Error parsing XML: unbound prefix Android

edit

I get an error in my XML code when I add the opencv:show_fps="true" to my XML code (cause i want to be able to see the FPS on the screen)

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >

    <org.opencv.android.JavaCameraView
        android:id="@+id/java_surface_view"
        android:layout_width="0dp"
        android:layout_height="fill_parent"
        android:layout_weight="0.80"
        opencv:show_fps="true" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="match_parent" 
        androi*emphasized text*d:drawableLeft="@drawable/camera"
        android:gravity="left|center_vertical"/>

</LinearLayout>

When I delete the line, the error also disappears, but I couldn't find another good alternative to read out the FPS. What am I doing wrong here?