BlueMap Chunk Trimmer

A BlueMap addon that helps you visualize and manage chunk activity on your Minecraft server. It reads the InhabitedTime NBT data from your .mca region files and renders it as a color-coded heatmap overlay directly on your BlueMap, so you can see at a glance where players have actually spent time. It also includes an interactive chunk selector that lets you pick chunks visually and export them for deletion with tools like MCA Selector — useful for trimming unused chunks to reduce world size or force terrain regeneration.

Both features are only available in BlueMap's flat (2D) view.

Heatmap data is read once at startup. The addon parses region files when BlueMap first loads, so the heatmap reflects the state of the world at that point. To pick up new activity, restart the server or run /bluemap reload.

Heatmap Overlay

The heatmap shows cumulative player activity per chunk. Each chunk that's been inhabited for more than 1 minute gets a colored overlay, with the color indicating how much total time players have spent there. Chunks are colored by tier:

• Faint blue — 1–5 minutes (just walked through)
• Blue — 5–10 minutes (lingered briefly)
• Brighter blue — 10–30 minutes (spent a bit of time)
• Teal/cyan — 30 minutes – 1 hour (properly explored)
• Yellow/amber — 1–3 hours (significant activity)
• Orange — 3–10 hours (major investment)
• Deep red — 10+ hours (base or major build)

Chunks with less than 1 minute of inhabited time don't appear on the heatmap at all, which keeps the map clean — chunks that were merely loaded or briefly passed through won't show up.

The heatmap is rendered client-side using Three.js InstancedMesh for performance. Toggle it on and off using the thermometer icon in the top-left control bar (only visible in flat view).

When the heatmap is visible, a small HUD appears at the bottom of the screen as you move your cursor. It shows the chunk coordinates, the inhabited time in a human-readable format (e.g. "2h 15m"), and the exact tick count underneath.

Chunk Selector

The chunk selector lets you visually pick chunks on the map for export. Enable it by clicking the chunk trimmer icon in the top-left toolbar (only visible in flat view).

Selecting chunks

• Ctrl/Cmd + click a chunk to select or deselect it. Selected chunks appear with a bright pink hatched pattern.
• Ctrl/Cmd + click and drag to paint across multiple chunks. Whether this selects or deselects depends on the state of the first chunk you click — if it's unselected, dragging selects; if it's already selected, dragging deselects.
• Shift click and drag to draw a rectangle. All chunks inside the rectangle get selected (or deselected, same logic as above). You'll see a white outline preview of the rectangle as you drag.

Selections are stored per-dimension (Overworld, Nether, The End) and persist in your browser's local storage, so they survive page refreshes.

Exporting selections

The panel in the bottom-right shows your current selection count and provides two export options:

JSON includes chunk coordinates, inhabited time data, and world metadata:

{
  "dimension": "overworld",
  "worldName": "world",
  "chunks": [
    { "chunkX": -9, "chunkZ": 2, "inhabitedTime": 161 },
    { "chunkX": 10, "chunkZ": 15, "inhabitedTime": 45000 }
  ]
}

The inhabitedTime values are in game ticks (20 ticks = 1 second).

CSV exports in MCA Selector's semicolon-delimited format, which you can use directly with MCA Selector's chunk filter to delete the selected chunks:

0;0;-9;2
0;0;10;15

The four columns are regionX;regionZ;chunkX;chunkZ (no header row).

Installation

Place the JAR in BlueMap's packs/ directory — not mods/. This is a BlueMap native addon, not a Fabric/Forge mod. Requires Java 21.

Development

Requires Java 21 and Gradle (wrapper included).

./gradlew shadowJar    # build fat JAR → build/libs/bluemap-chunk-trimmer-0.1.0.jar
./gradlew compileJava  # compile check only

The shadow JAR relocates the Querz NBT library to avoid classpath conflicts. BlueMap provides Gson at runtime, so it's not bundled.