Report empty tile children arrays as a warning

specs/TilesetValidationSpec.ts

@@ -393,6 +393,14 @@ describe("Tileset validation", function () {
     expect(result.get(0).type).toEqual("REQUIRED_VALUE_NOT_FOUND");
   });
 
+  it("detects issues in tileChildrenEmptyArray", async function () {
+    const result = await Validators.validateTilesetFile(
+      "specs/data/tilesets/tileChildrenEmptyArray.json"
+    );
+    expect(result.length).toEqual(1);
+    expect(result.get(0).type).toEqual("ARRAY_LENGTH_UNEXPECTED");
+  });
+
   it("detects issues in tileContentBoundingVolumeInvalidType", async function () {
     const result = await Validators.validateTilesetFile(
       "specs/data/tilesets/tileContentBoundingVolumeInvalidType.json"

specs/data/tilesets/tileChildrenEmptyArray.json

@@ -0,0 +1,13 @@
+{
+  "asset" : {
+    "version" : "1.1"
+  },
+  "geometricError" : 2.0,
+  "root" : {
+    "boundingVolume" : {
+      "box" : [ 0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5 ]
+    },
+    "geometricError" : 1.0,
+    "children": []
+  }
+}

src/issues/JsonValidationIssues.ts

@@ -158,6 +158,21 @@ export class JsonValidationIssues {
     return issue;
   }
 
+  /**
+   * A warning that indicates that an array SHOULD have had a
+   * certain length, but had a different length.
+   *
+   * @param path - The path for the `ValidationIssue`
+   * @param message - The message for the `ValidationIssue`
+   * @returns The `ValidationIssue`
+   */
+  static ARRAY_LENGTH_UNEXPECTED(path: string, message: string) {
+    const type = "ARRAY_LENGTH_UNEXPECTED";
+    const severity = ValidationIssueSeverity.WARNING;
+    const issue = new ValidationIssue(type, path, message, severity);
+    return issue;
+  }
+
   /**
    * Indicates that the length of a string does not match the length
    * that is specified via the JSON schema, using the `minLength`

src/validation/TileValidator.ts

@@ -347,19 +347,33 @@ export class TileValidator {
     const children = tile.children;
     const childrenPath = tilePath + "/children";
     if (defined(children)) {
-      // The children MUST be an array of objects with at least 1 element
+      // The children MUST be an array of objects
       if (
         !BasicValidator.validateArray(
           childrenPath,
           "children",
           children,
-          1,
+          0,
           undefined,
           "object",
           context
         )
       ) {
         result = false;
+      } else {
+        // The children are an array of objects.
+        // The case that the array has a length of 0 will cause a warning,
+        // due to https://github.com/CesiumGS/3d-tiles/issues/752
+        if (children.length === 0) {
+          const message =
+            `The 'children' array should contain at least 1 element, ` +
+            `but had a length of 0`;
+          const issue = JsonValidationIssues.ARRAY_LENGTH_UNEXPECTED(
+            childrenPath,
+            message
+          );
+          context.addIssue(issue);
+        }
       }
     }
     return result;