Android提高十六篇之使用NDK把彩图转换灰度图

  在Android上使用JAVA实现彩图转换为灰度图，跟J2ME上的实现类似，不过遇到频繁地转换或者是大图转换时，就必须使用NDK来提高速度了。本文主要通过JAVA和NDK这两种方式来分别实现彩图转换为灰度图，并给出速度的对比。

先来简单地介绍一下Android的NDK使用步骤：

以NDK r4为例，或许以后新版的NDK的使用方法略有不同。
1、下载支持C++的android-ndk-r4-crystax，支持C++的话可玩性更强......
2、下载cygwin，选择 ftp://mirrors.kernel.org 这个镜像，搜索  Devel Install 安装 gcc 和 make 等工具；

在搜索框里分别搜索gcc和make，必须是 Devel Install 栏的。

3、Cygwin安装目录下，找到home/username的目录下的.bash_profile文件，打开文件在最后加上：
    NDK=/cygdrive/d/android-ndk-r4-windows/android-ndk-r4
   export NDK
PS：假设安装在d:\android-ndk-r4-windows\android-ndk-r4。
4、运行cygwin，通过cd命令去到NDK\samples\例子目录\，运行$NDK/ndk-build来编译该目录下的Android.mk

以下是个人习惯.......
5、安装Eclipse的CDT，官方下载cdt安装包，解压缩后把plugins和feagures 复制覆盖到eclipse文件夹下即可
6、 去到系统属性->环境变量->Path添加"D:\cygwin\bin"（假设cygwin安装在D:下），重启计算机，然后就可以在 Eclipse里面建立基于cygwin的C/C++工程了，先通过这一步来验证NDK的程序能够编译成功，然后再通过第4步来生成SO文件。

接下来看看本文程序运行的效果：

从转换灰度图的耗时来说，NDK的确比JAVA所用的时间短不少。

main.xml源码如下：

 <? xml   version = "1.0"   encoding = "utf-8"   ?>
 - < LinearLayout   xmlns:android = "http://schemas.android.com/apk/res/android"   android:orientation = "vertical"   android:layout_width = "fill_parent"   android:layout_height = "fill_parent" >
   < Button   android:layout_height = "wrap_content"   android:layout_width = "fill_parent"   android:id = "@+id/btnJAVA"   android:text = "使用JAVA转换灰度图"   />
   < Button   android:layout_height = "wrap_content"   android:layout_width = "fill_parent"   android:id = "@+id/btnNDK"   android:text = "使用NDK转换灰度图"   />
   < ImageView   android:id = "@+id/ImageView01"   android:layout_width = "fill_parent"   android:layout_height = "fill_parent"   />
   </ LinearLayout >

<?xml version="1.0" encoding="utf-8" ?> - <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent"> <Button android:layout_height="wrap_content" android:layout_width="fill_parent" android:id="@+id/btnJAVA" android:text="使用JAVA转换灰度图" /> <Button android:layout_height="wrap_content" android:layout_width="fill_parent" android:id="@+id/btnNDK" android:text="使用NDK转换灰度图" /> <ImageView android:id="@+id/ImageView01" android:layout_width="fill_parent" android:layout_height="fill_parent" /> </LinearLayout>

主程序testToGray.java的源码如下：

 package  com.testToGray;

 import  android.app.Activity;
 import  android.graphics.Bitmap;
 import  android.graphics.Bitmap.Config;
 import  android.graphics.drawable.BitmapDrawable;
 import  android.os.Bundle;
 import  android.view.View;
 import  android.widget.Button;
 import  android.widget.ImageView;

 public   class  testToGray  extends  Activity {
     /** Called when the activity is first created. */
     Button btnJAVA,btnNDK;
     ImageView imgView;
     @Override
     public   void  onCreate(Bundle savedInstanceState) {
         super .onCreate(savedInstanceState);
         setContentView(R.layout.main);
         this .setTitle( "使用NDK转换灰度图---hellogv" );
         btnJAVA=(Button)this .findViewById(R.id.btnJAVA);
         btnJAVA.setOnClickListener(new  ClickEvent());

         btnNDK=(Button)this .findViewById(R.id.btnNDK);
         btnNDK.setOnClickListener(new  ClickEvent());
         imgView=(ImageView)this .findViewById(R.id.ImageView01);
     }
     class  ClickEvent  implements  View.OnClickListener{

         @Override
         public   void  onClick(View v) {
             if (v==btnJAVA)
             {
                 long  current=System.currentTimeMillis();
                 Bitmap img=ConvertGrayImg(R.drawable.cat);
                 long  performance=System.currentTimeMillis()-current;
                 //显示灰度图
                 imgView.setImageBitmap(img);
                 testToGray.this .setTitle( "w:" +String.valueOf(img.getWidth())+ ",h:" +String.valueOf(img.getHeight())
                         +" JAVA耗时 " +String.valueOf(performance)+ " 毫秒" );
             }
             else   if (v==btnNDK)
             {
                 long  current=System.currentTimeMillis();

                 //先打开图像并读取像素
                 Bitmap img1=((BitmapDrawable) getResources().getDrawable(R.drawable.cat)).getBitmap();
                 int  w=img1.getWidth(),h=img1.getHeight();
                 int [] pix =  new   int [w * h];
                 img1.getPixels(pix, 0 , w,  0 ,  0 , w, h);
                 //通过ImgToGray.so把彩色像素转为灰度像素
                 int [] resultInt=LibFuns.ImgToGray(pix, w, h);
                 Bitmap resultImg=Bitmap.createBitmap(w, h, Config.RGB_565);
                 resultImg.setPixels(resultInt, 0 , w,  0 ,  0 ,w, h);
                 long  performance=System.currentTimeMillis()-current;
                 //显示灰度图
                 imgView.setImageBitmap(resultImg);
                 testToGray.this .setTitle( "w:" +String.valueOf(img1.getWidth())+ ",h:" +String.valueOf(img1.getHeight())
                         +" NDK耗时 " +String.valueOf(performance)+ " 毫秒" );
             }
         }
     }

     /**
      * 把资源图片转为灰度图
      * @param resID 资源ID
      * @return
      */
     public  Bitmap ConvertGrayImg( int  resID)
     {
         Bitmap img1=((BitmapDrawable) getResources().getDrawable(resID)).getBitmap();

         int  w=img1.getWidth(),h=img1.getHeight();
         int [] pix =  new   int [w * h];
         img1.getPixels(pix, 0 , w,  0 ,  0 , w, h);

         int  alpha= 0xFF << 24 ;
         for  ( int  i =  0 ; i < h; i++) {
             for  ( int  j =  0 ; j < w; j++) {
                 // 获得像素的颜色
                 int  color = pix[w * i + j];
                 int  red = ((color &  0x00FF0000 ) >>  16 );
                 int  green = ((color &  0x0000FF00 ) >>  8 );
                 int  blue = color &  0x000000FF ;
                 color = (red + green + blue)/3 ;
                 color = alpha | (color << 16 ) | (color <<  8 ) | color;
                 pix[w * i + j] = color;
             }
         }
         Bitmap result=Bitmap.createBitmap(w, h, Config.RGB_565);
         result.setPixels(pix, 0 , w,  0 ,  0 ,w, h);
         return  result;
     }
 }

package com.testToGray; import android.app.Activity; import android.graphics.Bitmap; import android.graphics.Bitmap.Config; import android.graphics.drawable.BitmapDrawable; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.ImageView; public class testToGray extends Activity { /** Called when the activity is first created. */ Button btnJAVA,btnNDK; ImageView imgView; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); this.setTitle("使用NDK转换灰度图---hellogv"); btnJAVA=(Button)this.findViewById(R.id.btnJAVA); btnJAVA.setOnClickListener(new ClickEvent()); btnNDK=(Button)this.findViewById(R.id.btnNDK); btnNDK.setOnClickListener(new ClickEvent()); imgView=(ImageView)this.findViewById(R.id.ImageView01); } class ClickEvent implements View.OnClickListener{ @Override public void onClick(View v) { if(v==btnJAVA) { long current=System.currentTimeMillis(); Bitmap img=ConvertGrayImg(R.drawable.cat); long performance=System.currentTimeMillis()-current; //显示灰度图 imgView.setImageBitmap(img); testToGray.this.setTitle("w:"+String.valueOf(img.getWidth())+",h:"+String.valueOf(img.getHeight()) +" JAVA耗时 "+String.valueOf(performance)+" 毫秒"); } else if(v==btnNDK) { long current=System.currentTimeMillis(); //先打开图像并读取像素 Bitmap img1=((BitmapDrawable) getResources().getDrawable(R.drawable.cat)).getBitmap(); int w=img1.getWidth(),h=img1.getHeight(); int[] pix = new int[w * h]; img1.getPixels(pix, 0, w, 0, 0, w, h); //通过ImgToGray.so把彩色像素转为灰度像素 int[] resultInt=LibFuns.ImgToGray(pix, w, h); Bitmap resultImg=Bitmap.createBitmap(w, h, Config.RGB_565); resultImg.setPixels(resultInt, 0, w, 0, 0,w, h); long performance=System.currentTimeMillis()-current; //显示灰度图 imgView.setImageBitmap(resultImg); testToGray.this.setTitle("w:"+String.valueOf(img1.getWidth())+",h:"+String.valueOf(img1.getHeight()) +" NDK耗时 "+String.valueOf(performance)+" 毫秒"); } } } /** * 把资源图片转为灰度图 * @param resID 资源ID * @return */ public Bitmap ConvertGrayImg(int resID) { Bitmap img1=((BitmapDrawable) getResources().getDrawable(resID)).getBitmap(); int w=img1.getWidth(),h=img1.getHeight(); int[] pix = new int[w * h]; img1.getPixels(pix, 0, w, 0, 0, w, h); int alpha=0xFF<<24; for (int i = 0; i < h; i++) { for (int j = 0; j < w; j++) { // 获得像素的颜色 int color = pix[w * i + j]; int red = ((color & 0x00FF0000) >> 16); int green = ((color & 0x0000FF00) >> 8); int blue = color & 0x000000FF; color = (red + green + blue)/3; color = alpha | (color << 16) | (color << 8) | color; pix[w * i + j] = color; } } Bitmap result=Bitmap.createBitmap(w, h, Config.RGB_565); result.setPixels(pix, 0, w, 0, 0,w, h); return result; } }

封装NDK函数的JAVA类LibFuns.java的源码如下：

 package  com.testToGray;
 public   class  LibFuns {
     static  {
         System.loadLibrary("ImgToGray" );
     }

    /**
     * @param width the current view width
     * @param height the current view height
     */

     public   static   native   int [] ImgToGray( int [] buf,  int  w,  int  h);
 }

package com.testToGray; public class LibFuns { static { System.loadLibrary("ImgToGray"); } /** * @param width the current view width * @param height the current view height */ public static native int[] ImgToGray(int[] buf, int w, int h); }

彩图转换为灰度图的ImgToGray.cpp源码:

 #include <jni.h>
 #include <stdio.h>
 #include <stdlib.h>

 extern   "C"  {
 JNIEXPORT jintArray JNICALL Java_com_testToGray_LibFuns_ImgToGray(
         JNIEnv* env, jobject obj, jintArray buf, int  w,  int  h);
 }
 ;

 JNIEXPORT jintArray JNICALL Java_com_testToGray_LibFuns_ImgToGray(
         JNIEnv* env, jobject obj, jintArray buf, int  w,  int  h) {
     jint *cbuf;

     cbuf = env->GetIntArrayElements(buf, false );
     if  (cbuf == NULL) {
         return  0;  /* exception occurred */
     }
     int  alpha = 0xFF << 24;
     for  ( int  i = 0; i < h; i++) {
         for  ( int  j = 0; j < w; j++) {
             // 获得像素的颜色
             int  color = cbuf[w * i + j];
             int  red = ((color & 0x00FF0000) >> 16);
             int  green = ((color & 0x0000FF00) >> 8);
             int  blue = color & 0x000000FF;
             color = (red + green + blue) / 3;
             color = alpha | (color << 16) | (color << 8) | color;
             cbuf[w * i + j] = color;
         }
     }

     int  size=w * h;
     jintArray result = env->NewIntArray(size);
     env->SetIntArrayRegion(result, 0, size, cbuf);

     env->ReleaseIntArrayElements(buf, cbuf, 0);

     return  result;
 }

#include <jni.h> #include <stdio.h> #include <stdlib.h> extern "C" { JNIEXPORT jintArray JNICALL Java_com_testToGray_LibFuns_ImgToGray( JNIEnv* env, jobject obj, jintArray buf, int w, int h); } ; JNIEXPORT jintArray JNICALL Java_com_testToGray_LibFuns_ImgToGray( JNIEnv* env, jobject obj, jintArray buf, int w, int h) { jint *cbuf; cbuf = env->GetIntArrayElements(buf, false); if (cbuf == NULL) { return 0; /* exception occurred */ } int alpha = 0xFF << 24; for (int i = 0; i < h; i++) { for (int j = 0; j < w; j++) { // 获得像素的颜色 int color = cbuf[w * i + j]; int red = ((color & 0x00FF0000) >> 16); int green = ((color & 0x0000FF00) >> 8); int blue = color & 0x000000FF; color = (red + green + blue) / 3; color = alpha | (color << 16) | (color << 8) | color; cbuf[w * i + j] = color; } } int size=w * h; jintArray result = env->NewIntArray(size); env->SetIntArrayRegion(result, 0, size, cbuf); env->ReleaseIntArrayElements(buf, cbuf, 0); return result; }

Android.mk的源码：

 LOCAL_PATH: = $(call my-dir)

 include $(CLEAR_VARS)

 LOCAL_MODULE    : =  ImgToGray
 LOCAL_SRC_FILES : =  ImgToGray .cpp

 include $(BUILD_SHARED_LIBRARY)