# Perform hit-tests in your Android app

Perform a [hit-test](https://developers.google.com/ar/develop/hit-test) to determine the correct placement of a 3D object in your scene. Correct placement ensures that the AR content is rendered at the appropriate (apparent) size.

## Hit result types

A hit-test can yield four different types of hit results, as shown by the following table.

| Hit result type | Description | Orientation | Use case | Method calls |
|---|---|---|---|---|
| Depth ([`DepthPoint`](https://developers.google.com/ar/reference/java/com/google/ar/core/DepthPoint)) | Uses depth information from the entire scene to determine a point's correct depth and orientation | Perpendicular to the 3D surface | Place a virtual object on an arbitrary surface (not just on floors and walls) | **[`ArDepthMode`](https://developers.google.com/ar/reference/c/group/ar-config#ardepthmode) must be enabled for this to work.** [`Frame.hitTest(...)`](https://developers.google.com/ar/reference/java/com/google/ar/core/Frame#hitTest-motionEvent), check for [`DepthPoint`](https://developers.google.com/ar/reference/java/com/google/ar/core/DepthPoint)s in the return list |
| [`Plane`](https://developers.google.com/ar/reference/java/com/google/ar/core/Plane) | Hits horizontal and/or vertical surfaces to determine a point's correct depth and orientation | Perpendicular to the 3D surface | Place an object on a plane (floor or wall) using the plane's full geometry. Need correct scale immediately. Fallback for the Depth hit-test | [`Frame.hitTest(...)`](https://developers.google.com/ar/reference/java/com/google/ar/core/Frame#hitTest-motionEvent), check for [`Plane`](https://developers.google.com/ar/reference/java/com/google/ar/core/Plane)s in the return list |
| Feature point ([`Point`](https://developers.google.com/ar/reference/java/com/google/ar/core/Point)) | Relies on visual features around the point of a user tap to determine a point's correct position and orientation | Perpendicular to the 3D surface | Place an object on an arbitrary surface (not just on floors and walls) | [`Frame.hitTest(...)`](https://developers.google.com/ar/reference/java/com/google/ar/core/Frame#hitTest-motionEvent), check for [`Point`](https://developers.google.com/ar/reference/java/com/google/ar/core/Point)s in the return list |
| Instant Placement ([`InstantPlacementPoint`](https://developers.google.com/ar/reference/java/com/google/ar/core/InstantPlacementPoint)) | Uses screen space to place content. Initially uses estimated depth provided by the app. Works instantly, but pose and actual depth will change once ARCore is able to determine actual scene geometry | +Y pointing up, opposite to gravity | Place an object on a plane (floor or wall) using the plane's full geometry where fast placement is critical, and the experience can tolerate unknown initial depth and scale | [`Frame.hitTestInstantPlacement(float, float, float)`](https://developers.google.com/ar/reference/java/com/google/ar/core/Frame#hitTestInstantPlacement-xPx-yPx-approximateDistanceMeters) |

## Perform a standard hit-test

Call [`Frame.hitTest()`](https://developers.google.com/ar/reference/java/com/google/ar/core/Frame#hitTest-motionEvent) to perform a hit-test, using the [`TapHelper`](https://github.com/google-ar/arcore-android-sdk/blob/c684bbda37e44099c273c3e5274fae6fccee293c/samples/hello_ar_java/app/src/main/java/com/google/ar/core/examples/java/common/helpers/TapHelper.java) utility to obtain [`MotionEvent`](https://developer.android.com/reference/android/view/MotionEvent)s from the AR view.

### Java

```java
MotionEvent tap = tapHelper.poll();
if (tap == null) {
  return;
}

if (usingInstantPlacement) {
  // When using Instant Placement, the value in APPROXIMATE_DISTANCE_METERS will determine
  // how far away the anchor will be placed, relative to the camera's view.
  List<HitResult> hitResultList =
      frame.hitTestInstantPlacement(tap.getX(), tap.getY(), APPROXIMATE_DISTANCE_METERS);
  // Hit-test results using Instant Placement will only have one result of type
  // InstantPlacementResult.
} else {
  List<HitResult> hitResultList = frame.hitTest(tap);
  // TODO: Filter hitResultList to find a hit result of interest.
}
```

### Kotlin

```kotlin
val tap = tapHelper.poll() ?: return
val hitResultList =
  if (usingInstantPlacement) {
    // When using Instant Placement, the value in APPROXIMATE_DISTANCE_METERS will determine
    // how far away the anchor will be placed, relative to the camera's view.
    frame.hitTestInstantPlacement(tap.x, tap.y, APPROXIMATE_DISTANCE_METERS)
    // Hit-test results using Instant Placement will only have one result of type
    // InstantPlacementResult.
  } else {
    frame.hitTest(tap)
  }
```

Filter hit results based on the type you're interested in. For example, if you'd like to focus on `DepthPoint`s:

### Java

```java
// Returned hit-test results are sorted by increasing distance from the camera or virtual ray's
// origin.
// The first hit result is often the most relevant when responding to user input.
for (HitResult hit : hitResultList) {
  Trackable trackable = hit.getTrackable();
  if (trackable instanceof DepthPoint) { // Replace with any type of trackable type
    // Do something with this hit result. For example, create an anchor at this point of
    // interest.
    Anchor anchor = hit.createAnchor();
    // TODO: Use this anchor in your AR experience.
    break;
  }
}
```

### Kotlin

```kotlin
// Returned hit-test results are sorted by increasing distance from the camera or virtual ray's
// origin.
// The first hit result is often the most relevant when responding to user input.
val firstHitResult =
  hitResultList.firstOrNull { hit ->
    when (val trackable = hit.trackable!!) {
      is DepthPoint -> true // Replace with any type of trackable type
      else -> false
    }
  }
if (firstHitResult != null) {
  // Do something with this hit result. For example, create an anchor at this point of interest.
  val anchor = firstHitResult.createAnchor()
  // TODO: Use this anchor in your AR experience.
}
```

## Conduct a hit-test using an arbitrary ray and direction

Hit-tests are typically treated as rays from the device or device camera, but you can use [`Frame.hitTest(float[], int, float[], int)`](https://developers.google.com/ar/reference/java/com/google/ar/core/Frame#hitTest-origin3-originOffset-direction3-directionOffset) to conduct a hit-test using an arbitrary ray in world space coordinates instead of a screen-space point.

## Create an Anchor using the hit result

Once you have a hit result, you can use its pose as input to [place AR content](https://developers.google.com/ar/develop/anchors) in your scene. Use [`HitResult.createAnchor()`](https://developers.google.com/ar/reference/java/com/google/ar/core/HitResult) to create a new [`Anchor`](https://developers.google.com/ar/develop/anchors), ensuring that the content attaches to the underlying [`Trackable`](https://developers.google.com/ar/reference/java/com/google/ar/core/Trackable) of the hit result. For example, the anchor will remain attached to the detected plane for a Plane hit result, thus appearing to be part of the real world.

## What's next

- Check out the [`hello_ar_java`](https://github.com/google-ar/arcore-android-sdk/tree/master/samples/hello_ar_java) and [`hello_ar_kotlin`](https://github.com/google-ar/arcore-android-sdk/tree/master/samples/hello_ar_kotlin) sample apps on GitHub.