Thank you so much for the infor, gap
I tried sth about the TDW's GR2 editor as following,
open NSS_Uboat7c41.GR2. export the mesh "NSS_Uboat7c41_delt02", then I got the exported file.
NSS_Uboat7c41_det02.mtl
NSS_Uboat7c41_det02.obj
NSS_Uboat7c41_det02_AO.mtl
NSS_Uboat7c41_det02_AO.obj
Uboat7_body.Tga
Uboat7_body_N.Tga
Uboat7C41_O01.Tga
Uboat7C41_structure.Tga
Uboat7C41_structure_N.Tga
I import this exported obj file back into GR2 again then I got 2 errors about mtl files
********Mtllib NSS_Uboat7c41_det02.mtl contains an invalid map_bump/bump definition formaterial Uboat7C41_structure_subset0: map_bump Uboat7C41_structure_N.Tga. The app does not recognize some of the entries********
*************Failed to read in C:\NSS_Uboat7C41\NSS_Uboat7c41_det02.mtl and /or failed to update Mesh's material properties************
Seems editor cannot even handle the file exported by itself
