how to implement finger eraser in android?

Question

I have prepared one paint app.In my app we can draw any thing.It is working fine.Here i want prepare finger erase for erase paint.For that i have prepared following code,

this is on touch,

mMyPaint.setOnTouchListener(new OnTouchListener() {
            public boolean onTouch(View v, MotionEvent event) {
               if(paintAndEraserFlag==0){
                   if (event.getAction() == MotionEvent.ACTION_DOWN) {
                    // path = new Path();
                    mPath.moveTo(event.getX(), event.getY());
                    mPath.lineTo(event.getX(), event.getY());
                    mArryLstPath.add(mPath);
                } else if (event.getAction() == MotionEvent.ACTION_MOVE) {
                    mPath.lineTo(event.getX(), event.getY());

                } else if (event.getAction() == MotionEvent.ACTION_UP) {
                    mPath.lineTo(event.getX(), event.getY());
                }
               }else if(paintAndEraserFlag==1){
                   if (event.getAction() == MotionEvent.ACTION_DOWN) {
                    // path = new Path();
                       System.out.println("in path---");
                    mPath.moveTo(event.getX(), event.getY());
                    mPath.lineTo(event.getX(), event.getY());
                    mArryLstEarser.add(mEraserPath);
                } else if (event.getAction() == MotionEvent.ACTION_MOVE) {
                    mPath.lineTo(event.getX(), event.getY());

                } else if (event.getAction() == MotionEvent.ACTION_UP) {
                    mPath.lineTo(event.getX(), event.getY());
                }
               }

                mMyPaint.invalidate();
                return true;

            }
        });

my paint and eraser objects,

mPaint.setDither(true);
        mPaint.setColor(0xFFD2691E);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(30);

        mEraser.setDither(true);
        mEraser.setStyle(Paint.Style.STROKE);
        mEraser.setStrokeJoin(Paint.Join.ROUND);
        mEraser.setStrokeCap(Paint.Cap.ROUND);
        mEraser.setStrokeWidth(15);
        mEraser.setColor(0x00000000);

this is my on draw method in custom view,

public void onDraw(Canvas canvas) {

        if (myDrawBitmap == null) {
            myDrawBitmap = Bitmap.createBitmap(480, 800,
                    Bitmap.Config.ARGB_8888);
            mBmpDrawCanvas = new Canvas(myDrawBitmap);
            mIntDrawArray = new int[myDrawBitmap.getWidth()
                    * myDrawBitmap.getHeight()];
        }

        if (mBmpDrawCanvas != null) {
            myDrawBitmap.getPixels(mIntDrawArray, 0, myDrawBitmap.getWidth(),
                    0, 0, myDrawBitmap.getWidth(), myDrawBitmap.getHeight());
            if (MyEraserActivity.paintAndEraserFlag == 0) {
                for (Path path : MyEraserActivity.mArryLstPath) {

                    mBmpDrawCanvas.drawPath(MyEraserActivity.mPath, mPaint);

                }
            } else if (MyEraserActivity.paintAndEraserFlag == 1) {
                for (Path path : MyEraserActivity.mArryLstEarser) {

                    mBmpDrawCanvas.drawPath(MyEraserActivity.mEraserPath,
                            mEraser);
                    System.out.println("in eraser---");
                }
            }
            if (myDrawBitmap != null)
                canvas.drawBitmap(myDrawBitmap, 0, 0, null);

        }

    }

I want erase paint where i touch if drawn paint is there please help me.

Answer 1

Canvas does not support eraser while Bitmap does.

Basic workaround flow:

1. Create another canvas

2. Create a bitmap

3. Set that bitmap to that canvas

   public void init(int width, int height) {
       Log.i(TAG,"init with "+width+"x"+height);
       foreground = Bitmap.createBitmap(width, height, Config.ARGB_8888);
       cacheCanvas = new Canvas();
       cacheCanvas.setBitmap(foreground);
   }
   

4. Record the touches on that bitmap, including paint and eraser

   public boolean onTouchEvent(MotionEvent event) {
   // Log.i(TAG,"onTouch detected");
   float eventX = event.getX();
   float eventY = event.getY();
   
   switch (event.getAction()) {
   case MotionEvent.ACTION_DOWN:
       currentStroke = new Stroke();
       currentStroke.color = paint;
       currentStroke.path.moveTo(eventX, eventY);
       currentStroke.path.lineTo(eventX, eventY);
   
       synchronized (strokes) {
           strokes.add(currentStroke);
       }
       lastTouchX = eventX;
       lastTouchY = eventY;
       // There is no end point yet, so don't waste cycles invalidating.
       return true;
   
   case MotionEvent.ACTION_MOVE:
   case MotionEvent.ACTION_UP:
       // Start tracking the dirty region.
       resetDirtyRect(eventX, eventY);
   
       // When the hardware tracks events faster than they are delivered,
       // the
       // event will contain a history of those skipped points.
       int historySize = event.getHistorySize();
       for (int i = 0; i < historySize; i++) {
           float historicalX = event.getHistoricalX(i);
           float historicalY = event.getHistoricalY(i);
           expandDirtyRect(historicalX, historicalY);
           if (i == 0) {
               lastX = historicalX;
               lastY = historicalY;
               currentStroke.path.lineTo(historicalX, historicalY);
           } else {
               currentStroke.path.quadTo(lastX, lastY,
                       (historicalX + lastX) / 2,
                       (historicalY + lastY) / 2);
           }
       }
   
       // After replaying history, connect the line to the touch point.
       if(historySize==0){
           long duration=event.getEventTime()-event.getDownTime();
           float offset=0.1f;
           if(duration<300){
               offset=50.0f/duration;
           }
           currentStroke.path.lineTo(eventX+offset, eventY+offset);
       }else{
           currentStroke.path.lineTo(eventX, eventY);
       }
       synchronized (strokes) {
           strokes.add(currentStroke);
       }
   
       break;
   
   default:
       // Log.i(TAG,"Ignored touch event: " + event.toString());
       return false;
   }
   
   // Include half the stroke width to avoid clipping.
   float width = paint.getStrokeWidth() / 2;
   invalidate((int) (dirtyRect.left - width),
           (int) (dirtyRect.top - width), (int) (dirtyRect.right + width),
           (int) (dirtyRect.bottom + width));
   
   lastTouchX = eventX;
   lastTouchY = eventY;
   
   return true;
   }
   

5. Draw that bitmap on the canvas of your View

   protected void onDraw(Canvas canvas) {
   // Log.i(TAG,"onDraw called");
   synchronized (strokes) {
       if (strokes.size() > 0) {
           for (Stroke s : strokes) {
               cacheCanvas.drawPath(s.path, s.color);
           }
           canvas.drawBitmap(foreground, 0, 0, null);
           strokes.clear();
       }
   }
   }
   

Answer 2

It may helps you:

mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));