ConstraintLayout aspect ratio

Your understanding for the way app:layout_constraintDimensionRatio works is correct. If you set app:layout_constraintDimensionRatio="H,3:1" then it means width will be first computed from other constraints and then height will be adjusted according to the aspect ratio. The only problem with your implementation is that you added app:layout_constraintBottom_toBottomOf="parent" to the ImageView, so that it caused app:layout_constraintDimensionRatio to be ignored.

Here’s the layout to size your ImageView in 3:1 aspect ratio:

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#FF0000">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="16dp"
        android:layout_marginTop="16dp"
        android:layout_marginEnd="16dp"
        android:background="#0000FF"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintDimensionRatio="H,3:1" />

</android.support.constraint.ConstraintLayout>

and here’s the result view: