Add initial 3D Tiles output prototype.

[connormanning]

This pull request adds 3D Tiles output to Entwine. A sandbox project to view the tileset output within Cesium is located at connormanning/entwine-cesium-pages. Some samples of this output are publicly hosted: a New York City dataset of 4.7 billion points and the small but ubiquitous Autzen Stadium dataset.

Configuration file usage has been improved, and a Cesium template was added. This is used to easily generate tileset output if desired: entwine build config/cesium.json -i ~/data/something.laz -o ~/entwine/something is all that's needed to produce tileset output in addition to normal Entwine output. Tileset output will be written to the cesium subdirectory of the output path provided to Entwine.

For datasets without in-band color, you can modify that configuration file to color the tileset. formats.cesium.coloring: "intensity" (as seen in the NYC demo) will apply intensity coloring to the tileset, and formats.cesium.coloring: "tile" will apply a random color to each pnts tile for visual feedback about Cesium's loading. The geometric error and metadata split level may also be tweaked in this configuration.

There are some notable limitations of this initial implementation, the biggest one being that some Cesium-specific metadata is currently aggregated during build time and not serialized. This means that if the Cesium configuration is enabled, subset/merged builds and continuing previous builds are not supported. For more than 10 billion points or so this would be a big drawback, but for smaller datasets this shouldn't be much of an issue.

Input files must have accurate projection information for the configuration to work as advertised. Otherwise, -r <input SRS> EPSG:4978 should be added to the command line invocation.

Hopefully this is useful for building point cloud 3D Tiles datasets.

[zhenyanghua]

@connormanning Is there a support or plan for making greyhound served resources loaded into Cesium?

[connormanning]

There is no support for that currently - at the moment Entwine only supports 3D-Tiles output directly. 3D-Tiles support via Greyhound from normal Entwine builds is planned, but there is no set timeline for this work yet.