Add aging transparency support to MilStd2525 placemarks.

worldwind/src/commonMain/kotlin/earth/worldwind/shape/milstd2525/MilStd2525LevelOfDetailSelector.kt

@@ -3,6 +3,10 @@ package earth.worldwind.shape.milstd2525
 import earth.worldwind.render.RenderContext
 import earth.worldwind.shape.Placemark
 import earth.worldwind.shape.PlacemarkAttributes
+import kotlinx.datetime.Clock
+import kotlinx.datetime.Instant
+import kotlin.time.Duration.Companion.days
+import kotlin.time.Duration.Companion.hours
 
 /**
  * The [MilStd2525LevelOfDetailSelector] determines which set of [PlacemarkAttributes] to use for a [MilStd2525Placemark].
@@ -17,12 +21,38 @@ open class MilStd2525LevelOfDetailSelector : Placemark.LevelOfDetailSelector {
         protected const val MEDIUM_LEVEL_OF_DETAIL = 1
         protected const val HIGH_LEVEL_OF_DETAIL = 2
         protected const val HIGHEST_LEVEL_OF_DETAIL = 3
+
         /**
          * Controls the symbol modifiers visibility threshold
          */
         var modifiersThreshold = 5e4
+
+        /**
+         * Duration after which placemark becomes slightly transparent
+         */
+        var firstAgingThreshold = 1.hours
+
+        /**
+         * Fist level of transparency
+         */
+        var firstAgingAlpha = 0.75f
+
+        /**
+         * Duration after which placemark becomes moderate transparent
+         */
+        var secondAgingThreshold = 1.days
+
+        /**
+         * Second level of transparency
+         */
+        var secondAgingAlpha = 0.5f
     }
 
+    /**
+     * Base time to calculate transparency of aging Placemarks
+     */
+    var baseAgingTime = Instant.DISTANT_FUTURE
+
     protected var lastLevelOfDetail = -1
     protected var isHighlighted = false
     protected var isInvalidateRequested = false
@@ -89,6 +119,16 @@ open class MilStd2525LevelOfDetailSelector : Placemark.LevelOfDetailSelector {
         placemark.attributes.isDrawLeader = lastLevelOfDetail >= MEDIUM_LEVEL_OF_DETAIL
         placemark.attributes.imageScale = if (isHighlighted) HIGHLIGHTED_SCALE else NORMAL_SCALE
 
+        if (baseAgingTime != Instant.DISTANT_FUTURE) {
+            val agingTime = Clock.System.now() - baseAgingTime
+            // Make placemark translucent with time
+            placemark.attributes.imageColor.alpha = when {
+                !isHighlighted && agingTime > secondAgingThreshold -> secondAgingAlpha
+                !isHighlighted && agingTime > firstAgingThreshold -> firstAgingAlpha
+                else -> 1.0f // Opaque placemark by default
+            }
+        }
+
         return true
     }
 }