Odd render anomaly with multi-pass skylights

[R Reynolds]

I've discovered a strange thing when using 5X multi-pass rendering with skylights in v12.0n+.

 

As shown in the attached image I have two spheres in front of a flat shaded skydome. The first sphere has a diffuse colour of 128, 128 128 while the second is 255, 255, 255.

 

The first pair of images shows a comparison of lighting the spheres with a single klieg and rendering an image with and without multi pass. As expected, no noticeable difference.

 

However in the next image set, I turn off the klieg and turn on my 52 bulb skylights (a variation on one of Yves' setups). The first image shows the render with no multi-pass. The next five images are images after turning on 5X multi-pass and doing a screen grab after each pass. The first pass looks fine but with each subsequent pass the spheres' illumination drops and the overall shading becomes more generalized.

 

I'm worried that there is something unusual about my skylight rig. Would someone please confirm this anomaly using a different skylight setup?

 

Thanks.

[Zaryin]

This happens with me too. I just render small sections to see what my lighting will look like after 16 passes.

 

I don't know why it does this.

[Stuart Rogers]

The first pass looks fine but with each subsequent pass the spheres' illumination drops and the overall shading becomes more generalized.

I've also seen noticeable shading differences as each sub-frame accrues, more noticeable than I would have expected.

 

Although each subframe changes the accruing frame significantly, the shading from each completed frame to the next in an animation is smooth, so I'm not concerned enough yet to report it as a bug. Besides, I've yet to work out how to define it if it is a bug.

[Animus]

Hi!

 

I have had problems with skylights since version 12k. I use Yves skylight (25 lights) and i get very different results after 12k. I am not sure if your problem is related to this one, but it looks like shadows accumulate with passes. I don't know if it is a bug or a different way to handle shadows.Those images were multipassed 25 times [attachmentid=12184]

[R Reynolds]

Anomaly corrected.

 

After some testing it turns out that my problem was due to making the skylights very, very large in an over-zealous attempt to soften their resulting shadows. So large in fact that the volume of the bulbs near the horizon actually extended under the models. My testing showed that multi-pass treats a huge bulb as a true area light; randomly sampling rays from anywhere on it's surface. So it was quite possible to trace rays from below the models, hence putting them in shadow and making them darker with each pass. (the first pass was always the brightest because I assume the first sampled ray comes from the bulb's centre point, above the horizon). So it's important that the entire volume of a skylight bulb be above the horizon.

[ypoissant]

After some testing it turns out that my problem was due to making the skylights very, very large in an over-zealous attempt to soften their resulting shadows. So large in fact that the volume of the bulbs near the horizon actually extended under the models. My testing showed that multi-pass treats a huge bulb as a true area light; randomly sampling rays from anywhere on it's surface. So it was quite possible to trace rays from below the models, hence putting them in shadow and making them darker with each pass. (the first pass was always the brightest because I assume the first sampled ray comes from the bulb's centre point, above the horizon). So it's important that the entire volume of a skylight bulb be above the horizon.

Rodger,

 

I'd be currious to see how your skylight was setup to get such a result. I've been using skylights for years without getting this and the lower lights of my skylight rigs always extend below the horizon.

[R Reynolds]

I'd be currious to see how your skylight was setup

Yikes! I was kind of hoping no one would ask because looking at my old rig now, knowing what I know about how it works in multi-pass, it just looks ludicrous. I sort of stretched the the truth earlier when I said some of the bulbs' volumes went below the horizon.

 

However, confession is good for the soul, so here's a comparison of my former really-big-skylight setup (on the left) and my new, more proportional one (on the right), arrayed around my standard 3 mile diameter sky dome. The increase in the illumination of a scene using the new setup is quite noticeable.

 

Behold 19 mi. diameter bulbs! Let the derisive laughter begin.

[ypoissant]

OK. I see. Glad you found a solution.

 

BTW, When I was working on designing my skylights, I also did a few of those experiments that turned not quite like what I expected. But those were good learning experiences.