PDA

View Full Version : Volumetric and Specular Headache

tcastudios
03-20-2006, 10:16 PM
I have touched the volumetric/visible light problem I have a year or two ago and haven't really solved it other than using a complicated route via multirender and inverted layers in AE. This is doable for simple scenes but are now being to problematic for me in more complex setups with loads of light sources.
The "new" problem I have got is a issue with specularity and transparency that do give me headache atm.
http://homepage.mac.com/tcastudios/tempfiles/slp/VolSpots.jpg
This first pict shows the problem I have with volumetric/visible lights. On the left it is only the fill light and on the right two spots are turned on from -behind-. What happens is that the shadowed part of the cube is getting lighter(more visible) even if no additional light is getting there (there is no GI involved, just plain lights) as well as the wall that is far behind. The bricks are getting more visible thou no extra lights are hitting them. So the volumetric part is adding in a wrong way. To get it right I have to render out a separate layer of the visible lights and invert them to desaturate the backgound back to dark.
http://homepage.mac.com/tcastudios/tempfiles/slp/SpecFillLights.jpg
Next pict shows a similar behaviour of the specular part of the fill lights into a reflective/transparant glass object. It is partly hit from behind by spots. If the specular is turned off on the fill lights, it looks "right", but if the specular is turned on on the fill lights, the parts of the cube that is behind the glass again gets lighter/more visible than it should.

For simple scenes this sometimes can be fixed but it is impossible when things add up in a fullblown scenario.
http://homepage.mac.com/tcastudios/tempfiles/slp/resultproblem.jpg
This last pict shows what happens when it all is happening. Several glass objects and many lights. Things get totaly visible thru the glass since specular and visible sources are adding up as well as the specularity thru several glasses.

I would like to see some options how to add specularity/volumetric if possible.
Attached is the small example scene if anyone might find/know of a solution.
Or if I -totally- have missed something, but I have sat down and realy tried....
scene (http://homepage.mac.com/tcastudios/tempfiles/slp/speclightproblems.c4d.zip)
Cheers
Lennart

PS Does anyone else have problems using the "insert links" options when posting a thread? I had to manually type it into this text. DS
Simon Wicker
03-20-2006, 10:25 PM
i can't comment on the specular issue but the volumetric fog looks accurate to me - if the volumetric fog is bleeding over the edges of the cubes then it should lighten the areas in the shadows.

cheers, simon w.
Rantin Al
03-20-2006, 10:31 PM
Getting a 404 on that link, Lennart.
tcastudios
03-20-2006, 10:36 PM
i can't comment on the specular issue but the volumetric fog looks accurate to me - if the volumetric fog is bleeding over the edges of the cubes then it should lighten the areas in the shadows.

cheers, simon w.
I'm sorry, I have got that suggestion before and it is wrong. I've taken pictures of the behaviour in real life. A very little lit up room and spots pointing towards the camera. There are no results of the walls behind to be more visible. And the same for objects getting shadowed. They don't get more visible. I can't see more of a shadowed part of a object if a light is hitting it from behind. (Rather less, if exposure is taken in as a factor)

Cheers
Lennart
tcastudios
03-20-2006, 10:38 PM
Getting a 404 on that link, Lennart.
Thanks. Should be ok now.
Rantin Al
03-20-2006, 10:59 PM
Downloading OK now.
Simon Wicker
03-20-2006, 11:03 PM
I'm sorry, I have got that suggestion before and it is wrong. I've taken pictures of the behaviour in real life. A very little lit up room and spots pointing towards the camera. There are no results of the walls behind to be more visible. And the same for objects getting shadowed. They don't get more visible. I can't see more of a shadowed part of a object if a light is hitting it from behind. (Rather less, if exposure is taken in as a factor)

Cheers
Lennart

well cinemas volumetrics are additive just like the reflection channel so you can end up with more light going out than incoming (the additive check box in the volumetric light is for when you have multiple volumetrics stacking up on each other and doesn't affect how the volumtric is comped over the bg).

cheers, simon w.
Simon Wicker
03-20-2006, 11:37 PM
okay. scratch what i have written in the post above because it is wrong and instead try out this test:

make a cube and give it a texture that is exactly 50% grey in the luminance channel. now add in a volumetric omni light and place it in front of the cube. turn off illumination in the omni light and on the volumetric tab turn off falloff and turn the volumetric light brightness down to 50%.

now do some renders tweaking the dust value. what i am finding is that at 100% dust values the volumetric light is blended normally and at 0% dust levels the fog is blended in a fully additive way (i.e at 100% dust the cube renders 50% grey but at 0% dust the cube renders incorrectly at 100% brightness). i've enclosed the file.

looking at this i would still argue that the apparent brightening you are getting is still correct as long as your dust values are at 100% (as they are) because the render will never get brighter than the brightness added by the fog (which is the effect you get in the real world on a foggy day where the stronger the fog the more your image tends to the fog colour and luminosity.

cheers, simon w.
Rantin Al
03-20-2006, 11:38 PM
With the Visible Light Problem file:

1. Enabling the falloff on the Fill Lights stops the additive effect on the rear wall.
2. Or add the wall to the Scene Exclusion list for the Fill Lights.

It is the same with the Specular file. Enable the fall-off on the fill lights.

I don't know if this is the same as previous advice but it is all I could see that was a potential problem.

HTH, Al.
tcastudios
03-21-2006, 07:08 AM
Thanks both for looking at it.
Turning on the falloff for the fill light does get rid of the effect but that is only because the light is then not reaching the wall/glass. It is the same as turning the light off in essence.
This is not an option since a fill light is supposed to do just that, fill the scene.
Turning on the fall off and set the fall off to reach the wall/glass the problem is still the same.
This leads to what Simon shows. There is something "wrong" with the way specular and volumetric adds. The difference using 0 resp 100 percent dust is "only" the amount of desaturation of the background (wall). At 0 dust, the fog (in the spotray) is filled with red from the wall behind and at 100 percsent dust the spot ray is desaturated, the wall (behind the ray) is less affected by the red color of the wall.

So, in my opinion, what is missing is a "automask" of the volumetric light. When a volumetric/visible light is used a inverted mask should be added just behind the light to
block the faulty adding of nonrelevant colors (colors on objects that are not lit/affected by the light itself) This is what I have done in post in a few cases and is pretty hard/timeconsuming. I did then use 16bit renders and got it pretty good. I'll see if the newer option of higher bitdepths might be better, (giving less "halo" artifacts)

And just to clearify the volumetric problem. Look at the edges along the spot. That is where the problem shows most. You can see the bricks of the wall clearly -within- the spot ray.

The specular problem seems more complicated and I have no "post" trick I can think of to solve it.

Cheers
Lennart
Rantin Al
03-21-2006, 07:24 AM
This has just gone way over my capabilities.
Maybe Dr. Janine could help over at 3DFluff?

Cheers, Al.
CGTalk Moderation
03-21-2006, 07:24 AM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.