Android: Painting app with eraser not working

Question

I am working on a painting app, with undo/redo function and would like to add eraser function.

Code for MainActivity

case R.id.undoBtn:          
     doodleView.onClickUndo();          
     break;

case R.id.redoBtn:          
     doodleView.onClickRedo();          
     break;

case R.id.eraserBtn:            
     Constants.mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT)); break; 

DrawView

// Drawing Part    
    private Bitmap mBitmap;   
    private Paint  mBitmapPaint;
    private Canvas mCanvas; 
    private Path   mPath; 
    private int selectedColor = Color.BLACK;
    private int selectedWidth = 5;

    private ArrayList<Path> paths = new ArrayList<Path>();
    private ArrayList<Path> undonePaths = new ArrayList<Path>();
    private Map<Path, Integer> colorsMap = new HashMap<Path, Integer>();
    private Map<Path, Integer> widthMap = new HashMap<Path, Integer>();

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4; 
    Context context_new;

   public DoodleView(Context c, AttributeSet attrs) 
   {
       super(c, attrs);
       context_new = c;    
       setFocusable(true);
       setFocusableInTouchMode(true);
       setLayerType(View.LAYER_TYPE_SOFTWARE, null); // for solely removing the black eraser

       mPath = new Path(); 
       mCanvas = new Canvas();  
       mBitmapPaint = new Paint(Paint.DITHER_FLAG);   

       Constants.mPaint = new Paint();
       Constants.mPaint.setAntiAlias(true); // smooth edges of drawn line
       Constants.mPaint.setDither(true);
       Constants.mPaint.setColor(Color.BLACK); // default color is black
       Constants.mPaint.setStyle(Paint.Style.STROKE); // solid line
       Constants.mPaint.setStrokeJoin(Paint.Join.ROUND);
       Constants.mPaint.setStrokeWidth(20); // set the default line width
       Constants.mPaint.setStrokeCap(Paint.Cap.ROUND); // rounded line ends 
       Constants.mPaint.setXfermode(null);
       Constants.mPaint.setAlpha(0xFF);
   } 

   @Override
   public void onSizeChanged(int w, int h, int oldW, int oldH)
   {
       super.onSizeChanged(w, h, oldW, oldH);
       mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);

      Constants.DRAW_W = w;
      Constants.DRAW_H = h;
      Log.d("TAG", "onSizeChanged!!!" + Constants.DRAW_W + Constants.DRAW_H + Constants.SCREEN_W + Constants.SCREEN_H);   
      // bitmap.eraseColor(Color.WHITE); // erase the BitMap with white    
   }   

   @Override
   protected void onDraw(Canvas canvas) 
   {
       canvas.drawBitmap(mBitmap, 0, 0, null); // draw the background screen

       for (Path p : paths)
       {
           Constants.mPaint.setColor(colorsMap.get(p));
           Constants.mPaint.setStrokeWidth(widthMap.get(p));
           canvas.drawPath(p, Constants.mPaint);           
       }       
       Constants.mPaint.setColor(selectedColor);
       Constants.mPaint.setStrokeWidth(selectedWidth);
       canvas.drawPath(mPath, Constants.mPaint);                   
   } 

   @Override
   public boolean onTouchEvent(MotionEvent event) 
   {          
          float x = event.getX();
          float y = event.getY();

          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;
    }

   private void touch_start(float x, float y) 
   {
       undonePaths.clear();
       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;

           startdrawing = true;
       }
   }

   private void touch_up() 
   {
       mPath.lineTo(mX, mY);      
       mCanvas.drawPath(mPath, Constants.mPaint); 
       paths.add(mPath);
       colorsMap.put(mPath,selectedColor);
       widthMap.put(mPath,selectedWidth);
       mPath = new Path(); 
   }

   public void onClickUndo() 
   { 
       if (paths.size()>0) 
        { 
           undonePaths.add(paths.remove(paths.size()-1));
           invalidate();
        }      
       else Toast.makeText(getContext(), R.string.toast_nothing_to_undo, Toast.LENGTH_SHORT).show();  
    }

   public void onClickRedo()
   {
       if (undonePaths.size()>0) 
       { 
           paths.add(undonePaths.remove(undonePaths.size()-1)); 
           invalidate();
       } 
       else 
           Toast.makeText(getContext(), R.string.toast_nothing_to_redo, Toast.LENGTH_SHORT).show(); 
    }  

   public void setDrawingColor(int color)
   {       
       selectedColor = color;
       Constants.mPaint.setColor(color);
   } 

   public int getDrawingColor()
   {
      return Constants.mPaint.getColor();
   } 

Question:

Normal painting and undo / redo can be performed perfectly. However, the eraser does not work.

1. After pressing (touch_start) the eraser button and touch the screen, all the previous drawn line immediately turn black.

2. When using the eraser upon touch_move , the eraser itself is drawing black lines, even though it was on CLEAR mode.

3. Upon touch_up, All other drawn lines maintained at black color. The "erased" area was also in black color. However, when drawing a new path subsequently, the original line turned to their original color, the area "erased" by the eraser turn its color to the last color chosen and the paths retained in the View.

How could the code on eraser be written in a proper way? (keeping undo / redo)

Thanks a lot in advance!

Answer 1

It's hard to answer your question, but. I'd implement the erase functionality as follows:

The erase function will by simple white path. So the erase mode means that you are drawing white path which will be wider than the drawing path. This way you will be able to select even the width of the eraser and the undo/redo functionality will stay the same.

Answer 2

Try this type of application with samsung spen sdk, all this funtions are simplified there. SPen Sdk Tutorial

or

Make your erase paint color to your background color.

Answer 3

// Please use this is erasure concept . This is working and tested.

public void addErasure(){
        drawView.setErase(true);
        drawView.setBrushSize(20);
    }

    public void addPencil(){
        drawView.setErase(false);
        drawView.setBrushSize(20);
        drawView.setLastBrushSize(20);
    }
/// this my drawing view. you can add this view into your main layout.

    public class DrawingView extends View {

        //drawing path
        private Path drawPath;
        //drawing and canvas paint
        private Paint drawPaint, canvasPaint;
        //initial color
        private int paintColor = 0xFF660000;
        //canvas
        private Canvas drawCanvas;
        //canvas bitmap
        private Bitmap canvasBitmap;
        //brush sizes
        private float brushSize, lastBrushSize;
        //erase flag
        private boolean erase=false;

        private boolean isFirstTime = false;

        public DrawingView(Context context, AttributeSet attrs){
            super(context, attrs);
            //setBackgroundColor(Color.CYAN);
            setupDrawing();
        }

        //setup drawing
        private void setupDrawing(){

            //prepare for drawing and setup paint stroke properties
            brushSize = getResources().getInteger(R.integer.medium_size);
            lastBrushSize = brushSize;
            drawPath = new Path();
            drawPaint = new Paint();
            drawPaint.setColor(paintColor);
            drawPaint.setAntiAlias(true);
            drawPaint.setStrokeWidth(brushSize);
            drawPaint.setStyle(Paint.Style.STROKE);
            drawPaint.setStrokeJoin(Paint.Join.ROUND);
            drawPaint.setStrokeCap(Paint.Cap.ROUND);
            canvasPaint = new Paint(Paint.DITHER_FLAG);
            //canvasPaint.setColor(Color.GREEN);
        }

        //size assigned to view
        @Override
        protected void onSizeChanged(int w, int h, int oldw, int oldh) {
            super.onSizeChanged(w, h, oldw, oldh);
            canvasBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            Bitmap canvasBackGroundBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            canvasBackGroundBitmap = getResizedBitmap(BitmapFactory.decodeResource(getResources(), R.drawable.pop_up_big_bg),h,w);
    //      drawCanvas = new Canvas(canvasBackGroundBitmap);
    //      drawCanvas.drawColor(Color.GREEN);
            setBackgroundDrawable(new BitmapDrawable(canvasBackGroundBitmap));

            drawCanvas = new Canvas(canvasBitmap);
        }

        public static Bitmap getResizedBitmap(Bitmap bm, int newHeight, int newWidth) {
            int width = bm.getWidth();
            int height = bm.getHeight();
            float scaleWidth = ((float) newWidth) / width;
            float scaleHeight = ((float) newHeight) / height;
            // CREATE A MATRIX FOR THE MANIPULATION
            Matrix matrix = new Matrix();
            // RESIZE THE BIT MAP
            matrix.postScale(scaleWidth, scaleHeight);

            // "RECREATE" THE NEW BITMAP
            Bitmap resizedBitmap = Bitmap.createBitmap(bm, 0, 0, width, height, matrix, false);
            return resizedBitmap;
        }

        //draw the view - will be called after touch event
        @Override
        protected void onDraw(Canvas canvas) {
            canvas.drawBitmap(canvasBitmap, 0, 0, canvasPaint);
            canvas.drawPath(drawPath, drawPaint);
        }

        //register user touches as drawing action
        @Override
        public boolean onTouchEvent(MotionEvent event) {
            float touchX = event.getX();
            float touchY = event.getY();
            //respond to down, move and up events
            switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                drawPath.moveTo(touchX, touchY);
                break;
            case MotionEvent.ACTION_MOVE:
                drawPath.lineTo(touchX, touchY);
                drawCanvas.drawPath(drawPath, drawPaint);
                drawPath.reset();
                drawPath.moveTo(touchX, touchY);
                break;
            case MotionEvent.ACTION_UP:
                //drawPath.lineTo(touchX, touchY);
                //drawCanvas.drawPath(drawPath, drawPaint);
                drawPath.reset();
                break;
            default:
                return false;
            }
            //redraw
            invalidate();
            return true;

        }





        //update color
        public void setColor(String newColor){
            invalidate();
            paintColor = Color.parseColor(newColor);
            drawPaint.setColor(paintColor);
        }

        //set brush size
        public void setBrushSize(float newSize){
            float pixelAmount = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 
                    newSize, getResources().getDisplayMetrics());
            brushSize=pixelAmount;
            drawPaint.setStrokeWidth(brushSize);
        }

        //get and set last brush size
        public void setLastBrushSize(float lastSize){
            lastBrushSize=lastSize;
        }
        public float getLastBrushSize(){
            return lastBrushSize;
        }

        //set erase true or false
        public void setErase(boolean isErase){
            erase=isErase;
            if(erase) {
                drawPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
            }else {
                drawPaint.setXfermode(null);
            }
        }

        //start new drawing
        public void startNew(){
            drawCanvas.drawColor(0, PorterDuff.Mode.CLEAR);
            invalidate();
        }
    }

Answer 4

Try this solution - works great with undo/redo as well:

   private float blur = 0F;

    case R.id.eraserBtn:            
         Constants.mPaint.setColor(Color.WHITE); // or whatever color to match canvas color
         Constants.mPaint.setShadowLayer(this.blur, 0F, 0F, Color.WHITE);

With this solution, no need to set:

 Constants.mPaint.setXfermode(null);