This is a bit of a problem for me now actually. I don't know whether it is best practice to set the Tail Falloff lower and have a brighter texture/lower number in the Reflection Glossiness slot, or have more blur in the Reflection Glossiness slot but a lower tail falloff.

I've spent literally hours toying with this.

[Macker]

"whichever looks right"

Generally speaking, leaving the tail at it's default value seems to work the majority of the time for me - though not always.

[Oleg_Budeanu]

I wonder if there is a "table" of physical correct values of tail faloff for corresponding materials ?

[super gnu]

ive been wondering the same thing.

[dubcat]

Is there a script out there that can convert all my old materials over to GGX ? I've been changing all my materials by hand since it was released, very time consuming.

[leehumphries]

Is there a script out there that can convert all my old materials over to GGX ? I've been changing all my materials by hand since it was released, very time consuming.

VMC Script

[Macker]

Pretty sure the VMC script does it.

[joconnell]

I wonder if there is a "table" of physical correct values of tail faloff for corresponding materials ?

Same as all the tables for scanned metals, it'd only be as accurate as the sample object that people measured. Glossiness is driven by how rough or how polished a surface is so you can get a tonne of variation depending on how the object was used. Say for example I look at my keyboard in front of me, some of the keys still have a little bit of bump to them but the spacebar is totally smoothed off from all the use it gets - both are still made from the exact same type of plastic but they'd have different tail values. It's where the observation part of it has to come in and where you'll have to use some reference. Say for example you wanted material settings for an old, worn gold object you've got a picture of and ask the forum - what's the correct values for gold, then I give you gold values that I've made from a really new, mirror finish gold object and say that they're correct. My gold settings are only correct for the object I've used as reference and probably won't get you the look your after and this is what happens with lots of tables.

With shiny objects a huge factor is the lights and environments around them too. Highlights and tail falloffs are going to be heavily influenced by how large the light sources that are shining on them are, so ideally if you're trying to get a certain look, it's really handy to know what was around the object, and if not try to make educated guesses about the lighting using things like reflections in the surface of really mirrored objects or the softness of shadows that objects are casting.

[Rens]

Alternatively you could run these code lines, sets all VRay mats to GGX:

for oMat in (getclassinstances vrayMtl) do
(
oMat.brdf_type = 4
)

And you can set the tail falloff with this for all (!) VRay materials:

-- set tail falloff here
fTailFalloff = 2.0

for oMat in (getclassinstances vrayMtl) do
(
oMat.gtr_gamma = fTailFalloff
)

[joconnell]

Congrats on season 5 Rens btw, only a month's rest and right back into it for ya