TileMill : set minzoom to layers?

Question

i found in this post:
CartoCSS layer visibility
a solution to set a layer not visible for some zoom levels.

But in
https://github.com/gravitystorm/openstreetmap-carto
it seems that a minzoom parameter can be set (according to the project.mml file) :

   {
      "name": "coast-poly",
      "srs-name": "900913",
      "geometry": "polygon",
      "class": "",
      "id": "coast-poly",
      "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
      "Datasource": {
        "type": "shape",
        "file": "data/land-polygons-split-3857/land_polygons.shp"
      },
      "extent": [
        -180,
        -85.05112877980659,
        180,
        85.05112877980659
      ],
      **"properties": {
        "minzoom": 10
      },**
      "advanced": {}
    },

But in Tilemill i can't get it working (lines are drawn in red).

What would be the best/fastest solution to manage minzoom & maxzoom for layers in TileMill?

My TileMill output could be Mbtiles or Mapnik files. Does this output strategy have an impact on the solution?

Answer 1

Assuming that you have, for example, a layer with the ID rivers.

You could select each zoom level that the layer should be displayed at individually:

#rivers[zoom=4],[zoom=5],[zoom=6] {
 ...
}

This would display the dataset only at zoom levels 4, 5, and 6.

Or, and I think this is more what you were looking for, you can use a greater than:

#rivers[zoom>4] {
 ...
}

This would apply the rules to any zoom level greater than 4.

Note that you can also use greater or equal:

#rivers[zoom>=4]{
 ...
}

The important thing to know in CartoCSS is that if there are no rules defined a dataset will not be displayed. So if you only define rules for greater or equal than 4, then 4 will translate to the layer's minimum zoom, and levels 1, 2, and 3 will not be shown.