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Comment by 01HNNWZ0MV43FF | Hacker News Reader

My favorite shadow fact is that outdoor shadows are blue.

It's not an optical illusion or artistic vibe or anything. The sky is blue, shadows on a clear day are illuminated by bounced light from the sky, therefore shadows are blue.

If you look underneath cars you can see it - A sharp blue shadow where the sky is visible, that fades to true black where the car's body occludes light from the sky.

If you combine this sharp blue sun shadow with a soft and black "AO" sky shadow you can get very pretty shadows for cheap.

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It's true, and so subtle that a lot of people don't even notice it.

But a good graphics rendering engine will do it. Shadows should carry a slight tint from the color of the sky.

Which is why some old screenshots of No Man's Sky bothered me. Pretty sure I saw scenes where shadows were purple despite a green sky.

> But a good graphics rendering engine will do it.

Correlation/Causation: lots of things in graphics rendering work because of observed phenomena. A “good graphics engine” is only as good as the eyes that implemented it. Todays engines still fall short of what is in front of us, and not because of a technical limitation.

> Pretty sure I saw scenes where shadows were purple despite a green sky.

If it’s a stylistic effect then it’s a stylistic effect. But otherwise, purple shadows are literally everywhere. Shadows can have an immense amount of chroma and vibrancy to them, or they can be incredibly cool and muted. It all depends on the context.

> Correlation/Causation: lots of things in graphics rendering work because of observed phenomena.

As we are moving more and more towards physics-based rendering, engine are shifting from imitating what's observed to properly defining the world and its interactions and get realism as a byproduct.

The traditional (cheap) way of lighting stuff is to model ambiant lighting as some constant, regardless where light actually comes from, and render shadows as dark patches. It is not at all how physics work, so you need an artist with a good sense of observation to make it look right. The artist may suggest some blue tint for the shadows because it looks right.

The physical approach is to start with just the sun, no ambiant light, and simulate Rayleigh scattering, which will naturally give you a blue sky and blueish shadows. An artist can still be involved, but his job will be more about stylistic choices and evaluating corner-cutting techniques.

> As we are moving more and more towards physics-based rendering, engine are shifting from imitating what's observed to properly defining the world and its interactions and get realism as a byproduct.

Sigh. Anyone who’s done physically based rendering will tell you it’s all bullshit. Even Maxwell’s equations are a special case of a much broader set of interactions. But a few decades ago you’d think that’s all there is. You don’t get realism as a byproduct, you get “looks real, ship it” as a byproduct.

It’s approximations. And that’s perfectly fine.

This is how most graphics engines work by default. Shadows are ambient light only and outdoors ambient light will get a blue tint from cubemaps, SH, GI or whatever other technique is used.

Artists are often taught to paint outdoor shadows a bit purple, for maximum contrast with the yellow tint of sunlight.

Just saw this on Monet’s winter paintings: https://news.artnet.com/art-world/monet-the-magpie-three-fac...

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