feat: add object detection (android) (#52)

## Description
<!-- Provide a concise and descriptive summary of the changes
implemented in this PR. -->

### Type of change
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] Documentation update (improves or adds clarity to existing
documentation)

### Tested on
- [ ] iOS
- [x] Android

### Testing instructions
<!-- Provide step-by-step instructions on how to test your changes.
Include setup details if necessary. -->

### Screenshots
<!-- Add screenshots here, if applicable -->

### Related issues
<!-- Link related issues here using #issue-number -->

### Checklist
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have updated the documentation accordingly
- [ ] My changes generate no new warnings

### Additional notes
<!-- Include any additional information, assumptions, or context that
reviewers might need to understand this PR. -->

android/build.gradle

@@ -1,4 +1,8 @@
 buildscript {
+  ext {
+    agp_version = '8.4.2'
+  }
+
   // Buildscript is evaluated before everything else so we can't use getExtOrDefault
   def kotlin_version = rootProject.ext.has("kotlinVersion") ? rootProject.ext.get("kotlinVersion") : project.properties["RnExecutorch_kotlinVersion"]
 
@@ -9,7 +13,7 @@ buildscript {
   }
 
   dependencies {
-    classpath "com.android.tools.build:gradle:7.2.1"
+    classpath "com.android.tools.build:gradle:$agp_version"
     // noinspection DifferentKotlinGradleVersion
     classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
   }
@@ -95,7 +99,8 @@ dependencies {
   // For < 0.71, this will be from the local maven repo
   // For > 0.71, this will be replaced by `com.facebook.react:react-android:$version` by react gradle plugin
   //noinspection GradleDynamicVersion
-  implementation "com.facebook.react:react-native:+"
+  implementation "com.facebook.react:react-android:+"
+  implementation 'org.opencv:opencv:4.10.0'
   implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
   implementation 'com.github.software-mansion:react-native-executorch:main-SNAPSHOT'
   implementation 'org.opencv:opencv:4.10.0'

android/src/main/java/com/swmansion/rnexecutorch/ObjectDetection.kt

@@ -0,0 +1,61 @@
+package com.swmansion.rnexecutorch
+
+import android.util.Log
+import com.facebook.react.bridge.Arguments
+import com.facebook.react.bridge.Promise
+import com.facebook.react.bridge.ReactApplicationContext
+import com.facebook.react.bridge.WritableArray
+import com.swmansion.rnexecutorch.models.BaseModel
+import com.swmansion.rnexecutorch.utils.ETError
+import com.swmansion.rnexecutorch.utils.ImageProcessor
+import org.opencv.android.OpenCVLoader
+import com.swmansion.rnexecutorch.models.objectdetection.SSDLiteLargeModel
+import org.opencv.core.Mat
+
+class ObjectDetection(reactContext: ReactApplicationContext) :
+  NativeObjectDetectionSpec(reactContext) {
+
+  private lateinit var ssdLiteLarge: SSDLiteLargeModel
+
+  companion object {
+    const val NAME = "ObjectDetection"
+  }
+
+  init {
+    if(!OpenCVLoader.initLocal()){
+      Log.d("rn_executorch", "OpenCV not loaded")
+    } else {
+      Log.d("rn_executorch", "OpenCV loaded")
+    }
+  }
+
+  override fun loadModule(modelSource: String, promise: Promise) {
+    try {
+      ssdLiteLarge = SSDLiteLargeModel(reactApplicationContext)
+      ssdLiteLarge.loadModel(modelSource)
+      promise.resolve(0)
+    } catch (e: Exception) {
+      promise.reject(e.message!!, ETError.InvalidModelPath.toString())
+    }
+  }
+
+  override fun forward(input: String, promise: Promise) {
+    try {
+      val inputImage = ImageProcessor.readImage(input)
+      val output = ssdLiteLarge.runModel(inputImage)
+      val outputWritableArray: WritableArray = Arguments.createArray()
+      output.map { detection ->
+        detection.toWritableMap()
+      }.forEach { writableMap ->
+        outputWritableArray.pushMap(writableMap)
+      }
+      promise.resolve(outputWritableArray)
+    } catch(e: Exception){
+      promise.reject(e.message!!, e.message)
+    }
+  }
+
+  override fun getName(): String {
+    return NAME
+  }
+}

android/src/main/java/com/swmansion/rnexecutorch/RnExecutorchPackage.kt

@@ -21,7 +21,9 @@ class RnExecutorchPackage : TurboReactPackage() {
       StyleTransfer(reactContext)
     } else if (name == Classification.NAME) {
       Classification(reactContext)
-    } 
+    } else if (name == ObjectDetection.NAME) {
+      ObjectDetection(reactContext)
+    }
     else {
       null
     }
@@ -63,6 +65,15 @@ class RnExecutorchPackage : TurboReactPackage() {
         false,  // isCxxModule
         true
       )
+
+      moduleInfos[ObjectDetection.NAME] = ReactModuleInfo(
+        ObjectDetection.NAME,
+        ObjectDetection.NAME,
+        false,  // canOverrideExistingModule
+        false,  // needsEagerInit
+        false,  // isCxxModule
+        true
+      )
       moduleInfos
     }
   }

android/src/main/java/com/swmansion/rnexecutorch/models/object_detection/SSDLiteLargeModel.kt

@@ -0,0 +1,71 @@
+package com.swmansion.rnexecutorch.models.objectdetection
+
+import com.facebook.react.bridge.ReactApplicationContext
+import com.swmansion.rnexecutorch.utils.ImageProcessor
+import org.opencv.core.Mat
+import org.opencv.core.Size
+import org.opencv.imgproc.Imgproc
+import com.swmansion.rnexecutorch.models.BaseModel
+import com.swmansion.rnexecutorch.utils.Bbox
+import com.swmansion.rnexecutorch.utils.CocoLabel
+import com.swmansion.rnexecutorch.utils.Detection
+import com.swmansion.rnexecutorch.utils.nms
+import org.pytorch.executorch.EValue
+
+const val detectionScoreThreshold = .7f
+const val iouThreshold = .55f
+
+class SSDLiteLargeModel(reactApplicationContext: ReactApplicationContext) : BaseModel<Mat, Array<Detection>>(reactApplicationContext) {
+  private var heightRatio: Float = 1.0f
+  private var widthRatio: Float = 1.0f
+
+  private fun getModelImageSize(): Size {
+    val inputShape = module.getInputShape(0)
+    val width = inputShape[inputShape.lastIndex]
+    val height = inputShape[inputShape.lastIndex - 1]
+
+    return Size(height.toDouble(), width.toDouble())
+  }
+
+  override fun preprocess(input: Mat): EValue {
+    this.widthRatio = (input.size().width / getModelImageSize().width).toFloat()
+    this.heightRatio = (input.size().height / getModelImageSize().height).toFloat()
+    Imgproc.resize(input, input, getModelImageSize())
+    return ImageProcessor.matToEValue(input, module.getInputShape(0))
+  }
+
+  override fun runModel(input: Mat): Array<Detection> {
+    val modelInput = preprocess(input)
+    val modelOutput = forward(modelInput)
+    return postprocess(modelOutput)
+  }
+
+  override fun postprocess(output: Array<EValue>): Array<Detection> {
+    val scoresTensor = output[1].toTensor()
+    val numel = scoresTensor.numel()
+    val bboxes = output[0].toTensor().dataAsFloatArray
+    val scores = scoresTensor.dataAsFloatArray
+    val labels = output[2].toTensor().dataAsFloatArray
+
+    val detections: MutableList<Detection> = mutableListOf();
+    for (idx in 0 until numel.toInt()) {
+      val score = scores[idx]
+      if (score < detectionScoreThreshold) {
+        continue
+      }
+      val bbox = Bbox(
+        bboxes[idx * 4 + 0] * this.widthRatio,
+        bboxes[idx * 4 + 1] * this.heightRatio,
+        bboxes[idx * 4 + 2] * this.widthRatio,
+        bboxes[idx * 4 + 3] * this.heightRatio
+      )
+      val label = labels[idx]
+      detections.add(
+        Detection(bbox, score, CocoLabel.fromId(label.toInt())!!)
+      )
+    }
+
+    val detectionsPostNms = nms(detections, iouThreshold);
+    return detectionsPostNms.toTypedArray()
+  }
+}

android/src/main/java/com/swmansion/rnexecutorch/utils/ObjectDetectionUtils.kt

@@ -0,0 +1,196 @@
+package com.swmansion.rnexecutorch.utils
+
+import com.facebook.react.bridge.Arguments
+import com.facebook.react.bridge.WritableMap
+
+fun nms(
+  detections: MutableList<Detection>,
+  iouThreshold: Float
+): List<Detection> {
+  if (detections.isEmpty()) {
+    return emptyList()
+  }
+
+  // Sort detections first by label, then by score (descending)
+  val sortedDetections = detections.sortedWith(compareBy({ it.label }, { -it.score }))
+
+  val result = mutableListOf<Detection>()
+
+  // Process NMS for each label group
+  var i = 0
+  while (i < sortedDetections.size) {
+    val currentLabel = sortedDetections[i].label
+
+    // Collect detections for the current label
+    val labelDetections = mutableListOf<Detection>()
+    while (i < sortedDetections.size && sortedDetections[i].label == currentLabel) {
+      labelDetections.add(sortedDetections[i])
+      i++
+    }
+
+    // Filter out detections with high IoU
+    val filteredLabelDetections = mutableListOf<Detection>()
+    while (labelDetections.isNotEmpty()) {
+      val current = labelDetections.removeAt(0)
+      filteredLabelDetections.add(current)
+
+      // Remove detections that overlap with the current detection above the IoU threshold
+      val iterator = labelDetections.iterator()
+      while (iterator.hasNext()) {
+        val other = iterator.next()
+        if (calculateIoU(current.bbox, other.bbox) > iouThreshold) {
+          iterator.remove()  // Remove detection if IoU is above threshold
+        }
+      }
+    }
+
+    // Add the filtered detections to the result
+    result.addAll(filteredLabelDetections)
+  }
+
+  return result
+}
+
+fun calculateIoU(bbox1: Bbox, bbox2: Bbox): Float {
+  val x1 = maxOf(bbox1.x1, bbox2.x1)
+  val y1 = maxOf(bbox1.y1, bbox2.y1)
+  val x2 = minOf(bbox1.x2, bbox2.x2)
+  val y2 = minOf(bbox1.y2, bbox2.y2)
+
+  val intersectionArea = maxOf(0f, x2 - x1) * maxOf(0f, y2 - y1)
+  val bbox1Area = (bbox1.x2 - bbox1.x1) * (bbox1.y2 - bbox1.y1)
+  val bbox2Area = (bbox2.x2 - bbox2.x1) * (bbox2.y2 - bbox2.y1)
+
+  val unionArea = bbox1Area + bbox2Area - intersectionArea
+  return if (unionArea == 0f) 0f else intersectionArea / unionArea
+}
+
+
+data class Bbox(
+  val x1: Float,
+  val y1: Float,
+  val x2: Float,
+  val y2: Float
+) {
+  fun toWritableMap(): WritableMap {
+    val map = Arguments.createMap()
+    map.putDouble("x1", x1.toDouble())
+    map.putDouble("x2", x2.toDouble())
+    map.putDouble("y1", y1.toDouble())
+    map.putDouble("y2", y2.toDouble())
+    return map
+  }
+}
+
+
+data class Detection(
+  val bbox: Bbox,
+  val score: Float,
+  val label: CocoLabel,
+) {
+  fun toWritableMap(): WritableMap {
+    val map = Arguments.createMap()
+    map.putMap("bbox", bbox.toWritableMap())
+    map.putDouble("score", score.toDouble())
+    map.putString("label", label.name)
+    return map
+  }
+}
+
+enum class CocoLabel(val id: Int) {
+  PERSON(1),
+  BICYCLE(2),
+  CAR(3),
+  MOTORCYCLE(4),
+  AIRPLANE(5),
+  BUS(6),
+  TRAIN(7),
+  TRUCK(8),
+  BOAT(9),
+  TRAFFIC_LIGHT(10),
+  FIRE_HYDRANT(11),
+  STREET_SIGN(12),
+  STOP_SIGN(13),
+  PARKING(14),
+  BENCH(15),
+  BIRD(16),
+  CAT(17),
+  DOG(18),
+  HORSE(19),
+  SHEEP(20),
+  COW(21),
+  ELEPHANT(22),
+  BEAR(23),
+  ZEBRA(24),
+  GIRAFFE(25),
+  HAT(26),
+  BACKPACK(27),
+  UMBRELLA(28),
+  SHOE(29),
+  EYE(30),
+  HANDBAG(31),
+  TIE(32),
+  SUITCASE(33),
+  FRISBEE(34),
+  SKIS(35),
+  SNOWBOARD(36),
+  SPORTS(37),
+  KITE(38),
+  BASEBALL(39),
+  SKATEBOARD(41),
+  SURFBOARD(42),
+  TENNIS_RACKET(43),
+  BOTTLE(44),
+  PLATE(45),
+  WINE_GLASS(46),
+  CUP(47),
+  FORK(48),
+  KNIFE(49),
+  SPOON(50),
+  BOWL(51),
+  BANANA(52),
+  APPLE(53),
+  SANDWICH(54),
+  ORANGE(55),
+  BROCCOLI(56),
+  CARROT(57),
+  HOT_DOG(58),
+  PIZZA(59),
+  DONUT(60),
+  CAKE(61),
+  CHAIR(62),
+  COUCH(63),
+  POTTED_PLANT(64),
+  BED(65),
+  MIRROR(66),
+  DINING_TABLE(67),
+  WINDOW(68),
+  DESK(69),
+  TOILET(70),
+  DOOR(71),
+  TV(72),
+  LAPTOP(73),
+  MOUSE(74),
+  REMOTE(75),
+  KEYBOARD(76),
+  CELL_PHONE(77),
+  MICROWAVE(78),
+  OVEN(79),
+  TOASTER(80),
+  SINK(81),
+  REFRIGERATOR(82),
+  BLENDER(83),
+  BOOK(84),
+  CLOCK(85),
+  VASE(86),
+  SCISSORS(87),
+  TEDDY_BEAR(88),
+  HAIR_DRIER(89),
+  TOOTHBRUSH(90),
+  HAIR_BRUSH(91);
+
+  companion object {
+    private val idToLabelMap = values().associateBy(CocoLabel::id)
+    fun fromId(id: Int): CocoLabel? = idToLabelMap[id]
+  }
+}