So is there a function which converts an image folder to the corresponding output folder?
There is the AlbumSynchronizer API that helps you getting to the output folder structure, but it turns out that the nifty methods that gets you the final step to the corresponding thumbnails and closeup images is private, so here's a code snippet, that given an album object gives you the corresponding thumbnails and closeup images. The album object is called "selected" here (the code can be run from the system console, and then acts on the selected album object):
rootOutputDirectory = engine.getInterpretedOutputDirectory();
origFile = new File(rootOutputDirectory, selected.getPathFromRoot());
thumbDir = new File(origFile.getParentFile(), engine.getThumbnailDirectory());
thumbFile = new File(thumbDir, engine.getTargetThumbName(origFile.getName()));
slidesDir = new File(origFile.getParentFile(), engine.getSlideDirectory());
closeupFile = new File(slidesDir, engine.getTargetName(origFile.getName()));