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Old 07 July 2008   #16
Originally Posted by thev: There is currently no single analytic BRDF model that can accurately represent all real-world materials; the Cook-Torrance model comes fairly close, but it is not very convenient to work with from a sampling point of view. It would be most accurate to use measured BRDF data, but even then, a certain number of approximations and assumptions are typically involved.


Absolutely. All the work I've seen testing the validity of different BRDFs has been fitting to measured data (A&S came out top in the most recent paper I seem to remember) using multiple lobes. This isn't something a user's going to want to tweak.

What I mean is the BRDF models in maxwell and fry better represent conceptually what's really going on at the surface: where in nature is there a specular lobe plus a lambertian component? I find their model much more attractive for the purpose of artistically tuning shaders, although having said that, I've yet to see a realistic skin material from either of those renderers

EDIT: Just seen quite a nice one in the fry thread in general discussion. I stand corrected!
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Last edited by playmesumch00ns : 07 July 2008 at 12:27 PM.
 
Old 07 July 2008   #17
Man, the information you guys have provided here has to be some of the best and most detailed information that I've ever seen given on the whole cgtalk forum, ever.

I'd buy you guys a handful of hookers if it was appropriate.
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Old 07 July 2008   #18
Great, I'm gonna organise it into PDF then! Granted a proper credit for Playmesumch00ns.
 
Old 07 July 2008   #19
That's great!

I was thinking about that, I put it into a word file for personal reference but it would be too big a deal to download all the images and place them accordingly so I just got the text.

A PDF would be ideal though.
I can't thank you guys enough.
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Old 07 July 2008   #20
Dtox, this is kind of my interpretation of cg science, but if you want something interesting and understand it at a great depth, I suggest you this book, which I bought recently and it is simply amazing, it has everything you may need about surfaces, light and so on http://www.amazon.com/Langfords-Bas...16178159&sr=8-1 Excellent color illustrations, the 8-th(!) edition first in color I guess, it is a real gem for your cg book collection. And for this price, man.
 
Old 07 July 2008   #21
Thanks for that book reference!

Photography is also a subject that interests me.

I'm not really a practicing photographer but I do take some pictures sometimes for reference or texture purposes.
Light itself is something that fascinates me for some reason.
And especially where the possibility of merging real world lighting into CG lighting.

When I'm working with CG cameras I wonder what else could theoretically be possible with cg generated images.
Like would it be possible to add additional channels to an image that comes out of a render engine than just an alpha or depth channel.
Maybe something like an IR channel that you could generate from inside the cg cameras attributes.
Like having a camera that captures the image of the visible spectrum, and also the IR spectrum that you can edit to influence the visible spectrum of the final image.
That you can render to a seperate pass too.
It could not only allow for light control, it could also be used to more accurately simulate colors for explosion FX from simple bon fires to jet engines and even the formation of stars and space FX.
They use a similar technique to color the images that come from the hubble telescope.
What I'm talking about is a loose reversal of that in a way.
Except you generate a grayscale image of the heat information to simulate explosions and heat FX, it can also be represented with a multi-colored image to control the temperature of the illumination.
And maybe even a 3rd ultra-violet channel that could be used to control gamma dynamically.
Making the simulation of sun light much easier and more realistic.

I don't know what's actually possible with advanced render engines like Maxwell, Fry, I don't even have V-Ray yet.
I use Cinema4D 10 with the built in Advanced Render module.
And it's slow when doing GI, SSS and caustics.
So there's no chance of lighting a scene with accuracy of lighting in mind.
I do what I can.
But it's real easy to blow the image in the long run.
__________________
"Too much of anything makes you an addict!"
 
Old 07 July 2008   #22
Originally Posted by Dtox: Man, the information you guys have provided here has to be some of the best and most detailed information that I've ever seen given on the whole cgtalk forum, ever.

I'd buy you guys a handful of hookers if it was appropriate.


+1.
Awesome thread.
 
Old 07 July 2008   #23
I didn't wanna start a new thread about this, so i add this question here instead.. if it's ok.

When you guys create your shaders, which approach do you use ? do you take the physically accurate route ? meaning that you try to find measured data that matches the material you're recreating and use them in the creation of your shader.

Or do you search for lots of referencematerials (images and text) about the shader you're reacreating and then simply eye out the fine details from those reference pics to recreate the look and feel of a material on your own ? let's call it the shoot-from-the-hip approach.

Which is your favorite method to use ? or do you use a completly different approach to this ?

/ Magnus
 
Old 07 July 2008   #24
when creating shaders, i try to eyeball characteristics of material that are visible in realworld and at the same time trying to make it visualy pleasing even if its not physically correct sometimes. noone cares about IOR, BDRF and stuff, its only a help for you to make things look good.
 
Old 07 July 2008   #25
Agreed

I agree. I don't really pay attention to a physical correct approach. It just gives me a rough outline. Afterwards I tweak the settings of the shader, to get the look I want and that's not always a photorealistic look. The material has to work for the scene you want to use it for.
 
Old 07 July 2008   #26
That's interesting to hear that you guys pretty much shoot from the hip when creating materials instead of relying on a more physically correct approach. Mind if i ask which renderengines you use ?

I'm using both Maxwell and Fry for my work and both engines use the more physically accurate approach to shader creation, to be honest it feels better when working as you know what you get when you modify values and apply textures to them. I don't see the same simple logic when working with Vray or Finalrender or similar engines.

/ Magnus
 
Old 07 July 2008   #27
Render engines

In my case I started to love MODO and it's render-engine. It's easy to set-up and gives you pretty fast results. Due to the fact that I have also a MAYA license running here at work. Some of my works I'm doing in MAYA rendering with MENTAL RAY as engine.
 
Old 07 July 2008   #28
i mostly work on feature movies and using XSI / mental ray. to be more precise with my opinion, its nice to have physically correct rendering and im allways trying to keep as close to it as its possible. but reality is when director tells you that over there is too dark and that thing over there should be more reflective or something, you never tell him that you will not do that because this way its physically correct and many times i have experienced that he told us guys, i know that in reality it wouldnt be like that, but i need it to catch attention or i need it for better composition. an he is right. because what we do is art (at least from some point of view) and we should go for believable AND visualy attractive results rather than only for physical accuracy. reality isnt really attractive in many cases and without little of art it would look... you know, boring.
 
Old 08 August 2008   #29
Originally Posted by Magnus3D: That's interesting to hear that you guys pretty much shoot from the hip when creating materials instead of relying on a more physically correct approach. Mind if i ask which renderengines you use ?

I'm using both Maxwell and Fry for my work and both engines use the more physically accurate approach to shader creation, to be honest it feels better when working as you know what you get when you modify values and apply textures to them. I don't see the same simple logic when working with Vray or Finalrender or similar engines.

/ Magnus


Using PRMan for visual effects production, I try to keep physical correctness for as long as possible and only branch off into 'artistic licence' when I absolutely have to.

The reason for this is that, in my experience, doing things the right way (or as close to it as my limited understanding of physics and maths allows) means that the results are predictable.

Layering artistic hack after artistic hack into shaders quickly results in setups tha are unmanageable and hard to make changes to. Moreover, if you don't light those materials in exactly the right way (as the original shader designer intended) the results can often be bizarre or just plain broken. This is especially important when you need to share shading and lighting setups between multiple artists working on different shots and sequences.
__________________

You can have your characters photoreal, fast or cheap. Pick two.
 
Old 08 August 2008   #30
Originally Posted by Dtox:
When I'm working with CG cameras I wonder what else could theoretically be possible with cg generated images.
Like would it be possible to add additional channels to an image that comes out of a render engine than just an alpha or depth channel.
Maybe something like an IR channel that you could generate from inside the cg cameras attributes.
Like having a camera that captures the image of the visible spectrum, and also the IR spectrum that you can edit to influence the visible spectrum of the final image.
That you can render to a seperate pass too.
It could not only allow for light control, it could also be used to more accurately simulate colors for explosion FX from simple bon fires to jet engines and even the formation of stars and space FX.
They use a similar technique to color the images that come from the hubble telescope.
What I'm talking about is a loose reversal of that in a way.
Except you generate a grayscale image of the heat information to simulate explosions and heat FX, it can also be represented with a multi-colored image to control the temperature of the illumination.
And maybe even a 3rd ultra-violet channel that could be used to control gamma dynamically.
Making the simulation of sun light much easier and more realistic.



Firstly I just wanted to say thankyou to all those who have contributed to this thread; it's been highly educational and has certainly made me re think some of my workflow.

Secondly to Dtox, I think what you are asking for is called Spectral Rendering, that is rendering using a much larger energy range than visible light squeezed into the confines of RGB channels. Mental Ray supports it, but I have never had any actual experience of using it, I heard about it in a conversation with an employee of Mental Images who explained that he was helping a client create spectral shaders so that they could render images in 18 colour channels, describing the appearance of a surface as it would appear in different 'lights' for instance how ultra violet light would effect it, or what it would look like in infra red. Hope that is some use to your future research.
 
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