Android 进度条改变图片透明度

<?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"
    tools:context="com.example.administrator.jsq.MainActivity"
    android:orientation="vertical">
    <!--图片透明-->
    <SeekBar
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:max="250"
        android:progress="0"
        android:id="@+id/sar"/>
    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/f8"
        android:id="@+id/tp"
        />
</LinearLayout>

package com.example.administrator.jsq;

import android.media.Image;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.ImageView;
import android.widget.SeekBar;

public class MainActivity extends AppCompatActivity {
    SeekBar sar;
    ImageView tp;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        sar = (SeekBar)findViewById(R.id.sar);
        tp = (ImageView)findViewById(R.id.tp);
        tp.setImageAlpha(0);
        sar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
            @Override
            public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                tp.setImageAlpha(progress);
            }

            @Override
            public void onStartTrackingTouch(SeekBar seekBar) {
                Log.e("TAG", "进度条开始拖动");
            }

            @Override
            public void onStopTrackingTouch(SeekBar seekBar) {
                Log.e("TAG","进度条停止拖动");
            }
        });
    }
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.administrator.jsq">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>