I know that thumbnails of windows can be created, look at kpager2 (http://www.kde-look.org/content/show.php?content=7120) or Taskbar v2 (http://www.kde-look.org/content/show.php?content=16261), both of these small applets have a mouseover effect where a small thumbnail of the window pops up. You can also have it in the kasbar.
I also know that stuff can be drawn on the KDE desktop, i.e (super)karamba does this, and that actions can be performed on this items.
The thing I don't know is how to create a program that would do this, and this is where you, whoever might be reading this, comes in. If anyone knows how to implement this idea, or know someone who can, or will help me getting started (or already knows about someone who is implementing this
Well it could happen) I would be very grateful (or maybe Krateful 
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http://symphonyos.com/desktop.html . I was thinking about something similar. when you hold mouse over taskbar items, small screenshot of that window could be shown, like tooltips are now. I'll make mockup soon. In fact I am not sure if I've seen it somewhere already on kde-look.
do you mean something like the pager (not the kicker pager, but the seperate one), or are you talking about something like the kasbar (but with the window previews instead of the icons)?
I'm thinking about something like the pager window, only without the window, just a thumbnail of the currently minimized windows placed directly on the desktop. Look at the top left of the screenshot in the download.
Hey. That's a great idea, indeed! Instead of putting it on the desktop I was thinking about writing a kind of iconbox applet for kicker, similar to enlightenment's iconbox, but having thumbnails in it. On the fvwm info page I found a year back, when they introduced this feature, that they listen to a new XEvent which is emitted just before a window is minimized. The KWin API, however, does not support this event yet. Moreover I don't have any clue how to catch this XEvent directly (I even didn't find which one it is). If someone has a nice link with code examples or a tutorial I would start implementing this. Best regards Sebastian
I was thinking that an option could be to have all open windows drawn (both minimized and normal) that way one wouldn't have to mind about the XEvent you mentioned, just create the thumbnail when a window is created, and remove it when the window is closed.
You are wrong because of two reasons: (1) Live updates of thumbnails are only possible utilizing the new XComposite extension. XComposite, however, lacks of providing a thumbnail for minimized or shaded windows. It's not usable. We need to know when the window is going to be minimized and ask X for the screenshot. (2) Passive updates by taking screenshots of the actual visible screen are already possible. Examples are Kasbar, KPager(2) and TaskbarV2. These take screenshots of the current active window every 10 or 15 seconds. Problems: (a) Taking the screenshot takes processing time. The cpu hangs every 15 seconds :(. (b) The screenshots of other windows are outdated. (c) It works only in KWin's 'Click-to-focus' mode, which means that the token screenshots may look ugly if other windows cover the active one. (d) To have a better quality, take pictures even on window move and window resize events. And (e): As seen in TaskbarV2 and KPager2 (I am the author) checking all these things produced errors - even if they won't let kicker crash, they may have strong effects on the systems performance. No - if, then we should find a nice and perfect solution.
In your first post you said you didn't know how to catch XEvents, after studying the kde and qt api documentation for some time I think I may have found what you're looking for. From http://doc.trolltech.com/3.3/qapplication.html: bool QApplication::x11EventFilter ( XEvent * ) [virtual] This virtual function is only implemented under X11. If you create an application that inherits QApplication and reimplement this function, you get direct access to all X events that the are received from the X server. Return TRUE if you want to stop the event from being processed. Return FALSE for normal event dispatching. I'm no expert, but doesn't it say "direct access to all X events"?
It is a little more: WHICH XEvent? I believe there is none. In my opinion FVWM gets a WM specific event created by the own window decoration when hitting the "Iconify" button. I assume they first take a screenshot and then close it. You probably should find somebody who is willing to patch KWin to provide screenshots token at minimizing windows.