What is the fastest way to get QWidget pixel color under mouse?

c++, qt, qt4.8

Solution

The "idiomatic" way of doing this in Qt is completely different from what you're describing. You'd use the Graphics View Framework for this type of thing.

Graphics View provides a surface for managing and interacting with a large number of custom-made 2D graphical items, and a view widget for visualizing the items, with support for zooming and rotation.

You'd define your own `QGraphicsItem` type for the "cells" in the breadboard that would react to hover enter/leave events by changing their color. The connections between the cells (wires, resistors, whatever) would also have their own graphics item types with the features you need for those.

Here's a quick and dirty example for you. It produces a 50x50 grid of green cells that become red when the mouse is over them.

#include <QtGui>

class MyRect: public QGraphicsRectItem
{
    public:
        MyRect(qreal x, qreal y, qreal w, qreal h)
            : QGraphicsRectItem(x,y,w,h) {
            setAcceptHoverEvents(true);
            setBrush(Qt::green);
        }
    protected:
        void hoverEnterEvent(QGraphicsSceneHoverEvent *) {
            setBrush(Qt::red);
            update();
        }
        void hoverLeaveEvent(QGraphicsSceneHoverEvent *) {
            setBrush(Qt::green);
            update();
        }
};

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    QGraphicsScene scene;
    for (int i=0; i<50; i++)
        for (int j=0; j<50; j++)
            scene.addItem(new MyRect(10*i, 10*j, 8, 8));

    QGraphicsView view(&scene);
    view.show();
    return app.exec();
}

You could modify the hover event handlers to talk to your "main window" or "controller" indicating what's currently under the mouse so you can update your caption, legend box or tool palette.

Problem

Original source