View Full Version : Whats wrong with my GI+FG?

02 February 2011, 11:43 PM
My first real attempt to create a "real" interior. Pretty happy with the result so far, except for the strange (marked in the image) hot spots. I want to go on and add more detail and so on but first I want to get rid of those annoying hotspots..
Any suggestions?
btw Im using Maya + Mental Ray.
Any comments are wellcome

02 February 2011, 02:18 AM
What version of Maya?

Generally, you can get a "light leak" from thin geometry where it pulls irradiance from the back side. If your GI solution pulls a photon from the wrong side, it can cause that.

Easiest first step: Stop using photons. Necessary for caustics, but FG can generate additional diffuse bounces on it's own. (In more recent versions of Maya there is an input field to increase the diffuse count)

Try that and see what it looks like.

02 February 2011, 09:11 PM
thank you bitter!

Im using maya 2009. Im aware of light leaks but this semes to be something else. In fact this only show up when my "furniture layer" is included in the render. Can it be something that reflects the photons in a way that this happens?

Ive done a few test with only FG active. Very hard to control the brightness and color is bleeding over alot. For the example I had 4 secondary bounces and the diffuse scale set to 4.
Any good ways to control light distrubusion and color with FG?

02 February 2011, 05:14 AM
Now you may have some splotchiness from FG.

Start by putting the "filter" value under final gathering to 1. It will help eliminate some hotspots.

You may have excessive color bleed from
a. incorrect shader models (needs to be physically accurate) Use mia_material
b. small scene scale

Try scaling your scene to real world dimensions. Despite the relative scale being correct, making the scene larger and using correct decay in lights does cause the lighting to falloff more correctly and smoothly.

You may be able to smooth final gather variance by lowering the density parameter and increasing the interpolation. If the lighting then becomes too smooth, some occlusion can help bring back details and corners, etc.

02 February 2011, 10:07 PM
Bitter is wise.

Additionally, what kind of lights are you using? You should be using portal lights for those windows - this will often negate blotchies all by itself.

02 February 2011, 11:15 PM
with the GI+FG set up I had four (4) area lights as photon emiters. First behind camera where there is a window. second outside pointing at the glass doors en about 45 degree angle (x-y). third inside by the table by about where the lamp is hanging. fourth in the kitchen door wich you cant see in teh picture. All this to get an even distrubution of photons. No portals.

With the FG only (wich I have only just started to try out) I had a 2 portals one by the window behind the camera and a second outside (outside field of view). The second I placed outside fow because if I had it where I first put it (inside by the glass door, pointing towards the kitchen) it turnd up black and looked realy strange. ???
The blotches is no problem, they are in my example only because I had some low settings.
The problem is the color and distrubution off light. Ill look in to that and starts fiddeling with the fall off. Hopefully Ill be able to post a nice render on sunday.

thank you for the help!

02 February 2011, 11:19 PM
With portal lights, set to quadratic falloff, visible, and with the sun/sky as your environment, you should do just fine. GI is cool but mostly just for caustics - I rarely use it on interiors unless the lighting needs a little "kick" or there are multiple man-made lights involved.

Your portal lights shouldn't be outside your FOV. They should line up with and fill the portals precisely. They should also not be directed anywhere but perpendicular to your windows/doorways. The sun position will handle the angled lighting - consider Portal Lights as FG concentrators, really.

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