Translating C/C++ to Java ? Please, don't, performance will deteriorate significantly.

For this kind of computing, I don't think so. I would prefer to avoid it but if it's the only way to integrate the image generation in Sweet Home 3D, I'll have to do it one day.

[About OBJ files imported in Blender] Is it a known problem or I should report it ?

I don't think there's a problem in OBJ / MTL generation. Blender proposed many parameters at OBJ importation time. Did you try to play with them?