private void ensureSignatureBitmap() {
        if (mSignatureBitmap == null) {
            mSignatureBitmap = Bitmap.createBitmap(getWidth(), getHeight(),
                    Bitmap.Config.ARGB_8888);
            mSignatureBitmapCanvas = new Canvas(mSignatureBitmap);
        }
    }
Resets the dirty region when the motion event occurs.

@param eventX the event x coordinate.
@param eventY the event y coordinate.
signature-pad/src/main/java/com/github/gcacace/signaturepad/views/SignaturePad.java