How to wash with finger paint

Question

I want to remove the area where the finger is detected in the bitmap of the gray square, but it does not work.

I show a bitmap with ic_launch as an image, then I show a gray Paint square in the image where I can change the color to TRANSPARENT

What is the problem? Thanks you

private Bitmap mBitmap; private Canvas mCanvas; private Path mPath; private Paint mBitmapPaint; private Paint mPaint; public CaseView(Context c) { super(c); mPaint = new Paint(); mPaint.setAntiAlias(true); mPaint.setDither(true); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeWidth(40); mBitmap = Bitmap.createBitmap(100, 100, Bitmap.Config.ARGB_8888); for (int i = 0; i < 100; i++) { for (int j = 0; j < 100; j++) { mBitmap.setPixel(i, j, Color.GRAY); } } mCanvas = new Canvas(mBitmap); mPath = new Path(); mBitmapPaint = new Paint(Paint.DITHER_FLAG); } @Override protected void onSizeChanged(int w, int h, int oldw, int oldh) { super.onSizeChanged(0, 0, oldw, oldh); } @Override protected void onDraw(Canvas canvas) { canvas.drawRect(100, 100, 200, 200, mBitmapPaint); canvas.drawPath(mPath, mPaint); Bitmap _scratch = BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher); canvas.drawColor(Color.WHITE); // logo canvas.drawBitmap(_scratch, 0, 100, null); for (int i = 0; i < 100; i++) { for (int j = 0; j < 100; j++) { if ((mX < 100 && mX >= 0) && (mY < 100 && mY >= 0)) { mBitmap.setPixel((int) mY,(int) mX, Color.TRANSPARENT); } } } canvas.drawBitmap(mBitmap, 0, 100, mBitmapPaint); Bitmap mutableBitmap = Bitmap.createBitmap(_scratch.getWidth(), _scratch.getHeight(), Bitmap.Config.ARGB_8888); mutableBitmap.setPixel(50, 50, 124); canvas.drawBitmap(mutableBitmap, 0, 100, null); int pixelColor = mBitmap.getPixel(50, 50); int red = Color.red(pixelColor); int green = Color.green(pixelColor); Log.v("red", "red:" + red + " /green:" + green); } private float mX, mY; private static final float TOUCH_TOLERANCE = 4; private void touch_start(float x, float y) { mPath.reset(); mPath.moveTo(x, y); mX = x; mY = y; } private void touch_move(float x, float y) { float dx = Math.abs(x - mX); float dy = Math.abs(y - mY); if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) { mPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2); mX = x; mY = y; } } private void touch_up() { mPath.lineTo(mX, mY); // commit the path to our offscreen mCanvas.drawPath(mPath, mPaint); // kill this so we don't double draw mPath.reset(); } @Override public boolean onTouchEvent(MotionEvent event) { float x = event.getX(); float y = event.getY(); Log.v("onTouchEvent", "x:" + x + "/y:" + y); switch (event.getAction()) { case MotionEvent.ACTION_DOWN: touch_start(x, y); invalidate(); break; case MotionEvent.ACTION_MOVE: touch_move(x, y); invalidate(); break; case MotionEvent.ACTION_UP: touch_up(); invalidate(); break; } return true; } 

Answer 1

To draw, you need to draw a path using a single Paint class. But to erase again, you have to draw one path on your coordinated touch

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

In the android-sdk sample, they gave the FingerPaint class, which explains it very well