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phreakmonkey
04-23-2006, 02:27 AM
heyah,

i was wondering if anyone has any ideas or suggestions on how to get light to bleed around your object (see attached pictures - those aren't mine). if you look closely..there's a little white that wraps around the object. because of the backlight or white bg? any thoughts about how to achieve this in maya and mental ray?

thanks in advance. anything would be awesome.

pokoy
04-23-2006, 08:22 AM
i think it's the material with fresnel fallof reflection that reflects the white bg or ground plane colour. set up a scene with a rounded object, assign a material with fresnel reflections and set your background to white, you'll quite quickly have the effect you're looking for.
fresnel reflection means that the reflection depends on the viewing angle, hence the rounded edges seem to be glowing in a way.

gdimmrt
04-23-2006, 09:02 AM
I think what you're looking for can be done in post in photoshop using diffuse glow filter. What it simulates is light blowing out part of the foreground object.

Rens
04-23-2006, 09:18 AM
Yea, in the first image it seems like reflections causing it. In the second both reflections and bloom.
Which software are you using?
In 3ds max it's easy to set up fresnel reflections, in other programs it gets a bit trickier to do realistic reflections but still very doable. The glow is most easiest done in post, in programs like Photoshop, After Effects, etc.
Tell me if you need to know more.

Edit: Ah, you're using Maya, I missed that part. I think a mr dielectric node will do the trick for Fresnel reflections. If you're using standard shaders you'll want a ramp connected to the reflectivity. And a sampler info node connected to the V coord of the ramp using the facing ratio. You can search for the exact settings here. I could show you, but I don't have Maya here at home.
Anyway, the key is to use the correct ramp settings. See this (http://forums.cgsociety.org/showthread.php?t=344644) thread for more info.

Brian-Bradley
04-23-2006, 10:09 AM
The simplest way is to simply cheat it by adding fill lights
(poss area ones) that will provide the illusion of light wrap.
The obvious advantage is that you get full control over
what these lights are doing, whereas trying to get lights
and materials to produce a physically accurate render can
be very time consuming.

Bri

phreakmonkey
04-23-2006, 05:18 PM
thanks for the replies, very helpful. i don't know why i didn't think about doing it in post, but adding a glow works fine.

i'm also going to give the fresnel shader a try. i've never tried it before, but it sounds do-able. i'll give it a go and let you know how it turned out.

thanks

phreakmonkey
04-24-2006, 09:12 PM
thanks all for the help. i've tried it both ways. and i think the post production technique works the best. least amount of time for the best results.

the sampler face ratio technique works too. it does require a lot of tweaking. however, when i got it to work, it pretty much doubled my renderng time. is that normal when using the sampler node?

thanks-phi-

Rens
04-24-2006, 09:30 PM
I doubt it. Though adding raytraced reflections will noticably increase render times.
I'd suggest setting it up though. It's much more realistic and when you set it up once it will be much quicker to set up from then on.

Rens
04-24-2006, 10:20 PM
Argh, I closed the window after typing a whole post. Anyway, here goes again:

Just to clear things up a bit. You use bloom (the glow around bright spots) whenever the light is really intense. The bloom is mostly caused by internal reflections in the camera lens. Although the internal reflections always happen with any light source it will only be visible with very bright sources because only then the internal reflections will be bright enough to show up.
This is most easiest done in post. For more realism work with high range images such as HDRi and OpenEXR because only then you can be sure that only the brightest parts of the image will receive a bloom/glow.

You use the fresnel effect on smooth, non-metallic surfaces.
First lets look at the difference between specular and diffuse reflections. Specular reflection is light reflected off the smooth surface of an object with the light 'rays' staying all parallel to eachother, they don't scatter out. This way the reflected image will stay recognisable, like a mirror.
Diffuse reflection is light reflected off a surface with the outgoing rays scattering all over the place. This way the object will appear to have a uniform colour and you won't see any recognisable reflections from the surroundings. Matte paint or sanded woods are an example of this.
What happens with fresnel reflections is that the specular reflectivity of an object becomes higher when the incidence angle reaches 90 degrees. Take an image of a sphere for example. The center of the sphere will have a very low specular reflection compared to the amount of light hitting the surface, while when you go outwards to the sides it will reach about 100%.
The amount of specular reflection in the center of the sphere will depend on the object's Index of Refraction, aka Refractive Index. An object with an IOR of 1.5 (~glass) will have a specular reflection of about 4% in the 'center' (0 degree incidence).
Metallic surfaces differ from non-metallic surfaces in that their spec. reflection goes from ~95% to 100% (non-metallic with IOR 1.5 goes from ~4% to 100%).
These raytraced specular reflections have to be calculated inside the 3D program and it will increase rendertimes. But it will also drastically increase realism.

So use bloom where you have over-bright lights. Use the fresnel effect where you have smooth non-metallic surfaces. They are not interchangable and can occur at the same time.

Hope this is clear. I typed it pretty fast. :)

phreakmonkey
04-26-2006, 03:09 AM
rens,

thanks man for the info. that is very helpful..and now i understand what's going on and when to use what. thanks for the help and the info, very much appreciated.

the link you sent was helpful as well. i've been looking for some good resources about mental ray and mental ray mats and mental ray lighting. i need to learn more of the why :)

sweetness. thanks!

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