//获得Bitmap的高和宽
int bmpWidth=bmp.getWidth();
int bmpHeight=bmp.getHeight();

//设置缩小比例
double scale=0.8;
//计算出这次要缩小的比例
scaleWidth=(float)(scaleWidth*scale);
scaleHeight=(float)(scaleHeight*scale);

//产生resize后的Bitmap对象
Matrix matrix=new Matrix();
matrix.postScale(scaleWidth, scaleHeight);
Bitmap resizeBmp=Bitmap.createBitmap(bmp, 0, 0, bmpWidth, bmpHeight, matrix, true);

package com.jcodecraeer.stargallerry;
import android.content.Context;
import android.graphics.Matrix;
import android.graphics.PointF;
import android.util.AttributeSet;
import android.util.FloatMath;
import android.view.MotionEvent;
import android.widget.ImageView;
public class ImageTouchView extends ImageView {

private PointF startPoint = new PointF();
private Matrix matrix = new Matrix();
private Matrix currentMaritx = new Matrix();

private int mode = 0;//用于标记模式
private static final int DRAG = 1;//拖动
private static final int ZOOM = 2;//放大
private float startDis = 0;
private PointF midPoint;//中心点

/**
* 默认构造函数
* @param context
*/
public ImageTouchView(Context context){
super(context);
}
/**
* 该构造方法在静态引入XML文件中是必须的
* @param context
* @param paramAttributeSet
*/
public ImageTouchView(Context context,AttributeSet paramAttributeSet){
super(context,paramAttributeSet);
}

public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction() & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN:
mode = DRAG;
currentMaritx.set(this.getImageMatrix());//记录ImageView当期的移动位置
startPoint.set(event.getX(),event.getY());//开始点
break;
case MotionEvent.ACTION_MOVE://移动事件

if (mode == DRAG) {//图片拖动事件
float dx = event.getX() - startPoint.x;//x轴移动距离
float dy = event.getY() - startPoint.y;
matrix.set(currentMaritx);//在当前的位置基础上移动
matrix.postTranslate(dx, dy);

} else if(mode == ZOOM){//图片放大事件
float endDis = distance(event);//结束距离
if(endDis > 10f){
float scale = endDis / startDis;//放大倍数
//Log.v("scale=", String.valueOf(scale));
matrix.set(currentMaritx);
matrix.postScale(scale, scale, midPoint.x, midPoint.y);
}


}
break;

case MotionEvent.ACTION_UP:
mode = 0;
break;
//有手指离开屏幕，但屏幕还有触点(手指)
case MotionEvent.ACTION_POINTER_UP:
mode = 0;
break;
//当屏幕上已经有触点（手指）,再有一个手指压下屏幕
case MotionEvent.ACTION_POINTER_DOWN:
mode = ZOOM;
startDis = distance(event);

if(startDis > 10f){//避免手指上有两个茧
midPoint = mid(event);
currentMaritx.set(this.getImageMatrix());//记录当前的缩放倍数
}

break;
}
this.setImageMatrix(matrix);
return true;
}

/**
* 两点之间的距离
* @param event
* @return
*/
private static float distance(MotionEvent event){
//两根线的距离
float dx = event.getX(1) - event.getX(0);
float dy = event.getY(1) - event.getY(0);
return FloatMath.sqrt(dx*dx + dy*dy);
}
/**
* 计算两点之间中心点的距离
* @param event
* @return
*/
private static PointF mid(MotionEvent event){
float midx = event.getX(1) + event.getX(0);
float midy = event.getY(1) - event.getY(0);

return new PointF(midx/2, midy/2);
}
}

在xml中这样使用自定义的ImageView：

<com.jcodecraeer.stargallerry.ImageTouchView
android:id="@+id/image"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scaleType="matrix"
/>