Support HemisphereLight

[viridia]

What problem does this solve or what need does it fill?

three.js supports a HemisphereLight: https://threejs.org/docs/#api/en/lights/HemisphereLight - this is an ambient light which has a sky color and a ground color, and fades between them. It's a quick way to get outdoorsy-looking or dramatic lighting without the expense and trouble of creating an environment map.

What solution would you like?

I'd like Bevy to support a hemisphere light.

What alternative(s) have you considered?

It's possible to do this using an environment map, assuming you can get the env map command-line tools to run on your system (I haven't).

[viridia]

Part of the reason why I want this, BTW, is that I've never been able to build an environment map under MacOS, getting the Vulkan-based tooling to compile is just too difficult. Having a hemispherical ambient light is a very poor man's substitute for environment maps, but it would at least let me have lighting that doesn't look quite so stark and flat.

[viridia]

If there is a concern about adding too many different types of lights (each light type adds new code to the standard shader), note that an ambient light is just a hemispherical light with both hemispheres set to the same color. So you could replace ambient lights with hemispherical lights, and provide a convenient constructor for the ambient light case.

[viridia]

Closing in favor of #17468