With the magic group name prefix sweethome3d_window_mirror in the *.obj, the mirror's colour cast can still be controlled by the Kd floats in the *.mtl.

Expectation: because d is a float, d controls the opacity proportionally.

Observation: d = 0 makes the mirror invisible, but any d > 0, no matter how small, makes fully opaque.

When instead of d, a texture with alpha channel is applied, the latter gets even completely ignored. The mirror is opaque all over, even where alpha = 0.0

With transmission instead of reflection, d resp. texture alpha have the expected effect. Because colour works accordingly with reflection, too, I assume the observed transparency behaviour is not intended.

Have I missed something? Should I file a bug report?