Thanks for your answers. I'll try this. I don't know if it's the triangles, but it has definitely something to do with the mesh : on the pic above, I superimposed the render and the mesh as seen in Poser workspace, and the black spots always appear on a specific part of the mesh. I'm using MayaX render control light set. I've tried turning the shadows on and off, and I've tried the "displacement map" trick. None worked. May be it's something about welding vertices ? If anyone has an idea or knows a modelling guru, I'd be glad for your help.