[Qt-interest] QGraphicsItem ignore scene translation/transform?

[Thomas Fjellstrom]

On Fri January 22 2010, Samuel Rødal wrote:
> ext Thomas Fjellstrom wrote:
> > On Fri January 22 2010, Thomas Fjellstrom wrote:
> >> On Fri January 22 2010, you wrote:
> >>> ext Thomas Fjellstrom wrote:
> >>>> I'm trying to get a few items to completely ignore the scene's
> >>>> current translation and transforms. That is, I'd like the item to
> >>>> stay at a fixed position in the view, regardless of any zooming, or
> >>>> movement in the view itself. And do that without it looking like I'm
> >>>> manually moving the item with the view (ie: it lagging slightly
> >>>> behind the scrolling).
> >>>>
> >>>> I've played with the QGraphicsItem::ItemIgnoresTransformations flag,
> >>>> it definitely doesn't ignore translations in the scene at all, and
> >>>> it seems to mess up the "y rotate" animation I added to a couple
> >>>> items. It looks like its shifting the item's coordinates around a
> >>>> little. I use: setTransformOriginPoint(size().width()/2,
> >>>> size().height()/2); to set the transform point to the centre of the
> >>>> item, so my item (without the ignore flag) flips around the y axis
> >>>> (centre of the item) nicely. With the ignore flag, it flips around
> >>>> the right hand edge of the item, and seems to shift its x position
> >>>> as it flips.
> >>>>
> >>>> I'm using Qt 4.6.0 (from qt.nokia.com, not a distro package), on
> >>>> linux 2.6.32 + KDE 4.3.4.
> >>>>
> >>>> Is there any way to get the behaviour I'm looking for? Thank you.
> >>>
> >>> Have you considered not transforming the view, and instead making all
> >>> the transformable item children of a top-level invisible graphics
> >>> item? Then you could transform all the child items by transforming
> >>> the top-level, leaving the non-transformable items as they were.
> >>
> >> How would that mess up scrolling the items in that top level item? I
> >>  assume I'd have to do it manually myself now?
> >>
> >> Basically I have a 2D game board with each square as a separate
> >> QGWidget, with a large enough board it easily becomes larger than the
> >> view (even on my 24" 1920x1080 LCD), and will need to scroll the items
> >> around. I also want to enable scaling the board up and down so all the
> >> items can be seen at once, or only a small subset (but much larger).
> >> Along with that I plan to make the entire gui of the game be widgets
> >> in the QGV. Right now I have my login, register, and "game request"
> >> dialogs as themed QGWidgets, they work fairly well, I just want to
> >> clean up the user interaction a little. I also intend to make the game
> >> status information be a horizontal translucent bar at the top of the
> >> view, as well as moving my current "chat" and "userlist" interface
> >> inside the view as well, but all of them will need to have a static
> >> transform and translation relative to the view. I don't want any of
> >> the UI to zoom or move with the board.
> >>
> >> Given your email address, I can only assume that what I want probably
> >>  isn't possible without making some more custom widgets. The only
> >>  solution I've come up with so far (based on your suggestion) is to
> >> make the board a fancier widget, that handles its own scrolling and
> >> zooming, probably utilizing the animation framework to handle smooth,
> >> natural feeling scrolling (and zooming).
> >>
> >> Luckily the only bits I'll want to zoom, or translate are inside my
> >> "GameBoard" widget, so it may not require many changes, other than
> >> maybe a new "ScrollZoomItem" that handles scrolling and zooming one or
> >> more subwidgets, including scrollbars...
> >>
> >> But it would be nice if Qt supported something like this a little more
> >> directly.
> >>
> >> But then, maybe I'm just lazy.
> >
> > The fun part will probably be getting the board to properly accept
> > input (each square on the board accepts mouse clicks, and "grow" on
> > hover), as well as scroll and zoom.
> 
> I'm not sure why scrolling and zooming will be a problem.

Imagine scrolling and zooming on a touch based interface. Similar to all of 
the new smart phones with a big screen and no keyboard. But it should (also) 
work with a regular mouse.

> Basically the
> top-level item is just an invisible item whose bounding rect is the same
> as the combined bounding rect of the child items (game squares).
> Whenever you earlier scaled / scrolled the view itself you'd now do the
> same with the top level item. You might wish to use
> QGraphicsScene::setSceneRect() to control which part of the scene should
> be mapped to the view at any time.
>
> I don't think this way of doing things should be significantly harder
> than changing the view transformation.

But how to actually handle the events is something else. Right now the child 
widgets all accept mouse events, which means the parent widget doesn't get 
them, so it can't know when a person is trying to scroll or zoom.

I'm not sure how to have both, selecting the game elements, AND letting the 
game board accept events to handle scrolling and zooming.
 
> --
> Samuel
> 

I find it odd that your messages to the list aren't getting through right 
away while mine are. Is there something up with the mailing list?

-- 
Thomas Fjellstrom
tfjellstrom at shaw.ca