Why is my GI not doing anything? help please.

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Old 04 April 2006   #1
Why is my GI not doing anything? help please.

Ok as of recently I've found that when I render through maya with mental ray...that my renders with certain settings are identical and shouldn't be. For instance, when I render with global Illumination, it's the same as without it. And I can turn my final gather on or off and get the same result. In this scene I have two spotlights both emmitting photons (although turning photons on and off also makes no difference), and in this scene I've turned on ray tracing in the render globals as well as in my lights attributes, and there's no change in my renders. Does anybody know why this is happening?

P.S. I'd include a picture of the render to assure that there should be a change, however I don't know how to attach one. Could anybody tell me how to do that as well? And lastly how to get rid of the displayed information of my name and whereabouts on all my posts.
my site : Gabesportfolio.com

Last edited by daydreameroperter : 04 April 2006 at 07:16 AM.
Old 04 April 2006   #2
use www.imageshack.us to store your files
CG newsdump@wordpress
remember: Its no rocket science

Old 04 April 2006   #3

did that work? if not here's a direct link to the image.. http://img60.imageshack.us/img60/65...hallenge7fr.jpg

The scene file that this render was made from is the scene for the jewelry rendering challenge here on CGtalk.
my site : Gabesportfolio.com
Old 04 April 2006   #4
You don't mention in much detail how the scene is like so I'm just gonna take a stab at possible common solutions....

1. Did you turn on GI and FG in the render globals ? Set your renderer to mental ray ?(I know its stupid, but some people forget.)
2. You may be emmiting too few photons, or that your lights are so far away that your photons have no energy by the time it gets to your object. Don't be shy, mental ray can handle 100's of thousands of photons and whatever energy is in units.
3. There already seems to be some ambient lighting going on, or maybe its just another light with shadows turned off.
4. Your textures are a flat gray, try changing it to a lighter color blinn.
5. Are there other objects in your scene to be bouncing light around ?

Thats all i can think off right off the top of my head. Good luck with it.
Old 04 April 2006   #5
Usually it's the photon intensity set too low. If 8 thousands is too dim to see, try 800 thousand or 80 million. Just keep adding 0's until something shows up.

(You see the exponent of 2? That's like a Quadratic decay rate on a light, and requires very high numbers in just the same way that a point light with Quadratic decay will require a very high intensity to show up.)

Final Gathering works with or without photons in the scene. It does require raytracing, but otherwise works well on its own. You can make the final gather scale brighter (try a value of 3 or 4 instead of 1) if you want to see it by itself with no other GI.

Jeremy Birn
Author, Digital Lighting & Rendering, 3rd Edition
Old 04 April 2006   #6
Well first of all thanks to everybody for your replies. And I've made the suggested changes which did change the render a small bit, but not very much. Here is an uploaded image of what the new render looks like.


There is a small bit more of light in the shadows of the cylinder.

I can confirm that it was rendered with Mental ray, with ray tracing set in the render globals, and all the lights, also that my FG and GI was set on, and that there is geometry to reflect some light. Here are some pictures of the scene with the cylinder made a yellow to stand out, two reflecting walls blue, and the surface that cylinder sits on is green. You can clearly make out the two spot lights, and the small blue curve with the camera at one end looks directly down the other.


Here are the changes made since my last render.

1. Both lights are emmitting 200,000 photons
2. the exponent was changed to 10
3. Global Ilumm changed to 20,000

Still somehow I feel as though my renders aren't quite where they should be with the render tecniques used. You can see on the jewelry render challenge on this site's forum that everyone's render of these two simple components in themself look far more life like. I've asked them why without reply so far. Thanks again to everyone here. By the way, Jeremy pleasently surprised to see you on here, I had to read your book for Shading and Lighting at my school.
my site : Gabesportfolio.com
Old 04 April 2006   #7
Could you tell me what kind of look you want with this ring?I am asking because you dont need to use those high values for GI and FG.Also try to always position your objects in the midle of the viewport grid, as the size of the scene can make the difference.

Bucket Watcher
Old 04 April 2006   #8
Hi Gabe -

Do not change the exponent to 10. Exponents in MR are like decay settings in Maya lights (0=no Decay,1=Linear,2=Quadratic,3=Cubic). So your light is already decaying very rapidly with distance at 2, probably leaving it at 2 and using a very high photon intensity will make up for it. Even though 2 is physically correct, sometimes people cheat a bit and use a lower value like 1.8 if they want really uniform fill light out of their GI, but 2 should work well. Be sure you have turned up the photon intensity bright enough: try it TOO bright, so your scene goes white, then turn it back down a bit to the intensity you actually want. If you're using GI because you want light to inter-reflect around an environment, make sure you have built a complete environment around the scene. Putting the whole scene and the lights inside a cube or sphere even could help with that, or any other environment that surrounds your models.

Using more photons slows down your render, so don't start using more and more photons until you're sure you have the basic look and brightness you want. Your tests with fewer photons will look blotchy, but blotchy little quick tests should be good enough to get the intensity and colors right.

Jeremy Birn
Author, Digital Lighting & Rendering, 3rd Edition
Old 04 April 2006   #9
Well with the suggested changes I can see that Mental Ray is at least responding. I put a sphere around the scene as suggested, put my
photon intensity to 400,000, took my exponent to 1.8. And with that I got the exact same render...but brighter. So at least the GI is
working. And to slipknot, the scene is actually already predesigned with the correct placement of the cameras, lights, and objects.
Now to change the size I could throw everything in a group and scale it down to retain the photon's energy. Cool, simple idea.

However in this challenge you can see a noticeable difference in their renders, and mine. Here is a link to the first page of the


And now here is a link to a new render of mine. Using FG and GI, with production quality.


first of all notice that the edges of all the shadows are soft in their render. Mind are hard, despite me using my
penumbra angle. Also the light bouncing up from the ground surface helping give an ambient look. I believe that whatever is
lacking in my rendering process that's not giving those two effects will help...And I would use area lights, but those don't emit photons.
And I don't think I can render out stuff like they are without FG and/or GI. Thanks again for the help.
my site : Gabesportfolio.com
Old 04 April 2006   #10
I see. Looks like a good excercise.

It sounds like you are learning about several things at once, so take stuff in order. Soft shadows are something you can learn about without using GI, and so are setting up shaders with soft reflections. Then, work on the caustics. Other than the caustics, GI seems like the lowest priority thing visually, and also renders the slowest, so that should be the last thing you work on, after you have the shadows, reflections, and caustics all working well.

Jeremy Birn
Author, Digital Lighting & Rendering, 3rd Edition
Old 04 April 2006   #11
I see. so soft shadows can be expressed through the shader rather than the light's attributes, and render tecniques. so where would i begin with learning about that?
my site : Gabesportfolio.com
Old 04 April 2006   #12
Originally Posted by daydreameroperter: I see. so soft shadows can be expressed through the shader rather than the light's attributes, and render tecniques. so where would i begin with learning about that?

Was that a mis-reading of something I said?? Anyway, nobody is going to type the whole manual for you into this thred, but you can fix your lighting to get soft shadows. Then work on setting up soft reflections. Then caustics...

Jeremy Birn
Author, Digital Lighting & Rendering, 3rd Edition

Last edited by jeremybirn : 04 April 2006 at 04:59 AM.
Old 04 April 2006   #13
I just downloaded the file (obj) provided for this challenge, and all the objects are not visible in reflection and refraction. do this, select all the objects, then go to Window---General Editors---Attribute Spread Sheet, look for the render tab, you will see "Visible in Reflection" and "Visible in Refraction",click on the first and then shift click on the second, turn all that to "on". I think now your GI,FG and caustics will work fine.

Bucket Watcher
Old 04 April 2006   #14
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