viewer.addEventHandler(new osgViewer::ScreenCaptureHandler(
new osgViewer::WriteToFileCaptureOperation("filename", "jpg")));
and the filename will be what you want. The WriteToFileCaptureOperation will add the context ID and the file number (if in SEQUENTIAL_NUMBER mode) to the file name.
(The attached also clarifies some notify messages, and corrects the comment when adding the handler in osgviewer.cpp)
I also remembered, the current architecture could allow a different CaptureOperation for each context, but currently the API only allows setting one CaptureOperation for all contexts. This could be improved if need be.
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