Drawing transparent line on canvas of imageview in android

Issue

The problem is, I attempt to change the opacity to 100 which should be transparent, but when I try to draw the line it has some circle on the line. (ref to the screenshot) Highly appreciate if provide some sample code. Thanks a lot for helping.

Code from MainActivity

// set image
bitmap = downScale(view.getTag().toString(),1280,1024);
altered_bitmap = Bitmap.createBitmap(bitmap.getWidth(),bitmap.getHeight(), bitmap.getConfig());
draw_view.setNewImage(altered_bitmap,bitmap);

  pen.setOnClickListener(new OnClickListener(){
            @Override
            public void onClick(View arg0) {
                    draw_view.setAlpha(100);
                }
            }
        });

And Code from the custom imageView

public ScaleImageView(Context context) {
        super(context);
        sharedConstructing(context);
    }

public void sharedConstructing(Context context) {
        super.setClickable(true);
        this.context = context;

        mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
        matrix = new Matrix();
        m = new float[9];
        setImageMatrix(matrix);
        setScaleType(ScaleType.MATRIX);

        paint = new Paint();

        paint.setAntiAlias(true);
        paint.setStrokeWidth(width);
        paint.setColor(color);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeCap(Paint.Cap.ROUND);
        paint.setAlpha(alpha);


        drawListener = new OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                if (getDrawable() != null) {
                    int action = event.getAction();
                    switch (action) {
                        case MotionEvent.ACTION_DOWN:
                            downx = getPointerCoords(event)[0];// event.getX();
                            downy = getPointerCoords(event)[1];// event.getY();
                            break;
                        case MotionEvent.ACTION_MOVE:
                            upx = getPointerCoords(event)[0];// event.getX();
                            upy = getPointerCoords(event)[1];// event.getY();
                            canvas.drawLine(downx, downy, upx, upy, paint);
                            mPath = new Path();
                            paths.add(mPath);
                            invalidate();
                            downx = upx;
                            downy = upy;
                            break;
                        case MotionEvent.ACTION_UP:
                            upx = getPointerCoords(event)[0];// event.getX();
                            upy = getPointerCoords(event)[1];// event.getY();
                            canvas.drawLine(downx, downy, upx, upy, paint);
                            mPath = new Path();
                            paths.add(mPath);
                            invalidate();
                            break;
                        case MotionEvent.ACTION_CANCEL:
                            break;
                        default:
                            break;
                    }
                }
                return true;
            }
        };

        setOnTouchListener(drawListener);
    }

    //draw view start
    public void setNewImage(Bitmap alteredBitmap, Bitmap bmp) {
        canvas = new Canvas(alteredBitmap);
        matrix_draw = new Matrix();
        canvas.drawBitmap(bmp, matrix_draw, paint);
        setImageBitmap(alteredBitmap);  
        mPath = new Path();
        paths.add(mPath);
    }

    public void setBrushColor(int color) {
        this.color = color;
        paint.setColor(color);
        paint.setAlpha(alpha);
    }

    public void setAlpha(int alpha) {
        this.alpha = alpha;
        paint.setAlpha(alpha);
    }

    public void setWidth(float width) {
        this.width = width;
        paint.setStrokeWidth(width);
    }

    final float[] getPointerCoords(MotionEvent e) {
        final int index = e.getActionIndex();
        final float[] coords = new float[] { e.getX(index), e.getY(index) };
        Matrix matrix = new Matrix();
        getImageMatrix().invert(matrix);
        matrix.postTranslate(getScrollX(), getScrollY());
        matrix.mapPoints(coords);
        return coords;
    }

    public void setIsScale() {
        isScale = !isScale;
        setOnTouchListener(isScale ? zoomListener : drawListener);
    }

And the screenshot

Update: Project code

Since it is quite difficult to figure out the problem though code extract , I have uplod the project (<1 mb) along with the used library:

if you have some spare time , you are welcome to take a look it is a small drawing tool , first copy a folder with some images to the folder "HistoryTool" in your device

The path , for example, like:

sd card root/ HistoryTool/ folder1 / a.jpg 

, then you can draw on it, but the draw transparent line has circle on it. thats all

https://drive.google.com/file/d/0B9mELZtUJp0LZncwQVM4alExalE/view?usp=sharing

Solution

I have complete the task by using On-draw and Path

        drawListener = new OnTouchListener() {
        @Override
        public boolean onTouch(View v, MotionEvent event) {
            if (getDrawable() != null) {
                int action = event.getAction();
                switch (action) {
                    case MotionEvent.ACTION_DOWN:
                        downx = getPointerCoords(event)[0];// event.getX();
                        downy = getPointerCoords(event)[1];// event.getY();
                        holderList.add(new Holder(color, width, alpha));
                        holderList.get(holderList.size() - 1).path.moveTo(downx, downy);
                        break;
                    case MotionEvent.ACTION_MOVE:
                        upx = getPointerCoords(event)[0];// event.getX();
                        upy = getPointerCoords(event)[1];// event.getY();
                        holderList.get(holderList.size() - 1).path.lineTo(upx, upy);
                        invalidate();
                        downx = upx;
                        downy = upy;
                        break;
                    case MotionEvent.ACTION_UP:
                        break;
                    case MotionEvent.ACTION_CANCEL:
                        break;
                    default:
                        break;
                }
            }
            return true;
        }
    };

    setOnTouchListener(drawListener);
}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    if (img != null) {
        tmp_canvas.drawBitmap(img, 0, 0, null);
    }
    for (Holder holder : holderList) {
        tmp_canvas.drawPath(holder.path, holder.paint);
    }
}

//draw view start
public void setNewImage(Bitmap alteredBitmap, Bitmap bmp) {
    tmp_canvas = new Canvas(alteredBitmap);
    img = bmp;
    tmp_canvas.drawBitmap(bmp, 0, 0, null);
    setImageBitmap(alteredBitmap);
}

Answered By - user782104