fix: conversion of a NoData ConstantTile should result in a NoData ConstantTile

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 - MinResample should ignore Int NODATA values [#3590](https://github.com/locationtech/geotrellis/pull/3590)
+- Empty (NoData) ConstantTile objects are now correctly converted to empty ConstantTile objects of the desired CellType 
 
 ## [3.8.0] - 2025-04-23
 

raster/src/main/scala/geotrellis/raster/ConstantTile.scala

@@ -80,17 +80,22 @@ abstract class ConstantTile extends Tile {
     * @param   newType  The type of cells that the result should have
     * @return            The new Tile
     */
-  def convert(newType: CellType): Tile =
-    newType match {
-      case BitCellType => new BitConstantTile(if (iVal == 0) false else true, cols, rows)
-      case ct: ByteCells => ByteConstantTile(i2b(iVal), cols, rows, ct)
-      case ct: UByteCells => UByteConstantTile(iVal.toByte, cols, rows, ct)
-      case ct: ShortCells => ShortConstantTile(i2s(iVal), cols, rows, ct)
-      case ct: UShortCells => UShortConstantTile(i2us(iVal) , cols, rows, ct)
-      case ct: IntCells => IntConstantTile(iVal, cols, rows, ct)
-      case ct: FloatCells => FloatConstantTile(d2f(dVal), cols, rows, ct)
-      case ct: DoubleCells => DoubleConstantTile(dVal, cols, rows, ct)
+  def convert(newType: CellType): Tile = {
+    if (isNoDataTile) {
+      ConstantTile.empty(newType, cols, rows)
+    } else {
+      newType match {
+        case BitCellType => new BitConstantTile(if (iVal == 0) false else true, cols, rows)
+        case ct: ByteCells => ByteConstantTile(i2b(iVal), cols, rows, ct)
+        case ct: UByteCells => UByteConstantTile(iVal.toByte, cols, rows, ct)
+        case ct: ShortCells => ShortConstantTile(i2s(iVal), cols, rows, ct)
+        case ct: UShortCells => UShortConstantTile(i2us(iVal) , cols, rows, ct)
+        case ct: IntCells => IntConstantTile(iVal, cols, rows, ct)
+        case ct: FloatCells => FloatConstantTile(d2f(dVal), cols, rows, ct)
+        case ct: DoubleCells => DoubleConstantTile(dVal, cols, rows, ct)
+      }
     }
+  }
 
   def interpretAs(newCellType: CellType): Tile =
     withNoData(None).convert(newCellType)
@@ -218,6 +223,14 @@ abstract class ConstantTile extends Tile {
     }
     tile
   }
+
+  override def isNoDataTile: Boolean = {
+    if (cellType.isFloatingPoint) {
+      !isData(getDouble(0, 0))
+    } else {
+      !isData(get(0, 0))
+    }
+  }
 }
 
 object ConstantTile {

raster/src/test/scala/geotrellis/raster/ConstantTileSpec.scala

@@ -150,5 +150,43 @@ class ConstantTileSpec extends AnyFunSpec with Matchers with RasterMatchers with
     }
   }
 
+  describe("conversion of empty tiles of CellTypes that support NoData should result in NoData tiles") {
+    List(
+      // BitCellType,
+      ByteUserDefinedNoDataCellType(1.toByte),
+      ByteConstantNoDataCellType,
+      // ByteCellType,
+      UByteConstantNoDataCellType,
+      UByteUserDefinedNoDataCellType(1.toByte),
+      // UByteCellType,
+      ShortUserDefinedNoDataCellType(1.toShort),
+      ShortConstantNoDataCellType,
+      // ShortCellType,
+      UShortUserDefinedNoDataCellType(1.toShort),
+      UShortConstantNoDataCellType,
+      // UShortCellType,
+      IntUserDefinedNoDataCellType(1),
+      IntConstantNoDataCellType,
+      // IntCellType,
+      FloatUserDefinedNoDataCellType(1.0f),
+      FloatConstantNoDataCellType,
+      // FloatCellType,
+      DoubleUserDefinedNoDataCellType(1.0),
+      DoubleConstantNoDataCellType,
+      // DoubleCellType
+    ).foreach { cellType =>
+      it(s"should convert an empty tile of $cellType to an empty tile of the new cell type") {
+        val tile = ConstantTile.empty(cellType, 1, 1)
+        assert(tile.isNoDataTile)
+        assert(tile.cellType == cellType)
+        val newCellType = FloatUserDefinedNoDataCellType(666.0f)
+        val convertedNoDataTile = tile.convert(newCellType)
+        assert(convertedNoDataTile.isNoDataTile)
+        assert(convertedNoDataTile.cellType == newCellType)
+      }
+    }
+  }
+
+
   private def getClassName[T](obj: T): String = obj.getClass.getName.split("\\.").last
 }