Light Leaking

[barryk50]

I know this question has probably been asked before but Im gonna ask again. Why is it that sometimes I use GI i get light leaking through cracks where it should not be? How do I fix this?

[DjVoyager]

It's difficult to understand what you mean by light leaking. Do you mean the cracks are lighter when they should be darker? Can you post the render?

[barryk50]

Ok here is my image. Most notably is the where the floor meets the wall on the left side. Light is coming where it shouldn't. What is wrong? Why does it do this when I use GI?

[DjVoyager]

I've been trying for a while to get my scene to bleed lights like that but could not. Granted it's not as complex as your scene but there should be some indication from any mesh that the lights are bleeding. Btw, I'm also using default GI settings in MR.
To me, your light photons seem to be calculating the wall separate from the floor. You might need to play around with the global illumination's accuracy too beside the photons attribute.
I would suggest to check out this tutorial;
http://www.digitaltutors.com/chit_chat/showthread.php?t=238&highlight=caustics
Hope that helps.

[DjVoyager]

P.S: The scale of the scene also matters when using global illumination.
I will keep on checking why yours is turning up that way, but let me know if you are able to fix it.

[Jak_Carver]

The reason the photons are leaking for you is because the GI algorithm is trying to spread too little photons over a large area, thus the photons are shot inaccurately; this inaccuracy ends up rendering as "light leaks".

Your gonna need to do the following to fix this:

1) Shoot more photons from your photon emitter

2) Decrease the size of your photons and make them very small, this depends on your scene size but a size of 1 usually works well, unless your scene is very small, then you need to decrease your photon radius even smaller

3) Increase your photon accurracy

4) Use fine FG settings to interpolate your GI solution, thus speeding up render time while simultaneously adding more physical accuracy and quality

Remember that GI and FG were designed by mental images to work together on purpose to render hundreds of times faster than using brute force GI.

[DjVoyager]

Okay, I've played around with a simple scene(below) and it's definitely not as difficult, it just require a bit of playing with the attributes, depending on how you want your scene to look.
As I mentioned above, the size matters, and the smaller the objects the difficult it is for MR to compute the photons. Mine is more than 10 times larger than the grid.
As far as what values to go with, I'm sure that you know it's quite a difference from what we are given by default. There's one spot light in my scene with 100000000 photon Intensity, and 150000 GI Photons.
In Render Globals, I've only got GI turned on and nothing else. GI accuracy is 600, and GI radius is 45.
Now I didn't know what values would work well with my scene, so I kept increasing them for these attributes and watched the results by saving the renders in Render View, it gave me a fairly good idea if a larger value works or a smaller.;
Spotlight: Photon Intensity, GI Photons
GI Render Globals: GI Accuracy, GI Radius.
At one point, I also got those light bleeds or leaks.
Also keep in mind that texturing also helps overcome these smaller issue (your scene looks pretty descent), so you don't have to fix every nook & cranny :).
Keep us posted of your progress.

[Jak_Carver]

This may be off topic barryk50, but are you trying to copy this guy?

http://www.cgarena.com/gallery/3d/description/fullimages/stairs.jpg

Else, would be quite a coincidence eh?

This image was done by Yogesh Sherman of India.

[barryk50]

Im not copying. This is a lighting challenge at CGTalk. Ill check my settings and play with them to see what I can get. Right now I have a million photons with a intensity of 20000. My accuracy for GI is 1024 but the photon accuracy is default. I also used FG with this. I measured my scene and got a min of .3 and a max of 3. This is where I am now. I will try what you guys say and see if I can get some different results.

[yogeshsherman]

Hi everbody ,
thanks Jak Carvar for remembering my work ,I know barryk50 about the challange on cgtalk though I have made this image long before.
Barryk50 though I have not used any GI solution ,only used Maya's software render for the image.
But I think DjVoyager is right about the solution to your problem mesure your scene and place No of lights according to your needs ,don't rely on applying only one light and if posiible make a sphere covering your whole scene(don't forget to uncheck renderable option) as it helps photons to bounce instead of losing it in space.
also check whether your scene is too big or too small most of the time gi solution doesn't provide the rquired result according to the needs because of scene size.
Most commonly light leaking happens b'coz of lack of photons but it is not necessary that u raise no of photon out of proportion.also play with other options too like the exponent etc.
hope to see your finished work soon in the forum.

[Leotril]

barry im having the same problem but using renderman for maya RfM
i get those leaks using the same scene .. can anydbody help with RfM is kinda different not photon settings taht i recall .. i tweak some settings but i was able to fix the leaks but the Gi dissapear i think it was shadow bias.k

[DjVoyager]

Post your question under Renderman (http://www.digitaltutors.com/chit_chat/forumdisplay.php?f=100), Leotril.