Rainer,
I'm not familiar with Debian, nor with hardware. So, I don't have specific recommendations for you. I'm using Ubuntu for close to 10 years now, and I know SH3D works well. In Ubuntu there can be a conflict of Java; my solution was to uninstall all Java lookalikes and install Oracle Sun Java.
Most other problems I've seen here over the years have something to do with incompatible hardware or drivers. So, that's where I would look first. Did you read the suggestions in the FAQ?
Another suggestion: try to render a photo in Q3 or Q4. Probably the artefacts will disappear, as these renders depend on software, not hardware (like Q1 and Q2).
I also suggest upgrading to SH3D v4.6, it's easy enough.
Here are the results I get with quality setting 3. Also, perfectly reproducible. Q1 and Q2 also look as expected with closed ceilings.
kitchen
living
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Hans
new website - under constuction
hansdirkse.info
