Change refine to inherit from parent

README.md

@@ -109,7 +109,7 @@ The `boundingVolume.region` property is an array of six numbers that define the
 
 The `geometricError` property is a nonnegative number that defines the error, in meters, introduced if this tile is rendered and its children are not.  At runtime, the geometric error is used to compute _Screen-Space Error_ (SSE), i.e., the error measured in pixels.  The SSE determines _Hierarchical Level of Detail_ (HLOD) refinement, i.e., if a tile is sufficiently detailed for the current view or if its children should be considered.
 
-The `refine` property is an optional string that is either `"replace"` for replacement refinement or `"add"` for additive refinement.  When `refine` is omitted, it defaults to `"add"`.
+The `refine` property is a string that is either `"replace"` for replacement refinement or `"add"` for additive refinement.  It is required for the root tile of a tileset; it is optional for all other tiles.  When `refine` is omitted, it is inherited from the parent tile.
 
 The `content` property is an object that contains metadata about the tile's content and a link to the content.  `content.url` is a string that points to the tile's contents with an absolute or relative url.  In the example above, the url, `2/0/0.b3dm`, has a TMS tiling scheme, `{z}/{y}/{x}.extension`, but this is not required; see the [roadmap Q&A](#How-do-I-request-the-tiles-for-Level-n).
 

examples/tiles.json

@@ -39,6 +39,7 @@
       "url": "0/0/0.b3dm"
     },
     "geometricError": 268.37878244706053,
+    "refine": "add",
     "children": [
       {
         "boundingVolume": {

schema/tile.schema.json

@@ -17,10 +17,9 @@
         },
         "refine" : {
             "type" : "string",
-            "description" : "Specifies if additive or replacement refinement is used when traversing the tileset for rendering.",
-            "enum" : ["add", "replace"],
-            "default" : "add"
-        },        
+            "description" : "Specifies if additive or replacement refinement is used when traversing the tileset for rendering.  This property is required for the root tile of a tileset; it is optional for all other tiles.  The default is to inherit from the parent tile.",
+            "enum" : ["add", "replace"]
+        },
         "content" : {
             "extends" : { "$ref" : "tile.content.schema.json" },
             "description" : "Metadata about the tile's content and a link to the content. When this is omitted the tile is just used for culling."