Refactor transparency log

ground/src/main/java/com/google/android/ground/ui/map/gms/ClippingTileProvider.kt

@@ -16,17 +16,14 @@
 
 package com.google.android.ground.ui.map.gms
 
-import android.graphics.Bitmap
-import android.graphics.BitmapFactory
-import android.graphics.Color
 import com.google.android.gms.maps.model.LatLngBounds
 import com.google.android.gms.maps.model.Tile
 import com.google.android.gms.maps.model.TileProvider
 import com.google.android.gms.maps.model.TileProvider.NO_TILE
+import com.google.android.ground.ui.map.gms.mog.ImageEditor
 import com.google.android.ground.ui.map.gms.mog.TileCoordinates
 import com.google.android.ground.ui.map.gms.mog.toPixelBounds
 import com.google.android.ground.ui.map.gms.mog.toPixelCoordinate
-import java.io.ByteArrayOutputStream
 
 private const val MAX_ZOOM = 19
 
@@ -47,24 +44,11 @@ class ClippingTileProvider(
 
   private fun clipToBounds(tileCoords: TileCoordinates, tile: Tile): Tile {
     if (tile.data == null) return NO_TILE
-    val opts = BitmapFactory.Options()
-    opts.inMutable = true
-    val bitmap = BitmapFactory.decodeByteArray(tile.data, 0, tile.data!!.size, opts)
-    bitmap.setHasAlpha(true)
-    for (y in 0 until bitmap.height) {
-      for (x in 0 until bitmap.width) {
+    val output =
+      ImageEditor.setTransparentIf(tile.data!!) { _, x, y ->
         val pixelCoords = tileCoords.toPixelCoordinate(x, y)
-        if (pixelBounds.none { it.contains(pixelCoords) }) {
-          bitmap.setPixel(x, y, Color.TRANSPARENT)
-        }
+        pixelBounds.none { it.contains(pixelCoords) }
       }
-    }
-    // Android doesn't implement encoders for uncompressed format, so we must compress the returned
-    // tile so that it can be later encoded by Maps SDK. Experimentally, decompressing JPG and
-    // compressing WEBP each tile adds on the order of 1ms to each tile which we can consider
-    // negligible.
-    val stream = ByteArrayOutputStream()
-    bitmap.compress(Bitmap.CompressFormat.WEBP, 100, stream)
-    return Tile(tile.width, tile.height, stream.toByteArray())
+    return Tile(tile.width, tile.height, output)
   }
 }

ground/src/main/java/com/google/android/ground/ui/map/gms/mog/ImageEditor.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2023 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.android.ground.ui.map.gms.mog
+
+import android.graphics.Bitmap
+import android.graphics.BitmapFactory
+import android.graphics.Color
+import java.io.ByteArrayOutputStream
+
+/** Utility for modifying images. */
+object ImageEditor {
+  /**
+   * Decodes the provided source image, making all pixels for which the provided lambda returns
+   * `true` fully transparent (alpha = 0). Returns the modified images as a byte array.
+   */
+  fun setTransparentIf(
+    sourceImage: ByteArray,
+    isTransparent: (bitmap: Bitmap, x: Int, y: Int) -> Boolean
+  ): ByteArray {
+    val opts = BitmapFactory.Options()
+    opts.inMutable = true
+    val bitmap = BitmapFactory.decodeByteArray(sourceImage, 0, sourceImage.size, opts)
+    bitmap.setHasAlpha(true)
+    for (y in 0 until bitmap.height) {
+      for (x in 0 until bitmap.width) {
+        if (isTransparent(bitmap, x, y)) {
+          bitmap.setPixel(x, y, Color.TRANSPARENT)
+        }
+      }
+    }
+    // Android doesn't implement encoders for uncompressed format, so we must compress the returned
+    // tile so that it can be later encoded by Maps SDK. Experimentally, decompressing JPG and
+    // compressing WEBP each tile adds on the order of 1ms to each tile which we can consider
+    // negligible.
+    val stream = ByteArrayOutputStream()
+    bitmap.compress(Bitmap.CompressFormat.WEBP, 100, stream)
+    return stream.toByteArray()
+  }
+}

ground/src/main/java/com/google/android/ground/ui/map/gms/mog/MogTile.kt

@@ -16,11 +16,8 @@
 
 package com.google.android.ground.ui.map.gms.mog
 
-import android.graphics.Bitmap
-import android.graphics.BitmapFactory
 import android.graphics.Color
 import com.google.android.gms.maps.model.Tile
-import java.io.ByteArrayOutputStream
 
 /** Metadata and image data for a single tile. */
 class MogTile(val metadata: MogTileMetadata, val data: ByteArray) {
@@ -31,32 +28,15 @@ class MogTile(val metadata: MogTileMetadata, val data: ByteArray) {
       .toByteArray()
 
   fun toGmsTile(): Tile {
+    val noData = metadata.noDataValue
     val imageData =
-      if (metadata.noDataValue == null) buildJfifFile()
-      else maskNoDataValues(buildJfifFile(), metadata.noDataValue)
+      if (noData == null) buildJfifFile()
+      else maskNoDataValues(buildJfifFile(), Color.rgb(noData, noData, noData))
     return Tile(metadata.width, metadata.height, imageData)
   }
 
-  private fun maskNoDataValues(jfifData: ByteArray, noDataValue: Int): ByteArray {
-    val opts = BitmapFactory.Options()
-    opts.inMutable = true
-    val bitmap = BitmapFactory.decodeByteArray(jfifData, 0, jfifData.size, opts)
-    bitmap.setHasAlpha(true)
-    for (y in 0 until bitmap.height) {
-      for (x in 0 until bitmap.width) {
-        if (bitmap.getPixel(x, y) == Color.rgb(noDataValue, noDataValue, noDataValue)) {
-          bitmap.setPixel(x, y, Color.TRANSPARENT)
-        }
-      }
-    }
-    // Android doesn't implement encoders for uncompressed format, so we must compress the returned
-    // tile so that it can be later encoded by Maps SDK. Experimentally, decompressing JPG and
-    // compressing WEBP each tile adds on the order of 1ms to each tile which we can consider
-    // negligible.
-    val stream = ByteArrayOutputStream()
-    bitmap.compress(Bitmap.CompressFormat.WEBP, 100, stream)
-    return stream.toByteArray()
-  }
+  private fun maskNoDataValues(jfifData: ByteArray, noDataColor: Int): ByteArray =
+    ImageEditor.setTransparentIf(jfifData) { bitmap, x, y -> bitmap.getPixel(x, y) == noDataColor }
 
   private fun buildJfifFile(): ByteArray =
     START_OF_IMAGE +