Android编程实现只显示图片一部分的方法

package com.iwin.zzs;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Matrix;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.view.WindowManager;
public class ShowPoritionPictureActivity extends Activity {
  /** Called when the activity is first created. */
  Bitmap picRes;
  Bitmap showPic;
  //获取原图片的宽和高
  int picWidth;
  int picHeight;
  private PoritionView poritOnView= null;
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    // 不显示状态栏
    this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
    DisplayMetrics dm = new DisplayMetrics();
    this.getWindowManager().getDefaultDisplay().getMetrics(dm);
    // 得到屏幕的长和宽
    int screenWidth = dm.widthPixels; //水平分辨率
    int screenHeight = dm.heightPixels; //垂直分辨率
    picRes = BitmapFactory.decodeResource(this.getResources(), R.drawable.girl);
    // 得到图片的长和宽
    picWidth = picRes.getWidth();
    picHeight = picRes.getHeight();
    // 计算缩放率，新尺寸除原始尺寸
    float scaleWidth = ((float) screenWidth ) / picWidth;
    float scaleHeight = ((float) screenHeight ) / picHeight;
    // 创建操作图片用的matrix对象
    Matrix matrix = new Matrix();
    // 缩放图片动作
    matrix.postScale(scaleWidth, scaleHeight);
    // 新得到的图片是原图片经过变换填充到整个屏幕的图片
    Bitmap picNewRes = Bitmap.createBitmap(picRes, 0, 0,picWidth, picHeight, matrix, true);
    // bitmap = Bitmap.createBitmap(400, 480, Bitmap.Config.ARGB_8888);
    // canvas=new Canvas();
    // canvas.setBitmap(bitmap);
    showPic = Bitmap.createBitmap(picNewRes, screenWidth/2-50, screenHeight/2-50, 100, 100);
    poritOnView= new PoritionView(this);
    poritonView.setBitmapShow(showPic, screenWidth/2-50, screenHeight/2-50);
    setContentView(poritonView);
}

package com.iwin.zzs;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.view.View;
public class PoritionView extends View {
  private Bitmap showPic = null;
  private int startX = 0;
  private int startY = 0;
  public PoritionView(Context context) {
    super(context);
    // TODO Auto-generated constructor stub
  }
  @Override
  protected void onDraw(Canvas canvas) {
    // TODO Auto-generated method stub
    super.onDraw(canvas);
    canvas.drawBitmap(showPic, startX, startY, null);
  }
  public void setBitmapShow(Bitmap b, int x, int y)
  {
    showPic = b;
    startX = x;
    startY = y;
  }
}