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private fun Int.getTransform(): Mat4 {
   val tm = modelViewer.engine.transformManager
   return Mat4.of(*tm.getTransform(tm.getInstance(this), null))
}

private fun Int.setTransform(mat: Mat4) {
   val tm = modelViewer.engine.transformManager
   tm.setTransform(tm.getInstance(this), mat.toFloatArray())
}

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// Reset the root transform, then rotate it around the Z axis.
modelViewer.asset?.apply {
   modelViewer.transformToUnitCube()
   val rootTransform = this.root.getTransform()
   val degrees = 20f * seconds.toFloat()
   val zAxis = Float3(0f, 0f, 1f)
   this.root.setTransform(rootTransform * rotation(zAxis, degrees))
}

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private val frameCallback = object : Choreographer.FrameCallback {
   private val startTime = System.nanoTime()
   override fun doFrame(currentTime: Long) {
       val seconds = (currentTime - startTime).toDouble() / 1_000_000_000
       choreographer.postFrameCallback(this)
       modelViewer.animator?.apply {
           if (animationCount > 0) {
               applyAnimation(0, seconds.toFloat())
           }
           updateBoneMatrices()

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override fun onCreate(savedInstanceState: Bundle?) {
   // ...
  
   //loadGlb("DamagedHelmet")
   loadGltf("BusterDrone")
   loadEnvironment("venetian_crossroads_2k")
}

private fun loadGltf(name: String) {
   val buffer = readAsset("models/${name}.gltf")

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override fun onCreate(savedInstanceState: Bundle?) {
   // ...
   loadGlb("DamagedHelmet")
   loadEnvironment("venetian_crossroads_2k")
}

private fun loadEnvironment(ibl: String) {
   // Create the indirect light source and add it to the scene.
   var buffer = readAsset("envs/$ibl/${ibl}_ibl.ktx")
   KtxLoader.createIndirectLight(modelViewer.engine, buffer).apply {

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override fun onCreate(savedInstanceState: Bundle?) {
   // ...
   loadGlb("DamagedHelmet")
   modelViewer.scene.skybox = Skybox.Builder().build(modelViewer.engine)
}

private fun loadGlb(name: String) {
   val buffer = readAsset("models/${name}.glb")
   modelViewer.loadModelGlb(buffer)
   modelViewer.transformToUnitCube()

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private val frameCallback = object : Choreographer.FrameCallback {
   override fun doFrame(currentTime: Long) {
       choreographer.postFrameCallback(this)
       modelViewer.render(currentTime)
   }
}

override fun onResume() {
   super.onResume()
   choreographer.postFrameCallback(frameCallback)

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private lateinit var surfaceView: SurfaceView
private lateinit var choreographer: Choreographer
private lateinit var modelViewer: ModelViewer

override fun onCreate(savedInstanceState: Bundle?) {
   super.onCreate(savedInstanceState)
   surfaceView = SurfaceView(this).apply { setContentView(this) }
   choreographer = Choreographer.getInstance()
   modelViewer = ModelViewer(surfaceView)
   surfaceView.setOnTouchListener(modelViewer)

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dependencies {
   implementation 'com.google.android.filament:filament-android:1.6.0'
   implementation 'com.google.android.filament:filament-utils-android:1.6.0'
   implementation 'com.google.android.filament:gltfio-android:1.6.0'
   // ...
}

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import com.google.android.filament.utils.*

class MainActivity : Activity() {

   companion object {
       init { Utils.init() }
   }

   // ...