defaultConfig {
    ...
    renderscriptTargetApi 18
    renderscriptSupportModeEnabled true
  }

package com.fndroid.renderscriptdemo;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.renderscript.Allocation;
import android.renderscript.Element;
import android.renderscript.RenderScript;
import android.renderscript.ScriptIntrinsicBlur;
import android.support.v7.app.AppCompatActivity;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {
  private ImageView mImageView;
  private Bitmap sampleImg;
  private Bitmap gaussianBlurImg;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mImageView = (ImageView) findViewById(R.id.iv);
    sampleImg = BitmapFactory.decodeResource(getResources(), R.drawable.icon); // 获取原图
    gaussianBlurImg = blur(sampleImg, 25f);
    mImageView.setImageBitmap(gaussianBlurImg);
  }

  private Bitmap blur(Bitmap bitmap,float radius) {
    Bitmap output = Bitmap.createBitmap(bitmap); // 创建输出图片
    RenderScript rs = RenderScript.create(this); // 构建一个RenderScript对象
    ScriptIntrinsicBlur gaussianBlue = ScriptIntrinsicBlur.create(rs, Element.U8_4(rs)); // 创建高斯模糊脚本
    Allocation allIn = Allocation.createFromBitmap(rs, bitmap); // 创建用于输入的脚本类型
    Allocation allOut = Allocation.createFromBitmap(rs, output); // 创建用于输出的脚本类型
    gaussianBlue.setRadius(radius); // 设置模糊半径，范围0f<radius<=25f
    gaussianBlue.setInput(allIn); // 设置输入脚本类型
    gaussianBlue.forEach(allOut); // 执行高斯模糊算法，并将结果填入输出脚本类型中
    allOut.copyTo(output); // 将输出内存编码为Bitmap，图片大小必须注意
    rs.destroy(); // 关闭RenderScript对象，API>=23则使用rs.releaseAllContexts()
    return output;
  }
}

public class MainActivity extends AppCompatActivity implements SeekBar.OnSeekBarChangeListener {
  private ImageView mImageView;
  private ImageView mImageViewCover;
  private Bitmap sampleImg;
  private Bitmap gaussianBlurImg;
  private SeekBar mSeekBar;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mImageView = (ImageView) findViewById(R.id.iv);
    mSeekBar = (SeekBar) findViewById(R.id.sb);
    mImageViewCover = (ImageView) findViewById(R.id.iv_cover);
    sampleImg = BitmapFactory.decodeResource(getResources(), R.drawable.icon); // 获取原图
    gaussianBlurImg = blur(sampleImg, 25f);
    mImageView.setImageBitmap(gaussianBlurImg);
    mSeekBar.setOnSeekBarChangeListener(this);
  }

  private Bitmap blur(Bitmap bitmap, float radius) {
    Bitmap output = Bitmap.createBitmap(bitmap); // 创建输出图片
    RenderScript rs = RenderScript.create(this); // 构建一个RenderScript对象
    ScriptIntrinsicBlur gaussianBlue = ScriptIntrinsicBlur.create(rs, Element.U8_4(rs)); //
    // 创建高斯模糊脚本
    Allocation allIn = Allocation.createFromBitmap(rs, bitmap); // 开辟输入内存
    Allocation allOut = Allocation.createFromBitmap(rs, output); // 开辟输出内存
    gaussianBlue.setRadius(radius); // 设置模糊半径，范围0f<radius<=25f
    gaussianBlue.setInput(allIn); // 设置输入内存
    gaussianBlue.forEach(allOut); // 模糊编码，并将内存填入输出内存
    allOut.copyTo(output); // 将输出内存编码为Bitmap，图片大小必须注意
    rs.destroy(); // 关闭RenderScript对象，API>=23则使用rs.releaseAllContexts()
    return output;
  }

  @Override
  public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
    int alpha = 255 - progress;
    mImageViewCover.setImageAlpha(alpha);
  }

  @Override
  public void onStartTrackingTouch(SeekBar seekBar) {

  }

  @Override
  public void onStopTrackingTouch(SeekBar seekBar) {

  }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:id="@+id/activity_main"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical"
  tools:context="com.fndroid.renderscriptdemo.MainActivity">

  <FrameLayout
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1">

    <ImageView
      android:id="@+id/iv"
      android:layout_width="match_parent"
      android:layout_height="match_parent"/>

    <ImageView
      android:id="@+id/iv_cover"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:src="@drawable/icon"/>

  </FrameLayout>

  <SeekBar
    android:id="@+id/sb"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:max="255"/>

</LinearLayout>