Add tapered glowing lines as an option on PolylineGlowMaterial.

[emackey]

This adds a new uniform parameter to PolylineGlowMaterial called taperPower, to go alongside the existing glowPower parameter.

When set, this parameter will taper off the glowing line, using a strength falloff profile that matches the glowPower falloff for the equivalent number.

The default value is 1.0. Numbers 1.0 and higher produce no tapering effect, so, the default behavior will exactly replicate the look of PolylineGlowMaterial prior to the introduction of this new parameter.

The core of what actually changed here is in the file PolylineGlowMaterial.glsl.

The best sample usage of this new parameter is in the HeadingPitchRoll demo, where the aircraft now leaves behind a tapered glowing line, simply by adding the new parameter to its code. Use the arrow keys to steer the aircraft.

[cesium-concierge]

Thanks for the pull request @emackey!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works in IE11.

[emackey]

More demo fun.

[emackey]

I merged master in here, but the tests failed. The failures look like they might be related to #7579, createImageBitmap.

[emackey]

Merged master again, nothing test-related there, but tests pass this time.

[emackey]

This is ready.

[bagnell]

👍 Thanks @emackey.

[iamsee]

@emackey the demo page is 404

[OmarShehata]

@iamsee you can get the updated demo URL by taking the same query parameters with the sandcastle.cesium.com domain. Here's a working link.

[iamsee]

@OmarShehata thx