Maxon (the makers of Cinema) have e mailed me to say that 'incidents' always convert to polygons in an obj file. A fact that I didn't find in the manual. No, actually you are right about bad geometry. I've been relying on very complicated extrusions, made up of many grouped splines to make big walls full of doors and windows. I've now managed to get hold of an edge extrude plug-in which has helped me, it is naturally a slower process, but one achieves nice neatly divided polygons. Up till now I've had long messy spiders webs of poly lines which I've had to knife and stitch together to avoid them crossing window and door openings.