我已经使用以下代码行将图像作为我的Android应用程序的背景：
android:background="@drawable/background"
我现在想让它透明40％但是在xml文件中这怎么可能？我的exm文件如下所示：
xmlns:tools="/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/background"
android:alpha="0.6"
android:orientation="vertical"
android:padding="30dp"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".SecondActivity" >
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:paddingBottom="5dp"
android:text="Welcome"
android:textColor="#292421"
android:textSize="17sp"
android:textStyle="bold" />
android:id="@+id/TextView01"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center_vertical"
android:text="This Android application is used to track and record the movements of a person throughout the university of ulster, Magee Campus. The tracking phase of the application can only be initiated whilst within the campus and off campus tracking is unavailable at this moment in time."
android:textColor="#292421"
android:textSize="17sp" />
android:layout_width="match_parent"
android:layout_height="wrap_content" >
android:id="@+id/button1"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:layout_marginTop="85dp"
android:layout_weight="1"
android:background="#E6E6FA"
android:onClick="tracking"
android:text="@string/tracking_service" />
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:padding="20dp" >
android:id="@+id/button2"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:layout_marginTop="85dp"
android:layout_weight="1"
android:background="#E6E6FA"
android:onClick="help"
android:text="Help" />
