Custom light source

[lilleyse]

Fixes #6553

This PR adds more customizability to Cesium's lighting system. There is now a property scene.light that lets you swap the default sun light with a directional light of your choosing. For example:

viewer.scene.light = new Cesium.DirectionalLight({
    direction: new Cesium.Cartesian3(1, 0, 0),
    color: Cesium.Color.ORANGE,
    intensity: 0.5
});

What this PR doesn't add is multiple light sources. That will need to be a future PR.

Some more notes about this PR:

• I decided to go with the color + intensity paradigm. light.color is a Color instance and light.intensity is a scalar multiplier which can greater than 1.0 to support HDR lighting. This seems to be similar to what ThreeJS, BabylonJS, and Blender do. Note that only glTF 2.0 PBR models actually use the HDR color, everything else uses an equivalent SDR color for their lighting calculations.
• I removed moon specular lighting from phong.glsl and other areas. This concept doesn't fit in the new design. The default material shininess is pretty rough so the specular contribution was not that noticeable anyways. In the future this would be implemented with two light sources.
• scene.sunColor is deprecated and will be removed in 1.69. The recommended approach now is to set scene.light.color. This property only affected glTF 2.0 PBR models whereas scene.light affects all systems.
• Some glTF models won't look any different. These are glTF 1.0 or glTF 2.0 / KHR_techniques_webgl models. Since the shaders are hardcoded there's no good fix except to upgrade to glTF 2.0 (Upgrade all SampleData models to gltf 2.0 #7802). I upgraded the Cesium Balloon in this PR and @sanjeetsuhag will upgrade the rest as part of onramping. The Bentley BIM tileset uses custom shaders heavily and actually hardcodes czm_sunDirectionWC so we'd have to go through and change that to czm_lightDirectionWC.
• Added globe.dynamicAtmosphereLighting and globe.dynamicAtmosphereLightingFromSun to add more atmosphere and fog customization. This made the flashlight and moonlight demos look a lot better.

Some screenshots from the Sandcastle example:

Fixed lighting

Flashlight

Moonlight

Sunlight

Custom color

To do:

• Once merged, open issue to remove scene.sunColor in 1.69.

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[IanLilleyT]

This looks pretty thorough. Not having sun be the only light source is good.

My main API confusion is Globe.dynamicAtmosphereLightingFromSun, since the light source is the sun in most cases anyway, and so users might think they must set this property too. I can't think of a better name though. This isn't a big deal as it might be solved in the future by giving the atmosphere system its own light source, and then we can remove this custom behavior for the sun.

[lilleyse]

Does anyone else want to review this before we merge? Mainly looking for feedback on the API. From CHANGES.md:

Deprecated ⏳

• The property Scene.sunColor has been deprecated and will be removed in Cesium 1.69. Use scene.light.color and scene.light.intensity instead. #8493

Additions 🎉

• Add more customization to Cesium's lighting system #8493
  • Added Light, DirectionalLight, and SunLight classes for creating custom light sources.
  • Added Scene.light for setting the scene's light source, which defaults to a SunLight.
  • Added Globe.dynamicAtmosphereLighting for enabling lighting effects on atmosphere and fog, such as day/night transitions. It is true by default but may be set to false if the atmosphere should stay unchanged regardless of the scene's light direction.
  • Added Globe.dynamicAtmosphereLightingFromSun for using the sun direction instead of the scene's light direction when Globe.dynamicAtmosphereLighting is enabled. See the moonlight example in the Lighting Sandcastle example.
  • Primitives and the globe are now shaded with the scene light's color.

[mramato]

@lilleyse latest version looks good to me. Can't wait to use this, the flashlight is something we've wanted in Cesium since before it was even called Cesium.

The sooner we can get this in the better since it should get as much testing as possible before the next release. I say we merge today and we can always tweak the API based on feedback before we release on Feb. @IanLilleyT any objections?

Is there an updated issue with ideas for expanding this into an array of lights (and I assume adding different types of lights as well)? It would be could to capture your thoughts while still fresh in your head.

Thanks again @lilleyse!

[IanLilleyT]

Everything looks good!

[mramato]

🔥 🔥 🔥

[lilleyse]

Opened #8517 for removing scene.sunColor

Also opened #8518 for future lighting ideas.