Yes, that's it! (the vertex in the 0.0.0 point)
Thanks for finding it
After some investigations on multiple models, the behavior is always the same: holes in a mesh are resulting in black areas in the PositionToTexture picture.
So in the final renderer, all the holes of the model are linked to the 0.0.0 point.
When I use a complex object (and complex uv) without holes, everything is working perfectly.
Here is an example patch with a simple cube with holes and cubic UV:
PositionFromTextureFORUM2.zip (10.3 KB)
In the PositionFromTexture shader, would it be possible to ignore the black pixels?