Also note that the FG (i.e. final gather) was already saved and frozen while the back wall was visible to the camera.
Once the wall was made not visible to the camera, Max/mental ray weren’t able to read the hair opacity accurately due to the environment map (i.e. mr Physical Sky).
After having tried every possible avenue (i.e. assigning the matte/shadow/reflection (mi) shader to wall, make the wall not to cast shadows etc...) nothing seemed to have worked; apart from assigning the "environment/background switcher(mi") parameters shader to the wall(i.e. not visible to the camera).
To apply it, simply load the mental ray shader first,as explained in the "Mental ray_Using ambient occlusion (AO)as a separate pass" tutorial below.
1-Once loaded, click on its surface toggle and choose the "environment/background switcher (mi") shader from the "material/map browser" dialog list.
2-Next, open the "environment and effects" dialog box; drag and drop the environment map toggle content (i.e. mr physical sky) onto the environment/background switcher background toggle. Choose the copy instance method, to close the dialog.
Note that, mental ray doesn’t read properly the alpha channels generated from standard material shaders; therefore opacity bitmaps are required to be applied to the cutout toggle of the arch & design shader.
Rendered alpha channel generated by the cutout toggle
I hope you have found this post useful.
More tips and Tricks:
Tips & tricks for architectural Visualisation: Part 1
Essential tips & tricks for VRay & mental ray
Creating Customised IES lights
Creating a velvet/suede material