使用android中的矩阵缩放和拖动图像

Question

我得到了以下代码（来自Ed Burnette的Hello android）很好地工作，我可以拖动和缩放图像。

import android.app.Activity; 
import android.graphics.Matrix; 
import android.graphics.PointF; 
import android.os.Bundle; 
import android.util.FloatMath; 
import android.util.Log; 
import android.view.MotionEvent; 
import android.view.View; 
import android.view.View.OnTouchListener; 
import android.widget.ImageView; 

public class Touch extends Activity implements OnTouchListener { 
    private static final String TAG = "Touch"; 
    // These matrices will be used to move and zoom image 
    Matrix matrix = new Matrix(); 
    Matrix savedMatrix = new Matrix(); 

    // We can be in one of these 3 states 
    static final int NONE = 0; 
    static final int DRAG = 1; 
    static final int ZOOM = 2; 
    int mode = NONE; 

    // Remember some things for zooming 
    PointF start = new PointF(); 
    PointF mid = new PointF(); 
    float oldDist = 1f; 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.main); 
     ImageView view = (ImageView) findViewById(R.id.imageView); 
     view.setOnTouchListener(this); 
    } 

    @Override 
    public boolean onTouch(View v, MotionEvent event) { 
     ImageView view = (ImageView) v; 

     // Dump touch event to log 
     dumpEvent(event); 

     // Handle touch events here... 
     switch (event.getAction() & MotionEvent.ACTION_MASK) { 
     case MotionEvent.ACTION_DOWN: 
     savedMatrix.set(matrix); 
     start.set(event.getX(), event.getY()); 
     Log.d(TAG, "mode=DRAG"); 
     mode = DRAG; 
     break; 
     case MotionEvent.ACTION_POINTER_DOWN: 
     oldDist = spacing(event); 
     Log.d(TAG, "oldDist=" + oldDist); 
     if (oldDist > 10f) { 
      savedMatrix.set(matrix); 
      midPoint(mid, event); 
      mode = ZOOM; 
      Log.d(TAG, "mode=ZOOM"); 
     } 
     break; 
     case MotionEvent.ACTION_UP: 
     case MotionEvent.ACTION_POINTER_UP: 
     mode = NONE; 
     Log.d(TAG, "mode=NONE"); 
     break; 
     case MotionEvent.ACTION_MOVE: 
     if (mode == DRAG) { 
      // ... 
      matrix.set(savedMatrix); 
      matrix.postTranslate(event.getX() - start.x, 
        event.getY() - start.y); 
     } 
     else if (mode == ZOOM) { 
      float newDist = spacing(event); 
      Log.d(TAG, "newDist=" + newDist); 
      if (newDist > 10f) { 
       matrix.set(savedMatrix); 
       float scale = newDist/oldDist; 
       matrix.postScale(scale, scale, mid.x, mid.y); 
      } 
     } 
     break; 
     } 

     view.setImageMatrix(matrix); 
     return true; // indicate event was handled 
    } 

    /** Show an event in the LogCat view, for debugging */ 
    private void dumpEvent(MotionEvent event) { 
     String names[] = { "DOWN", "UP", "MOVE", "CANCEL", "OUTSIDE", 
      "POINTER_DOWN", "POINTER_UP", "7?", "8?", "9?" }; 
     StringBuilder sb = new StringBuilder(); 
     int action = event.getAction(); 
     int actionCode = action & MotionEvent.ACTION_MASK; 
     sb.append("event ACTION_").append(names[actionCode]); 
     if (actionCode == MotionEvent.ACTION_POINTER_DOWN 
      || actionCode == MotionEvent.ACTION_POINTER_UP) { 
     sb.append("(pid ").append(
       action >> MotionEvent.ACTION_POINTER_ID_SHIFT); 
     sb.append(")"); 
     } 
     sb.append("["); 
     for (int i = 0; i < event.getPointerCount(); i++) { 
     sb.append("#").append(i); 
     sb.append("(pid ").append(event.getPointerId(i)); 
     sb.append(")=").append((int) event.getX(i)); 
     sb.append(",").append((int) event.getY(i)); 
     if (i + 1 < event.getPointerCount()) 
      sb.append(";"); 
     } 
     sb.append("]"); 
     Log.d(TAG, sb.toString()); 
    } 

    /** Determine the space between the first two fingers */ 
    private float spacing(MotionEvent event) { 
     float x = event.getX(0) - event.getX(1); 
     float y = event.getY(0) - event.getY(1); 
     return FloatMath.sqrt(x * x + y * y); 
    } 

    /** Calculate the mid point of the first two fingers */ 
    private void midPoint(PointF point, MotionEvent event) { 
     float x = event.getX(0) + event.getX(1); 
     float y = event.getY(0) + event.getY(1); 
     point.set(x/2, y/2); 
    } 
} 

但是，有人能够清楚地解释上述代码中以下5行的功能吗？

savedMatrix.set(matrix); 
matrix.set(savedMatrix); 
matrix.postTranslate(event.getX() - start.x,event.getY() - start.y); 
float scale = newDist/oldDist; 
matrix.postScale(scale, scale, mid.x, mid.y); 

在此先感谢您的解释:)

Answer 1

savedMatrix.set(matrix); 

复制 “矩阵” 的内容为 “savedMatrix”

matrix.postTranslate(event.getX() - start.x,event.getY() - start.y); 

“矩阵” 决定了一些改造，如缩放和移动。 “postTranslate”将“移动”转换添加到已在“矩阵”中定义的转换。即“矩阵”包含一些缩放转换，然后在应用postTranslate之后，它将确定“缩放然后移动”转换。

float scale = newDist/oldDist; 
matrix.postScale(scale, scale, mid.x, mid.y); 

添加具有在“矩阵”

Answer 2

已经定义对于一个替代解决方案转化中心（mid.x，mid.y）缩放变换，我用从Android的dev的博客的例子。 http://android-developers.blogspot.com/2010/06/making-sense-of-multitouch.html

我使用此示例而不是实现的主要原因是他们编写了修改后的博客文章。本文介绍如何处理未安装Android 2.2或更高版本的设备上的多点触控事件。 http://android-developers.blogspot.com/2010/07/how-to-have-your-cupcake-and-eat-it-too.html