screen number and modal dialogs

**wysota** · 20th April 2007, 10:26

Originally Posted by high_flyer

As far as I understand it - Xinerama will allow you to span your desktop on all the creens - no Xinerama = all creens are clones of one screen.
But if I am wrong - please let me know!

I have no idea. I never managed to have more than one screen be it with or without Xinerama. I just think that if there is a way of configuring more than one screen and that Xinerama is an option, then it should be able to use multiple screens without that option.

**high_flyer** · 20th April 2007, 10:32

hmm - I looked around a bit, and it might be that you are right - that it is possible to have multiple not cloned screens - each as "stand alone" with out xinerama.
I need to read more though.
Thanks.

**high_flyer** · 20th April 2007, 10:41

Hmm - at least from Qt docs, this should not have been a problem to start with:

int QX11Info::appCells ( int screen = -1 ) [static]

Returns the number of cells used by the application on the given screen.

The screen argument is an X screen number. Be aware that if the user's system uses Xinerama (as opposed to traditional X11 multiscreen), there is only one X screen. Use QDesktopWidget to query for information about Xinerama screens.

Which I did as posted before - and screenNumber() returns 0 on both screens,whereas numScreens() returns correctly 2.
Maybe a Qt bug?

**wysota** · 20th April 2007, 10:59

According to the docs "correctly" should be "1", not "2", so if there is a bug, it's in numScreens(). The "gimp test" should shed some light on the case. If child windows are opened on the same screen as their parents, then something indeed may be wrong, but if they are opened "randomly", then I guess it's the way it should work. Besides, Qt should have nothing to do with choosing the screen to open the window on (at least not "by default").

**high_flyer** · 20th April 2007, 11:14

According to the docs "correctly" should be "1", not "2", so if there is a bug, it's in numScreens().

Why is that?

int QDesktopWidget::numScreens () const

Returns the number of available screens.

See also primaryScreen().

And I have 2...

Besides, Qt should have nothing to do with choosing the screen to open the window on (at least not "by default").

Well not with choosing the screens, but delivering the right information about the location of the widget - and allowing to choose the screen on which it should live.
And (specially) under KDE I would expect this to work - since KDE is built on Qt.
But I will read some more and look around.

**high_flyer** · 20th April 2007, 20:06

Ok, for the benefit of others who might run in to issues of that sort:
The reason I thought that its either Xinerama or clone mode is because under SUSE (which I am using) SaX2 gives only these two options.
So if you are under SUSE (currently I am on 10.1) you will have to edit your xorg.conf manually, so that the you have a "Device","Screen" and "Monitor" scetion for each screen you have.
At least with NVidia, which I use, there is an option "TwinView" which you have to comment out - and Xinerama ofcourse as well.

So now I have an X server running for each screen.
Obviously the above problem is gone but I still have to see how I will do this on the QuadView machine I am coding this for.
Its a Matrox Quad View card - not NVidia, and obviously it has no "TwinView" option.
But that is something that will have to wait for now.
At least I know what I will be looking for

Hope this is of use to someone.

**wysota** · 20th April 2007, 20:13

So it was Xinerama causing it or a yet unknown reason?

**high_flyer** · 20th April 2007, 20:27

Xinerama makes the whole desktop as one screen.
I think thats the reason why the screen number always returned 0, on both screens.
But the way I understand the docs, the screen number should return correctly even in xiberana mode.
Either I got it wrong, or Qt has a problem with this issue.

**wysota** · 20th April 2007, 20:58

I'd say it's a good candidate for a bug report. Either way something is incorrect here.

**high_flyer** · 20th April 2007, 21:07

Oh by the way - xinerama confuses gimp as well.
If I start gimp on screen 1 - all the other dialogs it wil open on screen 0.

**wysota** · 20th April 2007, 21:13

So it looks like the natural behaviour and the problem seems to be that Qt returns an incorrect number of screens or sth like that.

**high_flyer** · 23rd April 2007, 08:25

Well, yes and no.
As far as I understand the docs, Qt should return the number of physical screens, regardles if this is a viratual desktop (xinerama) or two desktops, as you can see in the illustration in the DesktopWidget docs.
Besides, the docs text it self makes no conditions on when numScreens() should return one screen (for xinerama) and N screens when not using xinerama (or virtual desktop).

All windows opened in the context of the application should be constrained to the boundaries of the primary screen; for example, it would be inconvenient if a dialog box popped up on a different screen, or split over two screens.

The text in the quote is talking from the exact problem I have (popping dialog box on another screen than parent).
So Qt is supposed to allow control over this situation, and allow the user to target physical screens on a virtual desktop - but it doesn't - not in the case I posed in this thread at least.