Hollowing Out a Map: Reverse Masking an Administrative Region in Cesium
Highlighting a single region while visually suppressing the rest of the map is a common GIS requirement for dashboards, election maps, and service-area displays. The polygon-hole approach avoids complex clipping or shader work and works directly with standard GeoJSON files, but the winding-order and coordinate-flattening pitfalls are easy to miss.
A Cesium map can spotlight a single administrative region by drawing a semi-transparent black polygon over the entire globe and then punching a hole through it using the district's GeoJSON boundary. The technique relies on Cesium's polygon hierarchy and its holes array, which accepts a flattened list of Cartesian coordinates. The result is a map where only the target area is brightly visible and everything outside is dimmed.
The implementation also supports toggling between a vector base map and an imagery base map with road annotations. Switching layers requires a full `removeAll()` call on the imagery collection before adding the new provider; otherwise, transparency artifacts and layer stacking occur. The GeoJSON coordinate arrays must be recursively flattened because Cesium's `fromDegreesArray` cannot parse the standard multi-level nesting that administrative boundaries use.
Three common failures are documented: the hole not appearing due to incorrect coordinate winding order, residual layers after a base map switch, and parsing errors from unflattened GeoJSON arrays. The code is built with Vue 3 and Element Plus, using Amap tile URLs for demonstration.
Using a global polygon with a hole is a simpler and more direct alternative to clipping planes or custom shaders for region highlighting, but it ties the mask to the entity API rather than the primitive level, which could limit performance with many dynamic holes.
The recursive array flattener is a one-liner with reduce, but it's a recurring friction point because GeoJSON's coordinate nesting depth varies by geometry type, and Cesium's API expects a flat list.
Hardcoding Amap tile URLs works for demos but the post explicitly warns against production use without proper licensing, which is a reminder that many Chinese map services require registered keys and compliance checks.
Ask how to do reverse selection? [grin]
Hello, if you want to implement reverse selection, that is, to mask the inside of an administrative region, first read the GeoJSON coordinates, then generate and draw a normal ground-clamped polygon based on the data, then fill the polygon with a semi-transparent mask. The external map can be displayed normally. You can refer to my newly published article, which has a specific implementation method.