Merge pull request #12978 from Beilinson/fix-clamped-polygon-picking

Fix clamped polygon picking

Author: lilleyse

CHANGES.md

@@ -17,6 +17,7 @@
 - Improved performance of `scene.drillPick`. [#12916](https://github.com/CesiumGS/cesium/pull/12916)
 - Improved performance when removing primitives. [#3018](https://github.com/CesiumGS/cesium/pull/3018)
 - Improved performance of terrain Quadtree handling of custom data [#12907](https://github.com/CesiumGS/cesium/pull/12907)
+- Fixed picking of `GroundPrimitive` with multiple `PolygonGeometry` instances selecting the wrong instance. [#12978](https://github.com/CesiumGS/cesium/pull/12978)
 
 ## 1.134.1 - 2025-10-10
 

packages/engine/Source/Shaders/ShadowVolumeAppearanceFS.glsl

@@ -65,21 +65,6 @@ void main(void)
 #endif // SPHERICAL
 #endif // TEXTURE_COORDINATES
 
-#ifdef PICK
-#ifdef CULL_FRAGMENTS
-    // When classifying translucent geometry, logDepthOrDepth == 0.0
-    // indicates a region that should not be classified, possibly due to there
-    // being opaque pixels there in another buffer.
-    // Check for logDepthOrDepth != 0.0 to make sure this should be classified.
-    if (0.0 <= uv.x && uv.x <= 1.0 && 0.0 <= uv.y && uv.y <= 1.0 || logDepthOrDepth != 0.0) {
-        out_FragColor.a = 1.0; // 0.0 alpha leads to discard from ShaderSource.createPickFragmentShaderSource
-        czm_writeDepthClamp();
-    }
-#else // CULL_FRAGMENTS
-        out_FragColor.a = 1.0;
-#endif // CULL_FRAGMENTS
-#else // PICK
-
 #ifdef CULL_FRAGMENTS
     // When classifying translucent geometry, logDepthOrDepth == 0.0
     // indicates a region that should not be classified, possibly due to there
@@ -89,6 +74,13 @@ void main(void)
     }
 #endif
 
+#ifdef PICK
+    out_FragColor.a = 1.0; // Explicitly set the alpha, otherwise this may be discarded by ShaderSource.createPickFragmentShaderSource
+#ifdef CULL_FRAGMENTS
+    czm_writeDepthClamp();
+#endif // CULL_FRAGMENTS
+#else // PICK
+
 #ifdef NORMAL_EC
     // Compute normal by sampling adjacent pixels in 2x2 block in screen space
     vec3 downUp = vectorFromOffset(eyeCoordinate, vec2(0.0, 1.0));

packages/engine/Specs/Scene/GroundPrimitiveSpec.js

@@ -1156,6 +1156,49 @@ describe(
       });
     });
 
+    // Test edge-case from #12978 of geometry instances z-fighting when rendered side-by-side
+    it("picks the correct instance", function () {
+      if (!GroundPrimitive.isSupported(scene)) {
+        return;
+      }
+      // This edge-case only occurs when the right instance is rendered before the left instance
+      const rectangles = [
+        {
+          rectangle: Rectangle.fromRadians(-1.0001, 0.5, -1.0, 0.5001),
+          id: "right",
+        },
+        {
+          rectangle: Rectangle.fromRadians(-1.0002, 0.5, -1.0001, 0.5001),
+          id: "left",
+        },
+      ];
+      const color = ColorGeometryInstanceAttribute.fromColor(Color.RED);
+      primitive = new GroundPrimitive({
+        geometryInstances: rectangles.map(
+          ({ rectangle, id }) =>
+            new GeometryInstance({
+              geometry: PolygonGeometry.fromPositions({
+                positions: Rectangle.subsample(rectangle),
+              }),
+              id,
+              attributes: {
+                color,
+              },
+            }),
+        ),
+        asynchronous: false,
+      });
+
+      // Slightly shift the rectangle such that without the proper discard,
+      // the right rectangle will render over the left during the picking pass
+      rectangle = Rectangle.fromRadians(-1.000201, 0.5, -1.0, 0.5001);
+      verifyGroundPrimitiveRender(primitive, color.value);
+
+      expect(scene).toPickAndCall(function (result) {
+        expect(result.id).toEqual("left");
+      });
+    });
+
     it("picking without depth texture", function () {
       if (!GroundPrimitive.isSupported(scene)) {
         return;