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

[Thomas Fjellstrom]

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.

> Regarding ItemIgnoresTransformations, the docs say "The item ignores
> inherited transformations (i.e., its position is still anchored to its
> parent, but the parent or view rotation, zoom or shear transformations
> are ignored)."
> 
> So yes, translation will not be ignored.

Thank you.

> --
> Samuel
> 


-- 
Thomas Fjellstrom
tfjellstrom at shaw.ca