Sunday, November 26, 2017

What happens to light in clouds?

Let's take a look at how sunlight interacts with clouds.

Photo New Scientist, photographer Mahrt Fabian
Although there is a light side and a shadow side on most clouds, most of the light enters the cloud rather than bouncing off the surface. If the cloud is thin enough or fragmentary enough, a light side and shadow side are not distinguishable.

But in a large, dense cloud like a thunderhead, the light that enters the cloud is scattered and dispersed within the mass of water vapor or ice crystals. After its random journey, the light eventually exits the cloud, lightening the shadow side.

 Note that the white cloud is darker than the white house.
Photo Home Buyer
This subsurface scattering gives clouds a different character than an opaque white surface such as plaster or painted stucco. The cloud's light side is darker than the solid surface (because it's absorbing light), and the shadow side is lighter (because of subsurface scattering).

The value of the shadow side of the cloud is therefore a combination of internal scattering and external sources, such as the blue light of the sky or reflected light from the ground.

Simulating clouds turns out to be a computational challenge for artists using 3D digital tools. A recent research paper by Disney's team of computer engineers discusses some improvements in the rendering of light behavior within clouds.

They used machine learning techniques to speed up the process of volumetric path tracing, following the complex pathways of the light within the cloud. (link to video)

Read more
Disney's research paper (PDF)
My book Color and Light: A Guide for the Realist Painter (link to Amazon) gets deep into this topic. If you live in the USA you can get the book signed on my website, shipped within 24 hours.
Previously: Subsurface Scattering


Rich said...

Quite "cloudy"
but still highly defined;-)

Tyler J said...

James, presumably the white cloud would be darker than the white house when viewed with human eyeballs in person? It's not just a limitation of the photo range?

I have become pretty distrusting of photos since you've done so many great posts on them. The realization that there are usually no black shadows in real life, for example, was a pretty big revelation for me. After that, I started to really see the blue in shadows everywhere (outside, on a relatively clear day).

James Gurney said...

Tyler, a good question. This morning I was just looking at a white barn in front of some white clouds and tried to photograph the relationship of the values. What I found was that yes, the white painted wood in sunlight had much more luminance than the brightest part of the clouds, and that, if anything, the cameras (I tried two different ones) had a hard time capturing that difference.