Threaded rendering question

mliberty

In your threaded rendering approach how do you finally draw the result. The original paint event that triggered the threads is over and you can't draw to a widget outside of a paint event. Do you use a timer to fire a paint event and then use the computed result?

Thanks!

Matthias

Yes, basically that is the way it is done.

The basic class implementing that in KLayout is lay::LayoutCanvas (layLayoutCanvas.cc) in LayoutCanvas::update_image. This class connects to the drawing threads through asynchronous callbacks (signal_transfer_done, signal_end_of_drawing).

"Asynchronous" means they can be called from any thread. They will be dispatched into the main thread by a mechanism called deferred execution in KLayout's source (tl::DeferredExecution object). That is nothing else than a Qt timer which is used to delegate the execution of a method to the main thread. Today there is "Qt::QueuedConnection" mode which to my understanding works the same way, but wasn't there when KLayout was initiated.

If you want to look further, the actual drawing happens inside a thread that is implemented in layRedrawThreadWorker.cc and layRedrawThread.cc (the names are slighly confusing as RedrawThread is actually a thread group while RedrawThreadWorker is one of the multiple drawing threads). RedrawThreadWorker regularly calles test_snapshot which itself calls wakeup from RedrawThread after having stashed the current bitmaps for image composition. The latter will trigger signal_transfer_done and this is the call that is delegated into the main thread and updates the screen image from the bitmaps.

Hope that helps,

Matthias