Velocity computed using negative time offset if no valid position at positive offset.

CHANGES.md

@@ -14,6 +14,7 @@ Change Log
 * Fix picking polylines that use a depth fail appearance. [#5337](https://github.com/AnalyticalGraphicsInc/cesium/pull/5337)
 * Fixed a crash when morphing from Columbus view to 3D. [#5311](https://github.com/AnalyticalGraphicsInc/cesium/issues/5311)
 * Fixed a bug which prevented KML descriptions with relative paths from loading. [#5352](https://github.com/AnalyticalGraphicsInc/cesium/pull/5352)
+* Fixed an issue where camera view could be invalid at the last frame of animation. [#4949](https://github.com/AnalyticalGraphicsInc/cesium/issues/4949)
 * Updated documentation for Quaternion.fromHeadingPitchRoll [#5264](https://github.com/AnalyticalGraphicsInc/cesium/issues/5264)
 
 ### 1.33 - 2017-05-01

CONTRIBUTORS.md

@@ -38,6 +38,7 @@ See [CONTRIBUTING.md](CONTRIBUTING.md) for details on how to contribute to Cesiu
    * [Austin Eng](https://github.com/austinEng)
    * [Shehzan Mohammed](https://github.com/shehzan10)
    * [Rachel Hwang](https://github.com/rahwang)
+   * [Joseph Klinger](https://github.com/klingerj)
 * [NICTA](http://www.nicta.com.au/)
    * [Chris Cooper](https://github.com/chris-cooper)
    * [Kevin Ring](https://github.com/kring)

Source/DataSources/EntityView.js

@@ -47,15 +47,24 @@ define([
         var cartesian = positionProperty.getValue(time, that._lastCartesian);
         if (defined(cartesian)) {
             var hasBasis = false;
+            var invertVelocity = false;
             var xBasis;
             var yBasis;
             var zBasis;
 
             if (mode === SceneMode.SCENE3D) {
                 // The time delta was determined based on how fast satellites move compared to vehicles near the surface.
                 // Slower moving vehicles will most likely default to east-north-up, while faster ones will be VVLH.
-                deltaTime = JulianDate.addSeconds(time, 0.001, deltaTime);
+                JulianDate.addSeconds(time, 0.001, deltaTime);
                 var deltaCartesian = positionProperty.getValue(deltaTime, updateTransformCartesian3Scratch1);
+
+                // If no valid position at (time + 0.001), sample at (time - 0.001) and invert the vector
+                if (!defined(deltaCartesian)) {
+                    JulianDate.addSeconds(time, -0.001, deltaTime);
+                    deltaCartesian = positionProperty.getValue(deltaTime, updateTransformCartesian3Scratch1);
+                    invertVelocity = true;
+                }
+
                 if (defined(deltaCartesian)) {
                     var toInertial = Transforms.computeFixedToIcrfMatrix(time, updateTransformMatrix3Scratch1);
                     var toInertialDelta = Transforms.computeFixedToIcrfMatrix(deltaTime, updateTransformMatrix3Scratch2);
@@ -114,6 +123,11 @@ define([
 
                         // Y is along the angular momentum vector (e.g. "orbit normal")
                         yBasis = Cartesian3.cross(zBasis, inertialDeltaCartesian, updateTransformCartesian3Scratch3);
+
+                        if(invertVelocity) {
+                            yBasis = Cartesian3.multiplyByScalar(yBasis, -1, yBasis);
+                        }
+
                         if (!Cartesian3.equalsEpsilon(yBasis, Cartesian3.ZERO, CesiumMath.EPSILON7)) {
                             // X is along the cross of y and z (right handed basis / in the direction of motion)
                             xBasis = Cartesian3.cross(yBasis, zBasis, updateTransformCartesian3Scratch1);