Deducing height on the move

.idea/deploymentTargetSelector.xml

@@ -4,7 +4,7 @@
     <selectionStates>
       <SelectionState runConfigName="app">
         <option name="selectionMode" value="DROPDOWN" />
-        <DropdownSelection timestamp="2025-08-26T11:06:03.805342749Z">
+        <DropdownSelection timestamp="2025-09-22T09:08:24.101225364Z">
           <Target type="DEFAULT_BOOT">
             <handle>
               <DeviceId pluginId="LocalEmulator" identifier="path=/home/secondbeam/.config/.android/avd/Virtual_Pixel_3.avd" />

app/src/main/java/com/mirenkov/ktheightmap/KhmParser.kt

@@ -4,6 +4,8 @@ import android.content.Context
 import android.util.Log
 import java.io.DataInputStream
 import java.io.FileInputStream
+import kotlin.math.max
+import kotlin.math.min
 
 private data class HeightInfo(
     val minLon: Float,
@@ -16,46 +18,63 @@ private data class HeightInfo(
 
 class KhmParser {
     companion object {
+        private var header: HeightInfo? = null
         const val HEIGHT_FILE: String = "height.khm"
         fun load(filePath: String, ctx: Context) {
             if (ctx.getFileStreamPath(HEIGHT_FILE).exists()) {
-                Log.i(TAG, "height.khm already exists")
                 return
             }
 
             val inp = FileInputStream(filePath)
             ctx.openFileOutput(HEIGHT_FILE, Context.MODE_PRIVATE).use {
                 it.write(inp.readBytes())
-                Log.i(TAG, "Copied %s to height.khm".format(filePath))
             }
         }
 
         private fun getOffset(header: HeightInfo, x: Int, y: Int): Int {
-            Log.i(TAG, "Offset for (%d, %d) = %d".format(x, y, y * header.height + x))
             return (x * header.width + y) * 2
         }
 
         private fun readHeader(dis: DataInputStream): HeightInfo {
-            return with(dis) {
+            if (header == null) {
-                val minLon = readFloat()
+                header = with(dis) {
-                val minLat = readFloat()
+                    val minLon = readFloat()
-                val lonPerValue = readFloat()
+                    val minLat = readFloat()
-                val latPerValue = readFloat()
+                    val lonPerValue = readFloat()
-                val width = readInt()
+                    val latPerValue = readFloat()
-                val height = readInt()
+                    val width = readInt()
-                Log.i(TAG, "Header: %.6f, %.6f, %.6f, %.6f, %d, %d".format(minLon, minLat, lonPerValue, latPerValue, width, height))
+                    val height = readInt()
-                HeightInfo( minLon, minLat, lonPerValue, latPerValue, width, height )
+                    HeightInfo(minLon, minLat, lonPerValue, latPerValue, width, height)
+                }
+            } else {
+                dis.skipBytes(4 * 6)
             }
+            return header!!
+        }
+
+        private fun inBounds(header: HeightInfo, lon: Float, lat: Float): Boolean {
+            val maxLon = header.minLon + (header.lonPerValue * header.width)
+            val maxLat = header.minLat + (header.latPerValue * header.height)
+            val miLon = min(header.minLon, maxLon)
+            val maLon = max(header.minLon, maxLon)
+            val miLat = min(header.minLat, maxLat)
+            val maLat = max(header.minLat, maxLat)
+            return lon > miLon && lat > miLat && lon < maLon && lat < maLat
         }
 
         fun getHeight(lon: Float, lat: Float, ctx: Context): UShort {
             val dis = DataInputStream(ctx.openFileInput(HEIGHT_FILE))
-            val header = readHeader(dis)
+            dis.use {
-            val x = ((lon - header.minLon) / header.lonPerValue).toInt()
+                val header = readHeader(dis)
-            val y = ((lat - header.minLat) / header.latPerValue).toInt()
+                if (!inBounds(header, lon, lat))
-            val offset = getOffset(header, x, y)
+                    return 0u
-            dis.skipBytes(offset)
+                val x = ((lon - header.minLon) / header.lonPerValue).toInt()
-            return dis.readUnsignedShort().toUShort()
+                val y = ((lat - header.minLat) / header.latPerValue).toInt()
+                val offset = getOffset(header, x, y)
+                dis.skipBytes(offset)
+                val height = dis.readUnsignedShort().toUShort()
+                return height
+            }
         }
     }
 }

app/src/main/java/com/mirenkov/ktheightmap/MapCanvas.kt

@@ -16,6 +16,7 @@ import androidx.compose.ui.graphics.Path
 import androidx.compose.ui.graphics.asImageBitmap
 import androidx.compose.ui.graphics.drawscope.Stroke
 import androidx.compose.ui.input.pointer.pointerInput
+import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.text.ParagraphStyle
 import androidx.compose.ui.text.SpanStyle
 import androidx.compose.ui.text.buildAnnotatedString
@@ -33,6 +34,7 @@ fun MapCanvas(
     tileContainer: TileContainer,
     modifier: Modifier = Modifier
 ) {
+    val ctx = LocalContext.current
     var offsetX by rememberSaveable { viewModel.mapOffsetX }
     var offsetY by rememberSaveable { viewModel.mapOffsetY }
     val scale by rememberSaveable { viewModel.scale }
@@ -170,13 +172,14 @@ fun MapCanvas(
                     topLeft = latLonOffset
                 )
 
+                val lon = SphereMercator.mercateX(centerTileX, level)
+                val lat = SphereMercator.mercateY(centerTileY, level)
+
                 drawText(
                     textMeasurer = textMeasurer,
                     text = buildAnnotatedString {
                         withStyle(ParagraphStyle(textAlign = TextAlign.Center)) {
                             withStyle(SpanStyle(color = gridColor)) {
-                                val lon = SphereMercator.mercateX(centerTileX, level)
-                                val lat = SphereMercator.mercateY(centerTileY, level)
                                 append("%.6f\n%.6f".format(lon, lat))
                                 if (debug) {
                                     append("\n%.0f\n%.0f".format(offsetX, offsetY))
@@ -187,6 +190,20 @@ fun MapCanvas(
                     size = latLonSize,
                     topLeft = latLonOffset
                 )
+
+                drawText(
+                    textMeasurer = textMeasurer,
+                    text = buildAnnotatedString {
+                        withStyle(SpanStyle(color = Color.White)) {
+                            append(
+                                "%dm".format(
+                                    KhmParser.getHeight(lon.toFloat(), lat.toFloat(), ctx).toInt()
+                                )
+                            )
+                        }
+                    },
+                    topLeft = Offset(halvedX + crossRadius, halvedY + crossRadius)
+                )
             }
         }
     }