Android - Touch to erase portions of foreground Im

Question

So I've been struggling with this for a better part of a day. Suppose I have a custom ImageView that I want to overlay over a background View (both within a RelativeLayout), which when touched, it erases portions of the View's source bitmap like an erase tool in MS Paint, exposing the View below it. I've checked pretty much all of the threads (like this one) and they suggest to use PorterDuff SRC Mode in the Paint object as well as creating a Canvas out out the ARGB_8888 shadow copy of the source bitmap to apply the masking.

Also, I can't set the source of the overlay ahead of time since I have to download it over the network so that the ImageView's scale type takes care of the scaling for me.

Every time I override onDraw, when I apply the erase on the IV's Bitmap, it unveils a black background instead of the view below it, even though I set the background to transparent. So I'm at my last rope as to what to do in order to unveil the background view.

Here's what I have so far:

view constructor:

    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));  
    paint.setColor(Color.TRANSPARENT);
    paint.setStyle(Paint.Style.STROKE);
    paint.setStrokeJoin(Paint.Join.ROUND);
    paint.setStrokeWidth(STROKE_WIDTH);
    paint.setAntiAlias(true);

overrode setImageBitmap to set my canvas from my re-configed source bitmap:

public void setImageBitmap(Bitmap bitmap){
    super.setImageBitmap(bitmap);
    Drawable bd = getDrawable();
    if(bd == null){
        return;
    }

    Bitmap fullSizeBitmap = ((BitmapDrawable) bd).getBitmap();
    overlay = fullSizeBitmap.copy(Config.ARGB_8888, true);
    c2 = new Canvas(overlay);
}

onDraw method:

protected void onDraw(Canvas canvas) {
    /*
     * Override paint call by re-drawing the view's Bitmap first, then overlaying our path on top of it
     */
    Drawable bd = getDrawable();
    if(bd == null){
        return;
    }
    Bitmap fullSizeBitmap = ((BitmapDrawable) bd).getBitmap();
    if(fullSizeBitmap != null && c2 != null){
        canvas.drawColor(Color.TRANSPARENT);
        c2.drawBitmap(fullSizeBitmap, 0, 0, null);
        c2.drawPath(path, paint);
        canvas.drawBitmap(overlay, 0, 0, null);
    }
}

Answer 1

I know this is a really old question but here is what I did to fix a similar problem I had. Maybe it helps someone in the future.

From API level 11 and up you have to specify this piece of code for your ImageView to unveil the image in the back and not a black area.

if (Build.VERSION.SDK_INT >= 11) {
    imageview.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
}

This helped me displaying the back image successfully

Answer 2

Can you try this solution it will help you with the erasing images in android on touch .. Or download a demo example

public class MainActivity extends Activity {

    Bitmap bp;
    Canvas bitmapCanvas;
    DrawView drawImg;
    LinearLayout ln1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);
        ln1 = (LinearLayout) findViewById(R.id.ln1);
        drawImg = new DrawView(this);
        ln1.addView(drawImg);

    }



    public class DrawView extends View implements View.OnTouchListener {

        private int x = 0;
        private int y = 0;

        Bitmap bitmap;
        Path circlePath;
        Paint circlePaint;

        private final Paint paint = new Paint();
        private final Paint eraserPaint = new Paint();


        public DrawView(Context context){
            super(context);
            setFocusable(true);
            setFocusableInTouchMode(true);
            this.setOnTouchListener(this);

            // Set background
            this.setBackgroundColor(Color.CYAN);
            bp = BitmapFactory.decodeResource(getResources(), R.drawable.bg);

            // Set bitmap
            bitmap = Bitmap.createBitmap(320, 480, Bitmap.Config.ARGB_8888);
            bitmapCanvas = new Canvas();
            bitmapCanvas.setBitmap(bitmap);
            bitmapCanvas.drawColor(Color.TRANSPARENT);
            bitmapCanvas.drawBitmap(bp, 0, 0, null);

            circlePath = new Path();
            circlePaint = new Paint();
            circlePaint.setAntiAlias(true);
            circlePaint.setColor(Color.BLUE);
            circlePaint.setStyle(Paint.Style.STROKE);
            circlePaint.setStrokeJoin(Paint.Join.MITER);
            circlePaint.setStrokeWidth(4f);

            // Set eraser paint properties
            eraserPaint.setAlpha(0);
            eraserPaint.setStrokeJoin(Paint.Join.ROUND);
            eraserPaint.setStrokeCap(Paint.Cap.ROUND);
            eraserPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
            eraserPaint.setAntiAlias(true);

        }

        @Override
        public void onDraw(Canvas canvas) {

            canvas.drawBitmap(bitmap, 0, 0, paint);
            bitmapCanvas.drawCircle(x, y, 30, eraserPaint);

            canvas.drawPath(circlePath, circlePaint);
        }

        public boolean onTouch(View view, MotionEvent event) {
            x = (int) event.getX();
            y = (int) event.getY();

            bitmapCanvas.drawCircle(x, y, 30, eraserPaint);

            circlePath.reset();
            circlePath.addCircle(x, y, 30, Path.Direction.CW);

            int ac=event.getAction();
            switch(ac){
                case MotionEvent.ACTION_UP:
                    Toast.makeText(MainActivity.this, String.valueOf(x), Toast.LENGTH_SHORT).show();
                    circlePath.reset();
                    break;
            }
            invalidate();
            return true;
        }
    }
}