通过在android中拖动一个圆圈来制作下面的图像

Question

在我的实现中，我有两张图像，一张叠放在另一张上。因为，我在顶部图像上移动了一个圆形对象，我想让圆形内部的那个区域透明，这样我就可以看到下面的图像。例如，我有两个图像 - 汽车图像及其框架图像。我将汽车图像覆盖在框架图像上，当我在汽车图像上拖动一个圆圈时，它应该显示下面的框架。

我尝试了很多搜索，但没有得到任何指示。在某处我读到我需要使用 alpha 遮罩或使用 porterduff 和 xfermode 的图像遮罩。但我没听懂。

具体来说，

我怎样才能使上面的图像透明，我怎样才能只使圆圈内的区域透明？

谢谢

Answer 1

我使用了来自问题 PorterduffXfermode: Clear a section of a bitmap 的有用信息。在下面的示例中，触摸区域变为“透明”，并且可以观察到up_image 下方的down_image 部分（两个图像都只是资源中的 jpg 可绘制对象）。

基本上有两种可能的实现方式：

1. 禁用绘图的硬件加速，这将使PorterDuff.Mode.CLEAR 使视图透明（有关硬件加速效果的更多详细信息，请参阅here）：

   活动：

   public class MyActivity extends Activity {
       /** Overlay image */
       private DrawingView mDrawingView = null;
   
       @Override
       public void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
   
           requestWindowFeature(Window.FEATURE_NO_TITLE);
   
           final RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
               ViewGroup.LayoutParams.MATCH_PARENT);
   
           // Create the view for the xfermode test
           mDrawingView = new DrawingView(this);
   
           params.addRule(RelativeLayout.CENTER_IN_PARENT);
           mDrawingView.setLayoutParams(params);
   
           final RelativeLayout relativeLayout = new RelativeLayout(this);
   
           relativeLayout.setBackgroundResource(R.drawable.down_image);
           relativeLayout.addView(mDrawingView);
   
           // Main part of the implementation - make layer drawing software
           if (android.os.Build.VERSION.SDK_INT >= 11) {
               mDrawingView.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
           }
   
           // Show the layout with the test view
           setContentView(relativeLayout);
       }
   }
   

   绘图视图：

   /**
    * View which makes touch area transparent
    */
   public class DrawingView extends View {
       /**  Paint to clear touch area */
       private Paint mClearPaint = null;
       /** Main bitmap */
       private Bitmap mBitmap = null;
   
       /** X coordinate of touch circle */
       private int mXTouch = -1;
       /** Y coordinate of touch circle */
       private int mYTouch = -1;
       /** Radius of touch circle */
       private int mRadius = 0;
   
       /**
        * Default constructor
        *
        * @param ct {@link Context}
        */
       public DrawingView(final Context ct) {
           super(ct);
   
           // Generate bitmap used for background
           mBitmap = BitmapFactory.decodeResource(ct.getResources(), R.drawable.up_image);
   
           // Generate array of paints
           mClearPaint = new Paint();
           mClearPaint.setARGB(255, 255, 255, 0);
           mClearPaint.setStrokeWidth(20);
           mClearPaint.setStyle(Paint.Style.FILL);
   
           // Set all transfer modes
           mClearPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
   
           mRadius = getResources().getDimensionPixelSize(R.dimen.radius);
       }
   
       @Override
       public void onDraw(final Canvas canv) {
           // Background colour for canvas
           canv.drawColor(Color.argb(255, 0, 0, 0));
   
           // Draw the bitmap leaving small outside border to see background
           canv.drawBitmap(mBitmap, null, new RectF(0, 0, getMeasuredWidth(), getMeasuredHeight()), null);
   
           // Loop, draws 4 circles per row, in 4 rows on top of bitmap
           // Drawn in the order of mClearPaint (alphabetical)
           if (mXTouch > 0 && mYTouch > 0) {
               canv.drawCircle(mXTouch, mYTouch, mRadius, mClearPaint);
           }
       }
   
       @Override
       public boolean onTouchEvent(final MotionEvent event) {
           boolean handled = false;
   
           // get touch event coordinates and make transparent circle from it
           switch (event.getActionMasked()) {
               case MotionEvent.ACTION_DOWN:
               case MotionEvent.ACTION_MOVE:
                   mXTouch = (int) event.getX();
                   mYTouch = (int) event.getY();
                   // TODO: Note, in case of large scene it's better not to use invalidate without rectangle specified
                   invalidate();
                   handled = true;
                   break;
   
               case MotionEvent.ACTION_UP:
               case MotionEvent.ACTION_POINTER_UP:
               case MotionEvent.ACTION_CANCEL:
                   mXTouch = -1;
                   mYTouch = -1;
                   // TODO: Note, in case of large scene it's better not to use invalidate without rectangle specified
                   invalidate();
                   handled = true;
                   break;
   
               default:
                   // do nothing
                   break;
           }
   
           return super.onTouchEvent(event) || handled;
       }
   }
   

2. 采用这个solution，但看起来它取决于 Android 版本，因为建议的解决方案根本不适用于我的 4.2 设备。