Qt: Why does my paintEvent() erase everything before doing its job?

Question

I'm trying to make my widget paint a rectangle with every paintEvent it receives. The rectangles are supposed to increase in size by 1px at a time, filling a square. What I get, however, is only the latest (and largest) rectangle.

void TestClass::paintEvent(QPaintEvent* e){
    static int size = 1;
    QStylePainter painter(this);
    painter.setPen(Qt::blue);
    painter.drawRect(QRect(50, 50, size, size));
    size++;
}

I don't understand why it would be that way. I expected the painter to just paint on top of what is already there. Instead it seems to delete the previously drawn rectangle, leaving me with a single rectangle at any time. Any ideas?

setAutoFillBackground(true/false) does not change anything but the color of the background.

To evoke a paintEvent I update() inside mousePressEvent(). So my rectangles grow with every click.

Thanks a lot.

You only see the largest square, because it completely covers all smaller squares drawn before. What do you expect to see? — Flopp, Mar 08 '16 at 21:08
@Flopp That's interesting, thanks. The rectangles are not filled, so I did not expect them to be able to cover anything. I'll see if I can verify that. — Jersey, Mar 08 '16 at 21:16
Hm, you're right; Your code is supposed to paint a non-filled rectangle. — Flopp, Mar 08 '16 at 21:32
I believe that if there is a single object instance of the `TestClass`, its `paintEvent()` will only perform one instance of the painting... therefore if you are not updating the widget, the painting will update the region affected - the lines of the new rectangle (which touch the old ones). Test this by making your rectangles smaller starting from some large one... (After reading Kuba Ober answer - that may also be true if your size increments are much larger - in which case there would be no reason for the old lines to be updated) — Thalia, Mar 08 '16 at 21:39

score 3 · Accepted Answer · answered Mar 10 '16 at 19:45

So, to answer my own question, I found out that:

(1) update(QRect area) erases the area specified by its argument before doing anything else. Calling update without argument erases the whole widget area.

(2) The area that has been cleared is the only area that any painting is done on, even if your paintEvent() looks to paint somewhere else. The untouched part of the widget is not affected.

For example, consider this paintEvent().

void myWidget::paintEvent(QPaintEvent* e){
    QPainter painter(this);
    static int counter = 1;

    if (counter % 2){
        painter.fillRect(0, 0, 199, 199, Qt::blue); //Fill with blue color
    } else {
        painter.fillRect(0, 0, 199, 199, Qt::green); //Fill with green color
    }
    counter++;
}

Calling update() repeatedly will toggle between green and blue color for every pixel. However, upon calling update(QRect(0, 0, 50, 50)) only the top left quarter of the widget's area will change its color, the other pixels will remain untouched, even if paintEvent(..) contains the instruction to always paint over the whole widget's area.

I do not know whether my answer is fully correct under any circumstances, but I expect more noobs to be confused about the relationship between update() and paintEvent() and so I'll offer this as a first aid.

Greets.

score 0 · Answer 2 · answered Mar 08 '16 at 21:34

0

The QWidget::paintEvent docs state that

When the paint event occurs, the update region has normally been erased, so you are painting on the widget's background.

So, probably that's the case in your example.

answered Mar 08 '16 at 21:34

Flopp

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Well, does "normally" refer to good programming standards or is there a Qt mechanism at work? I guess I misunderstood how painting is done in qt. I thought anything related to painting a widget happens inside the paintEvent(). – Jersey Mar 08 '16 at 21:47

score 0 · Answer 3 · answered Mar 08 '16 at 21:37

You're not guaranteed any sort of state preservation between invocations of paintEvent. That's the long and the short of it. The only correct semantics of your implementation of paintEvent are: paint (at least) the entire region that was passed to you, as in: touch every pixel, unless your widget was pre-cleared prior to painting.

Qt: Why does my paintEvent() erase everything before doing its job?

3 Answers3