title: How to center image in FloatingActionButton behind transparent background?
body: <p>I have a FloatingActionButton and I would like to make it transparent with a centered icon.</p>
  
  <p>I added a style:</p>
  
  <pre><code>&lt;style name="ButtonTransparent"&gt;
      &lt;item name="colorAccent"&gt;@android:color/transparent&lt;/item&gt;
  &lt;/style&gt;
  </code></pre>
  
  <p>which works so far. THe FAB got transparent. Then I added the FAB :</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;RelativeLayout 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"
      android:orientation="vertical"
      tools:context=".activities.CameraActivity"&gt;
      &lt;TextureView
          android:id="@+id/texture"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:layout_alignParentTop="true"/&gt;
  
      &lt;LinearLayout
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_alignParentBottom="true"
          android:layout_centerHorizontal="true"&gt;
  
          &lt;ImageView
              android:id="@+id/iv_last_image"
              android:layout_width="60dp"
              android:layout_height="60dp"
              android:layout_gravity="center_horizontal"
              android:layout_margin="10dp" /&gt;
  
          &lt;android.support.design.widget.FloatingActionButton
              android:id="@+id/btn_takepicture"
              android:layout_width="80dp"
              android:layout_height="80dp"
              android:layout_gravity="center_horizontal"
              android:layout_margin="20dp"
              android:theme="@style/ButtonTransparent"
              android:src="@drawable/selector_vector_camera_light" /&gt;
  
          &lt;android.support.design.widget.FloatingActionButton
              android:id="@+id/btn_back"
              android:layout_width="60dp"
              android:layout_height="60dp"
              android:layout_gravity="center_horizontal"
              android:layout_margin="20dp"
              android:theme="@style/ButtonTransparent"
              android:src="@drawable/selector_vector_go_back" /&gt;
      &lt;/LinearLayout&gt;
  &lt;/RelativeLayout&gt;
  </code></pre>
  
  <p>but as you can see on the screenshot below it is not well aligned.
  <strong>How can I correct that?</strong>
  <a href="https://i.stack.imgur.com/sU1HM.png" rel="noreferrer"><img src="https://i.stack.imgur.com/sU1HM.png" alt="screenshot"></a></p>