On-the-fly meshing of raster elevation tiles for the CesiumJS virtual globe

This package contains a preliminary but functional implementation of a Cesium TerrainProvider that uses right-triangular irregular networks (RTIN) pioneered by Mapbox's Martini to transform Terrain-RGB elevation tiles into quantized mesh terrain, for rendering in the CesiumJS digital globe. The module provides a general technique applicable to all raster imagery (although the Terrain-RGB format is near-ideal for streaming elevation data). Fixes for performance and better control of rendering quality are in progress.

This module was created to support our geologic map visualization work at Macrostrat and as a building block for future rich geoscience visualizations.

This package is listed on NPM as @macrostrat/cesium-martini. It can be installed using the command

npm install --save @macrostrat/cesium-martini

After cloning this repository, you can build the module (using Rollup) with npm run build, or build and watch for changes with npm run watch.

To run an example application, add MAPBOX_API_TOKEN=<your-mapbox-token> to a .env file. in the root of this repository. npm run dev bundles and runs the test application, which runs in the Webpack development server on http://localhost:8080.

Contributions in the form of bug reports and pull requests are welcome. These can be to add functionality (e.g. optional normal-map generation) or for performance. See list of known limitations below.

The Cesium digital globe is a powerful platform for visualization of geospatial data in 3D. Cesium maintains a global elevation dataset as a prebuilt terrain mesh, which caches the computationally-intensive step of meshing height-field data into a triangle irregular network (TIN). Unfortunately, this quantized mesh format is relatively new, narrowly supported and tailored to Cesium itself. Going forward, supporting a TIN format for elevation datasets requires maintenance of significant single-purpose processing pipelines and storage resources.

Mapbox maintains a multiscale global elevation dataset in their clever terrain-RGB format, which bridges web standard file formats (PNG images) with traditional raster GIS formats for representing terrain. Rasters are the standard representation of elevation data across the geosciences, and many pipelines are available to create and modify raster images. Basically any elevation dataset can be easily rescaled to the Terrain-RGB format, but the jump from there to a "Quantized mesh" is more complicated.

Recently, the MARTINI project by Vladimir Agafonkin at Mapbox demonstrated an elegant algorithmic approach that sidesteps this issue. MARTINI meshes based on right-triangulated irregular networks (RTIN, Evans et al., 1998) and is far quicker than the traditional TIN generation techniques.

A speedy meshing algorithm allows this data-preparation step to be handled in the browser after elevation tiles are loaded. Integrating this toolchain into the Cesium digital globe enables the usage of Mapbox global data and other raster terrain layers (e.g. planetary and bathymetric data!), without adding overhead of TIN processing and storage.

Cesium's implementations of the TerrainProvider interface are generally geared towards representing static terrain meshes. The RTIN algorithm used here can dynamically build meshes at a variety of error levels, and the input height field are data-dense and can represent extremely detailed meshes. Right now, meshes are generated at levels of detail that undersample the available structure in a terrain tile — levels of detail are calibrated to what Cesium needs to render visually pleasing output at a given zoom level.

A smarter and more parsimonious solution would use much lower zoom levels for terrain than imagery, using the full resolution of the dataset in mesh construction. Done correctly, this could lead to an extremely data-efficient and adaptive terrain render, but this seems to run somewhat counter to how Cesium internally manages levels of detail, and some thought will have to go into how to organize this. Ideally, someone familiar with the inner workings of Cesium would provide some guidance here.

• Right now, there is a bug with rendering tile bounding boxes when terrainExaggeration: 1 in the Cesium viewer (setting terrainExaggeration: 1.00001 works just fine). I'm uncertain why this is occurring, but it is likely easily fixable.
• High-resolution @2x tiles can be requested, but an indexing error prevents them from rendering properly.
• The increased resolution of @2x tiles can be used, but doing so forces the loading of high resolution overlay imagery across a wide area, so using them is not advisable until broader changes are made to the renderer.
• Tiles at low zoom levels must to respond to the curvature of the Earth, while their topographic range often yields only two triangles covering the entire tile. For zoom levels less than 5, we currently fall back to a basic height field, but we should ideally have a method that subdivides triangles to densify the mesh.
• There is no formal testing framework to catch regressions.
• TypeScript types are discarded on compilation rather than checked properly.

• Mapbox MARTINI
• MARTINI algorithm explanation
• Evans et al., Right-triangulated irregular networks, 1998 (journal link)
• Cesium quantized mesh specification
• Quantized mesh viewer
• Cesium globe materials example
• Cesium sky/atmosphere example