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I still frequently get a black outline of the object instead of the green profile image.
This happens with the STL of the gear I sent for testing on the false layering problem.
Problem appears to be STL related at the same ones tend to give the false black outline while others consistently show proper previews.
Problem seems to come and go with minor tweaks to the model as I revise it.
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I guess it is wrong normal, on slicing we do not use saved normal instead it recalculated onfly. But on 3D preview generation normal being used.
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