Specular Highlights of the Architectual (mia) Material

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Architectural Material Specular Highlight fixAuthor: si-community user moin
This shader compound modifies the mental ray archmat's specular highlights so they much closer approximate real world light reflections than does the factory behavior. The shader does not work with finalgathering. See the si-community thread for more details on this shader compound.

local backup: ArchMatSpec2015.xsirtcompound

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moin
Posts: 8
Joined: 21 Jul 2013, 21:30
Location: berlin

Specular Highlights of the Architectual (mia) Material

Post by moin » 10 Dec 2014, 15:10

something that gave me a lot of troubles in the past was the way the mia material handled specular reflections.
i always had the feeling that they where too "soft" in comparison with the raytraced reflections.
on top of that, they sometimes seemed to be too dark.

i just recently investigated the issue by comparing the specular reflections at different angles with the raytraced reflections and both the speculars and raytraced reflections of the mila material.

here is what i found out:

1. specular reflections of the mia shader appear to be too soft.
for example: if you want to mimic the results of a glossiness of 0.3 for raytraced reflections you would have to put the glossiness at around 0.75 for the specular reflections (wich is of course impossible within a mia shader since they share the same parameter)

2. the way that the intensity of the specular reflections is handled by the mia material seems to be bugged.
if the specular reflection is on a surface that your looking at from high angel between 90 and 60 degrees it looks fine compared to the raytraced reflections of the architectual shader and both the spec and raytraced reflections of the mila shader. But the lower the angel gets, the darker the specular reflections of the mia material become.

i measured that
at 90 -40 Degrees the "highligt vs reflection" value needs to 1
at 20 Degrees it should be 3
at 10 Degrees : 5
at 5 Degrees around 8

by default the spec of the mia material has a differen appearence than the one of the mila shader or the phong shader) so those values are just rough approximations.

3. the "dark spec at low angel" problem is actally worse if you use the default reflection and slightly better if you only use the "base reflection".


Workaround:
one could use a combination af two architectual shaders with different settings for the glossiness.
one arch mat only handles the raytraced reflections and is plugged into the "additional color" node of the other shader witch does all the rest including the specular reflections. the "highlight vs reflection" factor of this main shader would be set with an "incidence" node to get proper values for low angles. this main shader uses only the "base" reflecion.

i attached a compund that seems to do the trick. i have not yet tested it extensively so there still might be some issues. the compund uses the advanced architectual material so you might need that too in order for it to work.
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Last edited by moin on 10 Dec 2014, 19:33, edited 1 time in total.

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Daniel Brassard
Posts: 827
Joined: 18 Mar 2010, 23:38
Location: St. Thomas, Ontario

Re: Specular Highlights of the Architectual (mia) Material

Post by Daniel Brassard » 10 Dec 2014, 15:58

You are correct, this has been a known issue for quite a while.

Felix Geremus even provided a Mia_Material_Advanced shader to correct the problem.

You can download the shader from his website at:

http://felixgeremus.com/?p=108

Cheers!

Dan
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$endif

moin
Posts: 8
Joined: 21 Jul 2013, 21:30
Location: berlin

Re: Specular Highlights of the Architectual (mia) Material

Post by moin » 10 Dec 2014, 16:10

to my knowlege this is a totally unrelated problem to the specular problem that appears from the buggy anisotropy parameter.

i actually use the mentioned shader.

to illustrate the problem i attached an image that shows a scene setup and 3 renders with the architectural material, the mila shader, and my compound of 2 architectual shader

the specular reflection on the ground (right of the spere) and the raytraced reflection (left) should look the same.
with the mila material they do. with the architectual material they dont. at low angels (on the ground) it is not even close. my compound trys to fix this problem
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Maximus
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Joined: 09 Jun 2009, 15:45

Re: Specular Highlights of the Architectual (mia) Material

Post by Maximus » 19 Dec 2014, 00:15

I dont remember actually the old discussion and thread, but its a decision "mental images" took into designing the shader (actually not Mental images but more like Zap since hes the one).
There was a huge thread in CGSociety comparing Mia_Material vs Vray Material in terms of specular/glossy and it was very evident how they use different math behind it and Mia_material produces unrealistic and very hard to controll gloss/specular.

I have no clue why people still are using this dead engine, but hope you can find your more detailed answer in CGSociety forum. Try to look mia_material vs Vray material, probably something will pop up.

p.s.: i quickly found one of the bilions posts about this.

http://forum.nvidia-arc.com/showthread. ... discussion

you must be logged in to view images i think.

moin
Posts: 8
Joined: 21 Jul 2013, 21:30
Location: berlin

Re: Specular Highlights of the Architectual (mia) Material

Post by moin » 22 Dec 2014, 22:07

thanks maximus for the link i will defenetly read it.

by now i discovered that the compound i posted does not work with final gathering.
mia material has the feature that when it only renders specular reflections and not raytraced reflections, it fakes the raytraced reflections by using the light and color of the final gather map as a refection.
so by combining two mia material, one with raytraced reflections and one with specular reflections only, you end up getting too much reflection in total with final gather.

i guess i will do a simelar thing by combinig a mia material and a blinn or phong shader. but first i will do further resarch

Kzin
Posts: 432
Joined: 09 Jun 2009, 11:36

Re: Specular Highlights of the Architectual (mia) Material

Post by Kzin » 28 Dec 2014, 18:50

Maximus wrote:I dont remember actually the old discussion and thread, but its a decision "mental images" took into designing the shader (actually not Mental images but more like Zap since hes the one).
There was a huge thread in CGSociety comparing Mia_Material vs Vray Material in terms of specular/glossy and it was very evident how they use different math behind it and Mia_material produces unrealistic and very hard to controll gloss/specular.

I have no clue why people still are using this dead engine, but hope you can find your more detailed answer in CGSociety forum. Try to look mia_material vs Vray material, probably something will pop up.

p.s.: i quickly found one of the bilions posts about this.

http://forum.nvidia-arc.com/showthread. ... discussion

you must be logged in to view images i think.
the old discussion, yeah, can remember on that. i wrote some bullshit in this thread. :p

to make long things short, vray is using some kind of bsdf and mia not. that results is way better light reflections which mia cant without insanely high glossy ray samples. its a different kind of handling things. its not an excuse for mia, but by that time it was a user request while designing mia as it is. the problem in si is now that we have to stick with mia, we will not have access to mila or mdl in the near future. so try to workaround this, for example, use mia for what it is designed. or just use vray or arnold, the better choice for more modern rendering approaches (i would prefer arnold because of way better area light handling).

also take a look on my awnser here:
http://www.si-community.com/community/viewtopic.php?f=10&p=48588#p48588