Add example to TimelineRotation. Also fix bug where a non-interpolating timelinerotation would disappear before the first keyframe (#3310)

* Add example to TimelineRotation. Also fix bug where a non-interpolating timelinerotation would disappear before the first keyframe

---------

Co-authored-by: Emma Broman <emma.broman@liu.se>

data/assets/examples/rotation/timelinerotation/no-interpolate.asset

@@ -0,0 +1,52 @@
+-- No Interpolation
+-- This asset creates a SceneGraphNode that only displays coordinate axes. The rotation of
+-- the coordinate axes are determined by a timeline of individual rotations that are used
+-- without interpolating between the timeline entries. These rotations are keyframes that
+-- are used to change between different orientations. This example transitions between
+-- three rotations. In this example, the interpolation between entries is disabled, which
+-- will cause the coordinate axes to change their orientation abruptly when the rotation
+-- changes. If the interpolation were enabled, the orientation of the coordinate axes
+-- would transition seamlessly instead at the provided times. This example will only work
+-- if the in-game time is set to January 1st, 2000.
+
+local Node = {
+  Identifier = "TimelineRotation_Example_NoInterpolation",
+  Transform = {
+    Rotation = {
+      Type = "TimelineRotation",
+      Keyframes = {
+        -- The first timeline entry
+        ["2000 JAN 01 00:00:00"] = {
+          Type = "StaticRotation",
+          Rotation = { math.pi, 0.0, 0.0 }
+        },
+        -- The second timeline entry
+        ["2000 JAN 01 12:00:00"] = {
+          Type = "StaticRotation",
+          Rotation = { 0.0, 0.0, 0.0 }
+        },
+        -- The third timeline entry
+        ["2000 JAN 01 23:59:59"] = {
+          Type = "StaticRotation",
+          Rotation = { 0.0, 0.0, math.pi }
+        }
+      },
+      ShouldInterpolate = false
+    }
+  },
+  Renderable = {
+    Type = "RenderableCartesianAxes"
+  },
+  GUI = {
+    Name = "TimelineRotation - No Interpolation",
+    Path = "/Examples"
+  }
+}
+
+asset.onInitialize(function()
+  openspace.addSceneGraphNode(Node)
+end)
+
+asset.onDeinitialize(function()
+  openspace.removeSceneGraphNode(Node)
+end)

data/assets/examples/rotation/timelinerotation/timeline.asset

@@ -0,0 +1,47 @@
+-- Basic
+-- This asset creates a SceneGraphNode that only displays coordinate axes. The rotation of
+-- the coordinate axes are determined by a timeline of individual rotations. These rotations
+-- are keyframes that are used to seamlessly change between different orientations. This
+-- example transitions between three rotations over a long time span. This example will
+-- only work if the in-game time is set to January 1st, 2000.
+
+local Node = {
+  Identifier = "TimelineRotation_Example",
+  Transform = {
+    Rotation = {
+      Type = "TimelineRotation",
+      Keyframes = {
+        -- The first timeline entry
+        ["2000 JAN 01 00:00:00"] = {
+          Type = "StaticRotation",
+          Rotation = { math.pi, 0.0, 0.0 }
+        },
+        -- The second timeline entry
+        ["2000 JAN 01 12:00:00"] = {
+          Type = "StaticRotation",
+          Rotation = { 0.0, 0.0, 0.0 }
+        },
+        -- The third timeline entry
+        ["2000 JAN 01 23:59:59"] = {
+          Type = "StaticRotation",
+          Rotation = { 0.0, 0.0, math.pi }
+        }
+      }
+    }
+  },
+  Renderable = {
+    Type = "RenderableCartesianAxes"
+  },
+  GUI = {
+    Name = "TimelineRotation - Basic",
+    Path = "/Examples"
+  }
+}
+
+asset.onInitialize(function()
+  openspace.addSceneGraphNode(Node)
+end)
+
+asset.onDeinitialize(function()
+  openspace.removeSceneGraphNode(Node)
+end)

modules/base/rotation/timelinerotation.cpp

@@ -107,14 +107,8 @@ glm::dmat3 TimelineRotation::matrix(const UpdateData& data) const {
         return glm::dmat3(glm::slerp(prevRot, nextRot, t));
     }
     else {
-        if (prevTime <= now && now < nextTime) {
-            return prev->data->matrix(data);
-        }
-        else if (nextTime <= now) {
-            return next->data->matrix(data);
-        }
+        return now < nextTime ? prev->data->matrix(data) : next->data->matrix(data);
     }
-    return glm::dmat3(0.0);
 }
 
 } // namespace openspace