Fix disappearing terrain and sky

CHANGES.md

@@ -15,6 +15,7 @@ Change Log
 * Fixed an issue with drill picking at low frame rates that would cause a crash. [#3010](https://github.com/AnalyticalGraphicsInc/cesium/pull/3010)
 * Fixed a bug that prevented `setView` from working across all scene modes.
 * Fixed a bug that caused `camera.positionWC` to occasionally return the incorrect value.
+* Fixed issues causing the terrain and sky to disappear when the camera is near the surface. [#2415](https://github.com/AnalyticalGraphicsInc/cesium/issues/2415) and [#2271](https://github.com/AnalyticalGraphicsInc/cesium/issues/2271)
 
 ### 1.13 - 2015-09-01
 

Source/Core/EllipsoidalOccluder.js

@@ -129,6 +129,11 @@ define([
      * occluder.isScaledSpacePointVisible(scaledSpacePoint); //returns true
      */
     EllipsoidalOccluder.prototype.isScaledSpacePointVisible = function(occludeeScaledSpacePosition) {
+        // Disable occlusion culling when the viewer is under the ellipsoid to avoid false occulsion.
+        if (this._distanceToLimbInScaledSpaceSquared < 0.0) {
+            return true;
+        }
+
         // See http://cesiumjs.org/2013/04/25/Horizon-culling/
         var cv = this._cameraPositionInScaledSpace;
         var vhMagnitudeSquared = this._distanceToLimbInScaledSpaceSquared;

Source/Scene/ScreenSpaceCameraController.js

@@ -155,7 +155,7 @@ define([
          * @type {Number}
          * @default 20.0
          */
-        this.minimumZoomDistance = 20.0;
+        this.minimumZoomDistance = 1.0;
         /**
          * The maximum magnitude, in meters, of the camera position when zooming. Defaults to positive infinity.
          * @type {Number}

Source/Shaders/SkyAtmosphereFS.glsl

@@ -36,6 +36,9 @@
 const float g = -0.95;
 const float g2 = g * g;
 
+uniform float fCameraHeight;
+uniform float fInnerRadius;
+
 varying vec3 v_rayleighColor;
 varying vec3 v_mieColor;
 varying vec3 v_toCamera;
@@ -49,9 +52,35 @@ void main (void)
     vec3 direction = normalize(v_positionEC);
     czm_ray ray = czm_ray(vec3(0.0), direction);
 
-    czm_raySegment intersection = czm_rayEllipsoidIntersectionInterval(ray, ellipsoid);
-    if (!czm_isEmpty(intersection)) {
-        discard;
+    if (fCameraHeight > fInnerRadius) {
+	    czm_raySegment intersection = czm_rayEllipsoidIntersectionInterval(ray, ellipsoid);
+	    if (!czm_isEmpty(intersection)) {
+	        discard;
+	    }
+	} else {
+	    // The ellipsoid test above will discard fragments when the ray origin is
+	    // inside the ellipsoid.
+	    vec3 radii = ellipsoid.radii;
+	    float maxRadius = max(radii.x, max(radii.y, radii.z));
+	    vec3 ellipsoidCenter = czm_modelView[3].xyz;
+
+	    float t1 = -1.0;
+	    float t2 = -1.0;
+
+	    float b = -2.0 * dot(direction, ellipsoidCenter);
+	    float c = dot(ellipsoidCenter, ellipsoidCenter) - maxRadius * maxRadius;
+
+	    float discriminant = b * b - 4.0 * c;
+	    if (discriminant >= 0.0) {
+	        t1 = (-b - sqrt(discriminant)) * 0.5;
+	        t2 = (-b + sqrt(discriminant)) * 0.5;
+	    }
+
+	    if (t1 < 0.0 && t2 < 0.0) {
+	        // The ray through the fragment intersected the sphere approximating
+	        // the ellipsoid behind the ray origin.
+	        discard;
+	    }
     }
 
     // Extra normalize added for Android