lighting bug with Gauraud shading of 'inverted' models


I came across this problem: when I transform my model with Z scale = -1, my transformed model is much darker when I use Gauraud shading, compared to Flat shading. Two pictures and the VTK model attached. The ‘Flat’ inverted model looks similar to the original. Not sure, but I guess this could be a VTK problem.

Any ideas? Should I report it there instead?



Untitled.vtk (893.2 KB)

It is dark because the negative scaling flipped all of the normals, see if the generate surface normals filter flips them back or use the caclculator filter to do so.

You’re right. If I first flip the model, and then compute the normals - the lighting is correct. But if I flip the result of the Generate Normals filter, light is wrong.