Basically I'm drawing a lot of transparent JPanels; profiling shows that most time is spent in Component.paint(). It would be possible to optimize this quite radically since in most cases the real non-opaque area per JPanel is quite small, for example around the edges.
As it stands now, repainting() a component would trigger a repaint of all its parents, since the RepaintManager can't know the dirty parent area is in fact opaque and will go up the component hierarchy. I was thinking about calling markCompletelyClean() on all parents whenever a panel is invalidated and managing the dirty regions myself with addDirtyRegion().
However, is there a cleaner approach of marking only specific rectangles of a JPanel as opaque (or transparent, doesn't matter)?
Even if you do something fancy with markCompletelyClean() and addDirtyRegion(), I doubt you'll get much of a performance benefit. When swing goes for a repaint, it collates all of the dirty regions to paint and starts a repaint with the minimum bounding box of all of the dirty regions. Thus if you mark the perimeter of a JPanel as dirty, then the bounding box of the JPanel is the same as the entire JPanel, which means you'll be repainting the entire thing any way.