Add a uniform for whether the tile is in fog

packages/engine/Source/Scene/GlobeSurfaceTileProvider.js

@@ -2502,7 +2502,9 @@ function addDrawCommandsForTile(tileProvider, tile, frameState) {
       uniformMapProperties.localizedTranslucencyRectangle
     );
 
-    // For performance, use fog in the shader only when the tile is in fog.
+    // For performance, render fog only when fog is enabled and the effect of
+    // fog would be non-negligible. This prevents the shader from running when
+    // the camera is in space, for example.
     const applyFog =
       enableFog &&
       CesiumMath.fog(tile._distance, frameState.fog.density) >

packages/engine/Source/Scene/Model/FogPipelineStage.js

@@ -1,5 +1,7 @@
-import FogStageFS from "../../Shaders/Model/FogStageFS.js";
+import Cartesian3 from "../../Core/Cartesian3.js";
+import CesiumMath from "../../Core/Math.js";
 import ShaderDestination from "../../Renderer/ShaderDestination.js";
+import FogStageFS from "../../Shaders/Model/FogStageFS.js";
 
 /**
  * The fog color pipeline stage is responsible for applying fog to tiles in the distance in horizon views.
@@ -17,6 +19,24 @@ FogPipelineStage.process = function (renderResources, model, frameState) {
 
   shaderBuilder.addDefine("HAS_FOG", undefined, ShaderDestination.FRAGMENT);
   shaderBuilder.addFragmentLines([FogStageFS]);
+
+  // Add a uniform so fog is only calculated when the effect would
+  // be non-negligible For example when the camera is in space, fog density decreases
+  // to 0 so fog shouldn't be rendered. Since this state may change rapidly if
+  // the camera is moving, this is implemented as a uniform, not a define.
+  shaderBuilder.addUniform("bool", "u_isInFog", ShaderDestination.FRAGMENT);
+  renderResources.uniformMap.u_isInFog = function () {
+    // We only need a rough measure of distance to the model, so measure
+    // from the camera to the bounding sphere center.
+    const distance = Cartesian3.distance(
+      frameState.camera.position,
+      model.boundingSphere.center
+    );
+
+    return (
+      CesiumMath.fog(distance, frameState.fog.density) > CesiumMath.EPSILON3
+    );
+  };
 };
 
 export default FogPipelineStage;

packages/engine/Source/Scene/Model/ModelRuntimePrimitive.js

@@ -11,7 +11,6 @@ import CustomShaderMode from "./CustomShaderMode.js";
 import CustomShaderPipelineStage from "./CustomShaderPipelineStage.js";
 import DequantizationPipelineStage from "./DequantizationPipelineStage.js";
 import FeatureIdPipelineStage from "./FeatureIdPipelineStage.js";
-import FogPipelineStage from "./FogPipelineStage.js";
 import GeometryPipelineStage from "./GeometryPipelineStage.js";
 import LightingPipelineStage from "./LightingPipelineStage.js";
 import MaterialPipelineStage from "./MaterialPipelineStage.js";
@@ -200,7 +199,6 @@ ModelRuntimePrimitive.prototype.configurePipeline = function (frameState) {
   const mode = frameState.mode;
   const use2D =
     mode !== SceneMode.SCENE3D && !frameState.scene3DOnly && model._projectTo2D;
-  const fogRenderable = frameState.fog.enabled && frameState.fog.renderable;
   const exaggerateTerrain = frameState.verticalExaggeration !== 1.0;
 
   const hasMorphTargets =
@@ -305,10 +303,6 @@ ModelRuntimePrimitive.prototype.configurePipeline = function (frameState) {
 
   pipelineStages.push(AlphaPipelineStage);
 
-  if (fogRenderable) {
-    pipelineStages.push(FogPipelineStage);
-  }
-
   pipelineStages.push(PrimitiveStatisticsPipelineStage);
 
   return;

packages/engine/Source/Scene/Model/ModelSceneGraph.js

@@ -9,6 +9,7 @@ import SceneMode from "../SceneMode.js";
 import SplitDirection from "../SplitDirection.js";
 import buildDrawCommand from "./buildDrawCommand.js";
 import TilesetPipelineStage from "./TilesetPipelineStage.js";
+import FogPipelineStage from "./FogPipelineStage.js";
 import ImageBasedLightingPipelineStage from "./ImageBasedLightingPipelineStage.js";
 import ModelArticulation from "./ModelArticulation.js";
 import ModelColorPipelineStage from "./ModelColorPipelineStage.js";
@@ -606,6 +607,7 @@ ModelSceneGraph.prototype.configurePipeline = function (frameState) {
   modelPipelineStages.length = 0;
 
   const model = this._model;
+  const fogRenderable = frameState.fog.enabled && frameState.fog.renderable;
 
   if (defined(model.color)) {
     modelPipelineStages.push(ModelColorPipelineStage);
@@ -638,6 +640,10 @@ ModelSceneGraph.prototype.configurePipeline = function (frameState) {
   if (ModelType.is3DTiles(model.type)) {
     modelPipelineStages.push(TilesetPipelineStage);
   }
+
+  if (fogRenderable) {
+    modelPipelineStages.push(FogPipelineStage);
+  }
 };
 
 ModelSceneGraph.prototype.update = function (frameState, updateForAnimations) {

packages/engine/Source/Shaders/GlobeFS.glsl

@@ -523,8 +523,6 @@ void main()
 
             finalColor.rgb = mix(finalColor.rgb, finalAtmosphereColor.rgb, fade);
         #endif
-
-        //finalColor.rgb = computeEllipsoidPosition() / 1e7;
     }
 #endif
 

packages/engine/Source/Shaders/Model/FogStageFS.glsl

@@ -76,6 +76,12 @@ vec3 computeFogColor(vec3 positionMC) {
 }
 
 void fogStage(inout vec4 color, in ProcessedAttributes attributes) {
+    if (!u_isInFog) {
+        // Debugging
+        //color.rgb = vec3(1.0, 1.0, 0.0);
+        return;
+    }
+
     vec3 fogColor = computeFogColor(attributes.positionMC);
 
     // Note: camera is far away (distance > nightFadeOutDistance), scattering is computed in the fragment shader.