Exact correct size needed

JonB

@ollarch
Thank you. I will save what I have now and rewrite your way tomorrow, and report back :)

JonB

@ollarch
Firstly, I do understand what you are saying/your approach. I had been expecting to have to take the current scaling into account and do my own conversion of mouse coords to scene/pixmap coords. I agree that your way means the QGraphicsView will do this for me, and that makes me a happy bunny :)

I followed your instructions above exactly. It almost worked, but there is a little wrinkle!

When the window is first shown the pixmap is "small", does not fit whole area --- even though I had fitToView() in the constructor (where I add the pixmap) and a resizeEvent is generated. Once I then start interactive window resizing it jumps to correct, full size, and maintains that from then on correctly.

[Also your way I did not have to put in any "magic" - 2 on sizes, it never caused scrollbars to appear (even when I still had them allowed), so no need for "small pixel adjustment, which is an added plus point :) ]

I tried "everything" (I'm pretty thorough!) but nothing remedied this. So on a hunch I put in a showEvent() override to do the fitToView() there, and sure enough that, and only that, made it also be correct when first shown.

The essentials of my now-working code are:

void MapWindow::setupUi()
{
    // as per https://doc.qt.io/qt-5/qgraphicsview.html#fitInView
    // since we intend to always call `QGraphicsView::fitToView()` in `resizeEvent()`
    // we explicitly disable scrollbars to *ensure* no infinite recursion
    view->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    view->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    // create the map as a `QGraphicsPixmapItem`
    // `pixmapMap` will be fitted tightly in view in `showEvent()`/`resizeEvent()`
    this->pixmapMap = new QGraphicsPixmapItem(*pixmapOriginalMap);
    scene->addItem(pixmapMap);
}

/*virtual*/ void MapWindow::showEvent(QShowEvent *showEvent) /*override*/
{
    // call the base method
    QWidget::showEvent(showEvent);

    view->fitInView(pixmapMap, Qt::KeepAspectRatio);
}

/*virtual*/ void MapWindow::resizeEvent(QResizeEvent *event) /*override*/
{
    // call the base method
    QWidget::resizeEvent(event);

    view->fitInView(pixmapMap, Qt::KeepAspectRatio);
}

[I am unsure about the need to call QWidget::showEvent/resizeEvent() base class, and where, in my overrides. Some examples do, some don't. With/without, or moving/before after the fitInView() call, all make no difference.]

I don't know whether you have any comment/alternative suggestion about this need to do fitView() in showEvent(), but I couldn't get it to work first time without?

ollarch

@JonB said in Exact correct size needed:

When the window is first shown the pixmap is "small", does not fit whole area

This is because when the widget constructor is executed, the widget size is not the "final" widget size. After the constructor is called you call "show" and then it calculates the size of the ui elements.

[I am unsure about the need to call QWidget::showEvent/resizeEvent() base class

You don't need to call it.

Can you check the values on the debugger of "event->s" and "event->olds" ? The first one is the new size and the last is the old size.

A workarround for this could be to use a "QTimer::singleshot(100,this,&YourWindowClass::fitMyPixmapToView)" into constructor.
, but I have not need to use it.

JonB

@ollarch said in Exact correct size needed:

This is because when the widget constructor is executed, the widget size is not the "final" widget size.

Yep, I do realize this :)

After the constructor is called you call "show" and then it calculates the size of the ui elements.

It doesn't matter when I call show(), I find I have to do the "fit" in showEvent(), i.e. when it's actually shown.

Can you check the values on the debugger of "event->s" and "event->olds" ? The first one is the new size and the last is the old size.

I did that right from the start. In the very first resizeEvent() the old size is always (-1, -1), while the (new) size is always the correct, big one. This is still the case even after I have fitted in showEvent(). So, it looks like I get the first resizeEvent() before the showEvent(), and at that point fitting doesn't work in resizeEvent() but it does in showEvent().

12:33:20: Debugging starts
resizeEvent QSize(-1, -1) QSize(1492, 772)
showEvent
// now start dragging
resizeEvent QSize(1492, 772) QSize(1491, 772)
resizeEvent QSize(1491, 772) QSize(1490, 772)
resizeEvent QSize(1490, 772) QSize(1490, 771)
...

A workarround for this could be to use a "QTimer::singleshot(100,this,&YourWindowClass::fitMyPixmapToView)" into constructor.

Indeed I could have done that. But since it works via showEvent() I'm happy.

What else can I say? :)

ollarch

Hi, I just found that I use to call "showMaximized" on "main.cpp" instead of "show" to make the application full screen.
I've tested with calling "show" and yes, it does not fit the item to the view the first time.

I don't like too much this solution but you can use this

QTimer::singleShot(50, this, SLOT(fit()));

Define "fit" method as slot.

JonB

@ollarch
Thanks, it sounds like (if I understand correctly) you are saying you find the same issue as me first time? Though I haven't seen this commented on elsewhere.

As I said earlier, I have no problem with rectifying via your QTimer, but since I have rectified via showEvent() (as per my code) why are you not satisfied with that approach. any reason?

I have moved on now to mouseworks, and am happy with my coordinates :)

ollarch

As I said earlier, I have no problem with rectifying via your QTimer, but since I have rectified via showEvent() (as per my code) why are you not satisfied with that approach. any reason?

No, I have any problem and I'm satisfied that it works.

I have moved on now to mouseworks, and am happy with my coordinates :)

Now you can try to rotate the view 45 degrees and you will see that when you map the view coordinates to scene coordinates you still have the correct pixel coordinate.

JonB

@ollarch
Indeed, though rotation is not something I will want to do, but I take your point.

Overall I am very happy with the approach you stuck with and made me adopt for my situation. As I said, I thought I was going to have to do my own coordinate transformation arithmetic taking into account my current scale/zoom setting etc. I am much happier that all I have to do is call Qt's already-written mapToScene() to do it for me, and everything makes logical, "purist" sense to me.

So thank you for sticking to your guns and recommending the approach I was really looking for!

sainivedant41

Thanks for the clarification. mobdro

ollarch

Overall I am very happy with the approach you stuck with and made me adopt for my situation. As I said, I thought I was going to have to do my own coordinate transformation arithmetic taking into account my current scale/zoom setting etc. I am much happier that all I have to do is call Qt's already-written mapToScene() to do it for me, and everything makes logical, "purist" sense to me.

I'd rather not reinvent the wheel.

So thank you for sticking to your guns and recommending the approach I was really looking for!

I'm happy. ;)

JonB

@ollarch
Hi @ollarch , hope you're around and see this question (or anyone else who knows!)....

I am moving onto my requirement to draw a shape on the view to follow the mouse. It will be see-through but with a frame, so that user can see where on map the mouse is.

For now I will do that via a QAbstractGraphicsShapeItem on the QGrapcicsScene, because that's the only way I know how to do it; plus it will use scene coordinates, which is easy.

However, I am aware this is not "correct". There should be no permanent object on the scene for this. And, for example, although I do not have multiple views onto the scene, if I did it would show up in all views. Which is not correct. This shape is a temporary object which belongs/should be shown only in the view where the mouse is moved, not on the scene shared across views.

The "correct" thing to is to draw this on the GraphicsView only (with necessary coordinate calculations, but that's not the point). I came across QRubberBand which looks like the sort of thing which should be used (and I think that can be put on the view only). But unfortunately for me that only allows a line or a rectangle, and I will want a circle or a hexagon for my usage.

So.... what is the right way to do this, please? :)

EDIT
I have now come across, say, https://www.qtcentre.org/threads/4479-Adding-Rectangular-overlay-on-graphics-view [from 2006!], which is asking just my kind of question:

To simplify my problem, lets assume that we have two different graphics views. Both of these views have the same scene applied to it. When the user clicks on a specific point in the first graphics view I want to draw a rectangle related to the point she clicked, but I want it to only be visible in that graphics view.

I think this ends up saying to use QGraphicsView::drawForeground(QPainter *painter, const QRectF &rect) to achieve this. Is that the route I should be following? [And if it is I shall have further questions! Because the principle there is not good enough as-is for following the mouse and redrawing when it moves.]

UPDATE
Now that I am beginning to understand what facilities the graphics scene/view offer. I am now sailing along, using the foreground layer of the view via QGraphicsView::drawForeground() to draw my mouse-follow-shape, calling scene()->invalidate(QRectF(), QGraphicsScene::ForegroundLayer); to cause it to redraw from mouseMoveEvent(). I think I have got this right!

Which now makes me think: maybe I should be drawing the map on which all this happens via QGraphicsScene::drawBackground() instead of in a QGraphicsPixmapItem() on the scene? For one thing, I believe the scene's background layer is cached for speed by default, which sounds good for an unchanging map background?

ollarch

So you want to display a rect only on the first view? You could create two scenes and two views and add only the rect into the first one. If the second view is some kind of "only selected view" you can take the rect position and size to crop the image and set it to the QGraphicsPixmapItem on the second scene.

JonB

@ollarch said in Exact correct size needed:
Hi @ollarch. Your post has just crossed with the latest UPDATE I have typed into my post above!

So you want to display a rect only on the first view? You could create two scenes and two views and add only the rect into the first one.

Nooo, I don't think so! 2 scenes?! Just for a shape on a view?

I'm pretty sure now that I understand a bit more that what I am doing via QGraphicsView::drawForeground() is the correct, cheapest way to draw a temporary foreground shape --- which is not an object on the scene --- to follow the mouse.

You can see there that I am getting so excited now that I understand layers --- background -> any graphics objects -> foreground --- plus the fact that QGraphicsView draws its own layers, QGraphicsView::drawForeground

Reimplement this function to provide a custom foreground for this view.

The default implementation fills rect using the view's foregroundBrush. If no such brush is defined (the default), the scene's drawForeground() function is called instead.

So I now think I ought best move the map pixmap out from being a graphics object and into the scene's background layer instead. I am enjoying this as I begin to understand :)

ollarch

If I understand it. When the mouse enters the view you want to display a rect (or circle) centered to the mouse position?

JonB

@ollarch
Yes. (Only a touch more complex than that: it has to map to nearest rect/circle/shape where the map is notionally divided into a grid of distinctly-located shapes, rather than continuous, and "snap" to there. But that is a detail.) And this shape must only be drawn on the view where the mouse is being moved; it does not belong to the scene, and if there were multiple views it must not appear in views other than the one with the mouse move. That's why it has to be view-based, not scene-based.

But I am already at this point now, I have achieved what I need, using QGraphicsView::drawForeground() :)

ollarch

Hi,

Do you have a second view, for what?

JonB

@ollarch
As I wrote earlier, presently I do not, but there is no reason I should not introduce one at a later date. One could have multiple views onto the same scene, so that user could see different areas of map in different windows (views) if desired.

At which point I would only want the foreground shape following the user's mouse to appear in the one view where he is currently moving the mouse, not any other views (even if they happened to observe the same area of the scene as the current mouse view window).

This is why the shape should be drawn in QGraphicsView::drawForeground(), which is local to one view, not either in QGraphicsScene::drawForeground() nor via placing a QGraphicsItem on the scene (both of these would show the shape in all views observing that area of the scene, which I do not want.)

As I say, I have this working well now :)

ollarch

I was only thinking on a another way to to this.

JonB

@ollarch
:) No problem. You have really helpful getting me going on gfx scene + view.

I am finding that QGraphicsView::drawForeground() is the best way to draw a temporary shape on (the top of) the given view (only). I came across that somewhere (but can't remember where) as the suggested way to achieve this best.

ollarch

Happy to help.
;D