Add terrain tiles, with elevations encoded as RGB values.

Author: brawer

spec/v3/CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+## Version 3.6
+- Add RGB-shuffled terrain tiles.
+
 ## Version 3.5
 - add [MapLibre Vector Tile](https://github.com/maplibre/maplibre-tile-spec) `.mlt` enum by @msbarry [#596]
 
@@ -18,4 +21,4 @@
 ## Version 3.1
 - added `metadata` details about `vector_layers`.
 - Clarified that directory entry lengths must be nonzero, and directories must be non-empty.
-- add AVIF to TileTypes.
+- add AVIF to TileTypes.

spec/v3/spec.md

@@ -372,8 +372,25 @@ Additionally, this specification defines the following keys, which MAY be includ
 |`name`|A name describing the tileset|string|
 |`description`|A text description of the tileset|string|
 |`attribution`|An attribution to be displayed when the map is shown to a user. Implementations MAY decide to treat this as HTML or literal text. |string|
-|`type`|The type of the tileset |a string with a value of either `overlay` or `baselayer`|
+|`type`|The type of the tileset |a string with a value of either `overlay`, `baselayer` or `terrain`|
 |`version`|The version number of the tileset|a string containing a valid version according to [Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html) |
+|`rgb_shuffle`|An optional dictionary to re-interpret RGB pixels, as described in section 5.1. |
+
+
+## 5.1 RGB Shuffle
+
+If the JSON metadata contains a key `rgb_shuffle`, its value MUST be a JSON
+dictionary with floating-point numbers for `redFactor`, `greenFactor`,
+`blueFactor` and `baseShift`. To interpret an RGB pixel, compute
+`value = R * redFactor + G * greenFactor + B * blueFactor - baseShift`.
+This is useful to encode Digital Terrain Models, where values are
+elevations in fractional meters above ground, as WebP or PNG color images.
+
+Implementations MAY provide specially optimized code paths for the parameters
+`{"redFactor": 256.0, "greenFactor": 0.0, "blueFactor": 0.00390625, "baseShift": -32768.0}`.
+However, parameters that aren’t powers of two MUST be supported as well,
+such as a `blueFactor` of 0.1.
+
 
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