Qt: How to Find the Position of the Mouseclick Relative to an Image

Question

I would like to implement an image editor. I have a QPixmap in a QLabel in a QHBoxLayout. I have overriden the mousePressEvent in the parent widget. When the mousePressedEvent occurs, the

 event->pos() = QPoint(271,115)

points to a place which is displaced relative to the pointer (mouse). The displacement is the distance of the QLabel from the QWidget's corner. It gets bigger when I resize the window. How do I find this displacement vector? I want to draw a pixel on the QPixmap exactly where the mouse is. Note that the following methods give no remedy:

qDebug() << "event->pos()" << event->pos();
qDebug() << "this->pos() = " << this->pos();
qDebug() << "pm_imageLabel->pos() =" << pm_imageLabel->pos();
qDebug() << "pos = " << mapFromGlobal(QCursor::pos());

These give all different positions. No searching on the internet or in Qt's documentation brought me closer to the answer. Thank You in advance.

Answer 1

Finally I have figured it out partially with the help of vahancho. The the position of the QPixmap withing QLabel is difficult to determine, but I can forbid QLabel to resize. So I set the size of QLabel to the image size.

pm_imageLabel->setPixmap(m_pixmap);
pm_imageLabel->setFixedSize(m_pixmap.size());

and I override the mousePressed even inside QLabel class. This way the event->pos is correct.

Thanks.

Answer 2

I know it's been a while but i found a solution that works without having to resize the QLabel. The solution is in Python.

label = QLabel(...)
img_pix = QPixmap(...)
label.setPixmap(img_pix)

# now you can get mouse click coordinates on the label by overriding `label.mousePressEvent`

# assuming we have the mouse click coordinates
coord_x = ...
coord_y = ...

# calculating the mouse click coordinates relative to the QPixmap (img_pix)
img_pix_width = img_pix.width()
img_pix_heigth = img_pix.height()

label_width = label.width()
label_height = label.height()

scale_factor_width = label_width / img_pix_width
scale_factor_height = label_height / img_pix_heigth

relative_width_in_img_pix = coord_x / scale_factor_width 
relative_height_in_img_pix = coord_y / scale_factor_height

relative_coordinates_in_img_pix = QPoint(relative_width_in_img_pix, relative_height_in_img_pix)