rosu alin rosu alin - 2 months ago 35
Android Question

Eraser with PorterDuff.Mode.CLEAR always draws a black line, where i want to delete

Can i make it draw the path, where I move my finger to delete with a transparent line, or not draw at all?

This is how i instantiate my eraser:

OnClickListener eraseListener = new OnClickListener() {

@Override
public void onClick(View v) {
mPaint.setColor(0x00000000);
mPaint.setXfermode(clear);
mPaint.setAlpha(0x00);
myView.setPaint(mPaint);
LogService.log("PaintActivity", "------in eraseListener");

}
};


This sets the paint from my View that contains the canvas.
Here i have the following motion events:

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, dy;
dx = Math.abs(x - mX);
dy = Math.abs(y - mY);
if ((dx >= TOUCH_TOLERANCE) || (dy >= TOUCH_TOLERANCE)) {
undoPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
mPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
mX = x;
mY = y;
}
}
private void touch_up() {
mPath.lineTo(mX, mY);
mPath.moveTo(mX, mY);
canvas.drawPath(mPath, paint);
mPath.reset();
}
public boolean onTouchEvent(MotionEvent event) {
float x = event.getX();
float y = event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
touch_start(x, y);
break;
case MotionEvent.ACTION_MOVE:
touch_move(x, y);
invalidate();
break;
case MotionEvent.ACTION_UP:
touch_up();
invalidate();
break;
}
return true;
}


Now if i want to erase, as i said, when i move my finger, it draw a black line on that path. and when i take my finger off, on touch_up, it erases what is on the back of the black line it drawn. If i comment the invalidate(); line from the touch_move function, then it will not draw that black line, but also, it will only erase on touch_up. Can't i make it erase in real time?

Answer

I resolved it like this:

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);

        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        mPath.reset();
        mPath.moveTo(mX, mY);

        mX = x;
        mY = y;
    }
}

Here I added a line to the path, drew the path, reset the path and used moveTo in the touch_move method.

On touch_up I only use mPath.reset().

private void touch_up() {

    // kill this so we don't double draw
     mPath.reset();

}

This make my eraser transparent — no black line draw on erasing.