Draw transparent lines on bitmap via touch input [duplicate]

This question already has answers here: Making Overlaid image transparent on touch in Android? (3 answers) Closed 7 years ago.

I am working on a paint application for Android. Now I want to implement an eraser to allow erasing parts of a loaded bitmap by using touch input and making the bitmap transparent along the finger's path.

A very good example of what I try to achieve is shown in the application Steamy Window for Android. Steamy Window simulates a fogged window, where a user can wipe parts of the fog via touch input.

Any suggestions how I could implement the feature?

UPDATE: I have posted the most important sections of my current code below. I am not really happy with it for the following reasons:

1. Drawing is quite sluggish. What can I improve here?

2. I am looking for a way to use an alpha mask in order to set the pixel transparency/alpha of the pixels as I want to simulate a paint brush. Any suggestions how this could be achieved?

        public DrawView(Context context, String imageName) {

        super(context);

        // Get the overlay image and its dimensions
        Bitmap overlayImageOriginal = BitmapFactory.decodeResource(
                getResources(), R.drawable.overlay);
        width = overlayImageOriginal.getWidth();
        height = overlayImageOriginal.getHeight();

        // Get a copy of the overlay image and recycle the original
        overlayImage = overlayImageOriginal.copy(Config.ARGB_8888, true);
        overlayImageOriginal.recycle();

        // Get background image of this level
        int resId = getResources().getIdentifier(imageName, "drawable",
                "com.xxx");
        backgroundImage = BitmapFactory.decodeResource(getResources(), resId);

        // Copy pixels
        overlayImagePixels = new int[width * height];
        overlayImage.getPixels(overlayImagePixels, 0, width, 0, 0, width,
                height);

        getHolder().addCallback(this);
        _thread = new DrawThread(getHolder(), this);
        setFocusable(true);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        synchronized (_thread.getSurfaceHolder()) {
            switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touchStart(event.getX(), event.getY());
                return true;
            case MotionEvent.ACTION_MOVE:
                touchMove(event.getX(), event.getY());
                return true;
            case MotionEvent.ACTION_UP:
                touchStop();
                return true;
            }
            return false;
        }
    }

    @Override
    public void onDraw(Canvas canvas) {
       // draw the image to guess
       canvas.drawBitmap(backgroundImage, 0, 0, null);
       // apply user input to the overlay-image
       setRegionTransparency((int) mTouchX, (int) mTouchY, 20, 0);
       // draw the overlay-image (hiding the image to guess)
       canvas.drawBitmap(overlayImage, 0, 0, null);
    }


    /**
     * 
     * @param posX
     * @param p开发者_开发问答osY
     * @param radius
     * @param alpha - 0: full transparent, 255: full opaque
     */
    private void setRegionTransparency(int posX, int posY, int radius, int alpha) {
        int left = posX - radius;
        int right = posX + radius;
        int top = posY - radius;
        int bottom = posY + radius;
        int color = 0;

        // onDraw() is called before any user input
        // -> assume 0/0 as invalid arguments
        if (posX == 0 && posY == 0) {
              return;
        }

        // TODO currently only rectangular area is supported
        // but we need circular
        // we should use a pre-made alpha mask with smoothed edges
        // we should take into account: pressure and/or duration of touch

        for (int y = top; y = 0 && y = 0 && x = TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mTouchX, mTouchY, (x + mTouchX) / 2, (y + mTouchY) / 2);
            mTouchX = x;
            mTouchY = y;
        }
    }
}

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Take a look at

Making Overlaid image transparent on touch in Android?