Volumetric Light Oddity

[largento]

This was brought up in the Live Answer Time today and I wanted to mess around a little to see if I could figure out what was happening.

 

What's happening, you can see in this image, is the cones of the lights are breaking through the lids of the boxes. At least the edges of the cones are.

 

Adjusting "width softness" does factor into it somehow. In this example, it's set at 100% for the red light, 50% for the blue and 0% for the green. The oddity is more pronounced in the light set to 0%.

 

 

Anybody got a clue what's causing this?

 

I'm attaching the project.

 

light_test_1.zip

 

[EDIT] I forgot to mention that there's another oddity. The boxes all penetrate the floor, yet there are areas of light there at the floor, breaking up the boxes' shadows.

[robcat2075]

 

[EDIT] I forgot to mention that there's another oddity. The boxes all penetrate the floor, yet there are areas of light there at the floor, breaking up the boxes' shadows.

 

 

That is a limitation of Z-buffered lights. The "bias" sets how far a light travels past a surface before a shadow is drawn. Bias can be made smaller to reduce that effect, but for some math reason it can never be zero.

 

One solution is to model the box walls with thickness so the shadow calculation gets started on the inner surface and a shadow is already happening when the light reaches teh outside surface.

[largento]

That's what I get for using the default cho lights. :-)

 

So, changing the spot took care of that, but the main issue is still the edge of the volumetric lights. I've played with various settings and intensity seems to help a little. In this render, the red is set to 10%, the blue 50% and the green 100%. The red breakthough is probably subtle enough that a viewer wouldn't spot it, but it's hardly a dramatic effect. At this point, the only thing I can think is to try to limit your camera movement and use a render without the lights to mask out the areas that are breaking through.

 

[robcat2075]

The green result is weird.

 

This issue is now #6796. I've added your example in the comments.

[largento]

Yeah, I think we can say it's a bug at this point. The green one is the worst-case scenario.

[robcat2075]

BTW, i got a rotoscope to work on a light. You have to add it in the chor, not in the objects folder.

However, it neither stopped the rim of the volumetric light nor shaped the volumetric effect at all. I think that's a bug also.

[Rodney]

My first thought is to check the direction of the normals on the box lids as I suspect they are pointing outward.

I can't test with your project file at this time but will as I get the chance.

[largento]

I tried that, Rodney and it made no difference. The original instance that we were discussing was a box with depth and all of the normals facing outwards ...even an extra patch set to reflect the light was added but didn't solve the problem.

[Rodney]

This problem sounds familiar but I can't put my finger on where it was reported before.


Mark also said:

The boxes all penetrate the floor, yet there are areas of light there at the floor, breaking up the boxes' shadows.

 

There have been a few times when I actually wanted a similar effect!