Rendering Swing Components to an Offscreen buffer
- by Nick C
I have a Java (Swing) application, running on a 32-bit Windows 2008 Server, which needs to render it's output to an off-screen image (which is then picked up by another C++ application for rendering elsewhere). Most of the components render correctly, except in the odd case where a component which has just lost focus is occluded by another component, for example where there are two JComboBoxes close to each other, if the user interacts with the lower one, then clicks on the upper one so it's pull-down overlaps the other box.
In this situation, the component which has lost focus is rendered after the one occluding it, and so appears on top in the output. It renders correctly in the normal Java display (running full-screen on the primary display), and attempting to change the layers of the components in question does not help.
I am using a custom RepaintManager to paint the components to the offscreen image, and I assume the problem lies with the order in which addDirtyRegion() is called for each of the components in question, but I can't think of a good way of identifying when this particular state occurs in order to prevent it. Hacking it so that the object which has just lost focus is not repainted stops the problem, but obviously causes the bigger problem that it is not repainted in all other, normal, circumstances.
Is there any way of programmatically identifying this state, or of reordering things so that it does not occur?
Many thanks,
Nick
