Fix speed-colored track: pre-compute hex color per segment, use get("color")

Instead of asking MapLibre to interpolate/step colors at render time
(which was silently failing), pre-compute the CSS hex color for each
segment in speedSegments() and store it as a "color" property.
speedColorExpression() is now Expression.get("color") — a simple
property lookup that MapLibre parses as a CSS color for line-color.

https://claude.ai/code/session_01YUbuZNDAoLea4cf9UGQ9qn

1 files changed, 19 insertions, 12 deletions

diff --git a/android-app/app/src/main/kotlin/org/terst/nav/track/TrackColors.kt b/android-app/app/src/main/kotlin/org/terst/nav/track/TrackColors.kt
index 145a4dd..292240a 100644
--- a/android-app/app/src/main/kotlin/org/terst/nav/track/TrackColors.kt
+++ b/android-app/app/src/main/kotlin/org/terst/nav/track/TrackColors.kt
@@ -5,7 +5,18 @@ import org.maplibre.geojson.Feature
 import org.maplibre.geojson.LineString
 import org.maplibre.geojson.Point
 
-/** Breaks [points] into consecutive 2-point LineString Features, each tagged with "speed" (knots). */
+private fun speedToHex(speedKt: Double): String = when {
+    speedKt >= 13 -> "#F44336"  // red
+    speedKt >= 10 -> "#FFC107"  // amber
+    speedKt >= 7  -> "#4CAF50"  // green
+    speedKt >= 4  -> "#00BCD4"  // cyan
+    else          -> "#2196F3"  // blue
+}
+
+/**
+ * Breaks [points] into consecutive 2-point LineString Features.
+ * Each feature carries "speed" (knots) and "color" (CSS hex) properties.
+ */
 fun speedSegments(points: List<TrackPoint>): List<Feature> =
     (0 until points.size - 1).map { i ->
         val sog = (points[i].sogKnots + points[i + 1].sogKnots) / 2.0
@@ -14,18 +25,14 @@ fun speedSegments(points: List<TrackPoint>): List<Feature> =
                 Point.fromLngLat(points[i].lon, points[i].lat),
                 Point.fromLngLat(points[i + 1].lon, points[i + 1].lat)
             ))
-        ).apply { addNumberProperty("speed", sog) }
+        ).apply {
+            addNumberProperty("speed", sog)
+            addStringProperty("color", speedToHex(sog))
+        }
     }
 
 /**
- * MapLibre expression that step-interpolates line-color from the "speed" feature property.
- * Scale: 0 kt → blue · 4 kt → cyan · 7 kt → green · 10 kt → amber · 13+ kt → red.
+ * Returns the lineColor expression for speed-colored segments.
+ * Reads the pre-computed "color" string property from each feature.
  */
-fun speedColorExpression(): Expression = Expression.step(
-    Expression.get("speed"),
-    Expression.literal("#2196F3"),   // < 4 kt: blue
-    Expression.stop(4,  Expression.literal("#00BCD4")),  // cyan
-    Expression.stop(7,  Expression.literal("#4CAF50")),  // green
-    Expression.stop(10, Expression.literal("#FFC107")),  // amber
-    Expression.stop(13, Expression.literal("#F44336"))   // red
-)
+fun speedColorExpression(): Expression = Expression.get("color")