Per-feature randomness: `random()`

random(min, max) returns a number in [min, max) that differs from one feature to the next and is the same every time the same feature is drawn — across frames, tiles and zoom levels. It is mapbox's ["random", min, max, seed], and serves the same purpose: varying a size, a heading or a tint so that a row of identical symbols stops looking stamped.

#poi[class='tree'] {
  model-file: step(random(0, 3), (0, url('tree-spreading.glb')), (1, url('tree-layered.glb')), (2, url('tree-clumped.glb')));
  model-rotation: random(0, 360);
  model-scale: random(0.85, 1.15);
}

What it is seeded with

Call	Seed
`random(min, max)`	the feature id; in a `model` rule, each placed point (below)
`random(min, max, [field])`	the field's value — numbers as they are, strings hashed
`random(min, max, 7)`	the constant: the same value everywhere

The default seed is the variable [mapnik::seed], which a style can also read directly.

A tileset may pack many points into one MultiPoint feature — OpenMapTiles-style trees and wind turbines are one feature per tile. Seeded by the feature id, every point of it would draw the same value. A model rule therefore seeds each point from its own position, so each tree in the pack gets its own model, size and heading, and keeps them when the tile is re-decoded at another zoom.

Two calls with the same bounds and seed return the same value. To draw two independent values for one feature, give them different seeds: random(0, 360, [mapnik::seed] + 1).

What it is seeded with​

What it is seeded with