How to get value with mouse event drag - c++

Question

I have in my UI one QScrollArea with an image, and I want to get some value when I click on the image.
Being more explict, I need to change the brightness of an image, and I will get the value with the mouse. I already see the MouseMoveEvent but I don't know how to use it.
If I get the position of the mouse when I click and drag, I can extract one value to change the bright of my image, that I know. I just don't know how I will get the position.
Does anyone know how I can do this?
Ps.: My QScrollArea was created on Design, so I don't have any code writed by me with the specifications of the QScrollArea.

Answer 1

there is a small correction . when i programmed like this i was getting the movements a liitle jerky . so i tried to modified the code and now working perfect .
heres how i changed
const int deltaX = event->scenePos().x() - m_lastClickPosition.x();
if ( deltaX > 10 )// to the right
{
moveBy(10,0);
m_lastClickPosition = event->scenePos();
}
else if ( deltaX <-10 )
{
moveBy(-10,0);
m_lastClickPosition = event->scenePos();
}

Answer 2

All the information you need is in the QMouseEvent object that is sent to the mouseMoveEvent handler of your widget.
QMouseEvent::buttons()
QMouseEvent::pos()
A simple way to do what you're after is to change the brightness of the image whenever you receive a "mouse movement event" and the QMouseEvent object reports a button down (this means the user is moving the mouse while holding down a button).
void MyWidget::mousePressEvent( QMouseEvent* event )
{
if ( event->button() == Qt::LeftButton )
{
// Keep the clicking position in some private member of type 'QPoint.'
m_lastClickPosition = event->pos();
}
}
void MyWidget::mouseMoveEvent( QMouseEvent* event )
{
// The user is moving the cursor.
// See if the user is pressing down the left mouse button.
if ( event->buttons() & Qt::LeftButton )
{
const int deltaX = event->pos().x() - m_lastClickPosition.x();
if ( deltaX > 0 )
{
// The user is moving the cursor to the RIGHT.
// ...
}
else if ( deltaX < 0 ) // This second IF is necessary in case the movement was all vertical.
{
// The user is moving the cursor to the LEFT.
// ...
}
}
}