Draw a smooth line and a little bit gradually in android

I am trying to implement a simple accessible view. Right now I am using the Path quadTo method to draw a smooth line.

And the result is like this:

I don't know how to draw the line gradually when the user moves his finger quickly. It's the same with this example:

Do you know how I can get this result? (powered or open source anyway). Right now, I'm thinking about implementing my own "quadTo" method. But I think it will be slow (or is it over my ability). Because it is a native method in the Android SDK.

Thanks for any help.

this is my app for my simple accessible view for anyone who needs it:

public class TestView extends LinearLayout{

private static final String TAG = "TestView";
private PointF previousPoint;
private PointF startPoint;
private PointF currentPoint;
private static final float STROKE_WIDTH = 5f;

private static final float HALF_STROKE_WIDTH = STROKE_WIDTH / 2;

private Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
private Paint paintBm = new Paint(Paint.ANTI_ALIAS_FLAG);
private Bitmap bmp;
private Canvas canvasBmp;
private Path path;
private int paintSize = 25;

public TestView(Context context, AttributeSet attrs) {
    super(context, attrs);

    this.setWillNotDraw(false);
    paint.setAntiAlias(true);

    paint.setStyle(Paint.Style.STROKE);
    paint.setStrokeJoin(Paint.Join.ROUND);
    paint.setStrokeCap(Paint.Cap.ROUND);
    paint.setStrokeWidth(STROKE_WIDTH);
    paint.setColor(Color.BLACK);
    //paint.setAlpha(100);

    paintBm.setAntiAlias(true);

    paintBm.setStyle(Paint.Style.STROKE);
    paintBm.setStrokeJoin(Paint.Join.ROUND);
    paintBm.setStrokeCap(Paint.Cap.ROUND);
    paintBm.setStrokeWidth(STROKE_WIDTH);
    paintBm.setColor(Color.BLACK);
    paintBm.setAlpha(100);
    path = new Path();
    //paint.setPathEffect(new CornerPathEffect(2));
}

public TestView(Context context) {
    super(context);
    // TODO Auto-generated constructor stub
    paint.setAntiAlias(true);

    paint.setStyle(Paint.Style.STROKE);
    paint.setStrokeJoin(Paint.Join.ROUND);
    paint.setStrokeCap(Paint.Cap.ROUND);
    paint.setStrokeWidth(STROKE_WIDTH);
    paint.setColor(Color.BLACK);

    path = new Path();
    //paint.setPathEffect(new CornerPathEffect(2));
}



@Override
protected void onLayout(boolean changed, int left, int top, int right,
        int bottom) {
    // TODO Auto-generated method stub
    super.onLayout(changed, left, top, right, bottom);
    if(bmp == null){
        bmp = Bitmap.createBitmap(right-left,bottom-top,Bitmap.Config.ARGB_8888);
        canvasBmp = new Canvas(bmp);

    }
}
@Override
public boolean onTouchEvent(MotionEvent event) {
    //printSamples(event);
    switch (event.getAction()) {
    case MotionEvent.ACTION_DOWN:
        currentPoint = new PointF(event.getX(), event.getY());
        previousPoint = currentPoint;
        startPoint = previousPoint;
        path.reset();
        break;

    case MotionEvent.ACTION_MOVE:
        startPoint = previousPoint;
        previousPoint = currentPoint;
        currentPoint = new PointF(event.getX(), event.getY());
        int historySize = event.getHistorySize();
        for(int i = 0; i < historySize; i++){

        }



        drawLine(canvasBmp, path, paint, previousPoint, currentPoint);
        //path.moveTo(currentPoint.x, currentPoint.y);
        break;
    case MotionEvent.ACTION_UP:
        startPoint = previousPoint;
        previousPoint = currentPoint;
        currentPoint = new PointF(event.getX(), event.getY());
        drawLine(canvasBmp, path, paint, previousPoint, currentPoint);
        paintSize = 25;
        break;
    default:
        break;
    }

    invalidate();
    return true;// super.onTouchEvent(event);
}


@Override
protected void onDraw(Canvas canvas) {
    Log.v("pichan", "dasd");
    //canvas.drawBitmap(bmp, 0,0, null);
    //canvas.drawColor(Color.BLUE);
    //canvas.drawPath(path, paint); 
    canvas.drawBitmap(bmp, 0, 0, paintBm);

}





private void drawLine(Canvas canvas, Path path, Paint paint, PointF start, PointF end)
{
    PointF mid1 = midPoint(previousPoint, startPoint);
    PointF mid2 = midPoint(end, start);

    path.reset();
    paint.setStrokeWidth(paintSize);
    path.moveTo(mid1.x, mid1.y);
    path.quadTo(previousPoint.x, previousPoint.y, mid2.x, mid2.y);
    canvas.drawPath(path, paint);
    //canvas.
    //paintSize -= 1;
}

private PointF  midPoint(PointF p1, PointF p2)
{
    return new PointF((p1.x + p2.x) / 2.0f ,  (p1.y + p2.y) * 0.5f);
}

}