Colored Shadow Penumbra | Hacker News Reader

Colored Shadow Penumbra

chosker.github.io

37 pointsibobev16 comments

What's the physical basis for this effect? Does it happen in reality or is it a style choice?

Came to ask this. I suppose if the edge of the sun glows in a different color than the rest, it would tint the edge of the shadow too? So maybe appropriate for sunsets, where the sky near the sun is red but the sun itself still glows bright white. Honestly just guessing.

This seems more like a chromatic aberration "hack" for HDR landscapes (intensely-lit portions of the scene would have color fringing apparent at the boundaries of light/dark due to dispersion in the observer's lens).

(And it's def a style choice, looks cool when done right! :))

https://en.wikipedia.org/wiki/Chromatic_aberration

Not sure it happens with the sun, but if you have differently located light sources of different colors you can get shadows of different colors (because the shadow area is one source being blocked but it is still illuminated by the other sources)

I had a phone that produced this effect with its torch: https://twitter.com/rmbalt/status/1200397150003302403?s=20

Simple answer: There is no physical basis, it's style

Pedantic answer: Unless the light source has different colors on different sides

Complex answer: Kind of. Even a linear color fade (from reality) can turn non-linear (and therefore induce color effects) when pushed through a color grading pipeline. So if you count e.g. film emulation as a "physical effect", then yes.

> when pushed through a color grading pipeline. So if you count e.g. film emulation as a "physical effect", then yes.

I've seen some footage from a particular Red camera body that introduced some very interesting effects. This particular camera had an issue with the Green channel. The camera was used in a commercial shoot for some fast food chain's shakes. The whip cream would turn magenta when the exposure was pushed because the green channel just wouldn't get there as fast as the red and blue channels. The secondaries had to go dig out extra green channel data plus other tricks to get the whip cream to end up white. After pushing other footage, the magenta tint could be seen else where as well.

TL;DR it's not just film emulsion issues where weird edge case things like this happen.

wasnt that magenta highlights thing a pretty well known deficiency of the reds for a long time? (unsure if they resolved it in recent ones)

Not sure. I never had to work with Red footage to that extent. I just remember the colorist showing it to me. Maybe he knew about it and just pointed it out to me because it was such an extreme example. I wasn't a colorist, just someone that worked with a colorist. An assistant would even be glorifying it.

The post links another that goes into the theory a little: https://shahriyarshahrabi.medium.com/in-the-valley-of-gods-s...

Apparently a combination of Mie and Rayleigh scattering.

- https://en.wikipedia.org/wiki/Mie_scattering

- https://en.wikipedia.org/wiki/Rayleigh_scattering

I think the effect the author is talking about is definitely caused by atmospheric scattering, but the painted effects are different. Those are more likely inspired by overexposure, aberration, HDR, etc. Makoto Shinkai specifically is a filmmaker and often emulates camera effects like lens flare.

It happens in reality, though I've only noticed it with desert sunlight. It's caused by light cast into the penumbra from scattering and diffuse reflection. You can't see this in the lit area because your photoreceptors saturate, which looks white.

This page helped me understand what they’re going for: https://patapom.com/blog/Lighting/Colored%20Penumbra/Colored....

The way I’m intuiting it: some things will “glow” when strongly illuminated, and the glow is more colored than the reflected light, so if the illumination has a hard edge then the penumbra can end up saturated by the more strongly illuminated part’s glow.

OP’s rendition isn’t quite landing for me, though, and I’m not enough of an artist to be sure why. Maybe it’s just that saturation is cranked way up for the demo, but it might also be that it shouldn’t occur on rock, or that color seems sometimes to not react to a change in which material is glowing.

How refreshing it is to see an old school “find line X in file Y and insert this snippet of code” style tutorial.

That’s how I got my start, at least. Minecraft mods, specifically - figuring out how to tweak those little behind the scenes values.

Cool!

FYI the arrows on both photos only actually control the top photo.

causes the external display I have plugged into my MBP (clamshell mode) to flicker quite badly

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