GLTFRecorder: fix issue where resulting animation would have linear i…

…nterpolation for cases where a jump was expected

Runtime/Scripts/Timeline/GLTFRecorder.cs

@@ -230,7 +230,8 @@ internal class Track
 				private AnimationData tr;
 				private ExportPlan plan;
 				private Dictionary<double, object> samples;
-				private object lastSample;
+				private Tuple<double, object> lastSample = null;
+				private Tuple<double, object> secondToLastSample = null;
 
 				public Track(AnimationData tr, ExportPlan plan, double time)
 				{
@@ -245,11 +246,34 @@ public void SampleIfChanged(double time)
 					var value = plan.Sample(tr);
 					if (value == null || (value is Object o && !o))
 						return;
-					if (!value.Equals(lastSample))
-					{
-						samples[time] = value;
-						lastSample = value;
+					// As a memory optimization we want to be able to skip identical samples.
+					// But, we cannot always skip samples when they are identical to the previous one - otherwise cases like this break:
+					// - First assume an object is invisible at first (by having a scale of (0,0,0))
+					// - At some point in time, it is instantaneously set "visible" by updating its scale from (0,0,0) to (1,1,1)
+					// If we simply skip identical samples on insert, instead of a instantaneous
+					// visibility/scale changes we get a linearly interpolated scale change because only two samples will be recorded:
+					// - one (0,0,0) at the start of time
+					// - (1,1,1) at the time of the visibility change
+					// What we want to get is
+					// - one sample with (0,0,0) at the start,
+					// - one with the same value right before the instantaneous change,
+					// - and then at the time of the change, we need a sample with (1,1,1)
+					// With this setup, now the linear interpolation only has an effect in the very short duration between the last two samples and we get the animation we want.
+
+					// How do we achieve both?
+					// Always sample & record and then on adding the next sample(s) we check
+					// if the *last two* samples were identical to the current sample.
+					// If that is the case we can remove/overwrite the middle sample with the new value.
+					if (lastSample != null
+						&& secondToLastSample != null
+						&& lastSample.Item2.Equals(secondToLastSample.Item2)
+						&& lastSample.Item2.Equals(value)) {
+
+						samples.Remove(lastSample.Item1); 
 					}
+					samples[time] = value;
+					secondToLastSample = lastSample;
+					lastSample = new Tuple<double, object>(time, value);
 				}
 
 			}